Acoustic Design

May 20, 2018 | Author: John Erfe | Category: Hertz, Sound, Hearing, Pitch (Music), Waves
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acoustic design

Acoustic design

- sound of architecture

Part of the


2008 Acoustic Design – sound of architecture Author: Erik Ipsen, Marie Leth Rasmussen Editors: Peter Boe, Vesna Hodzic-Mehic, Marie Leth Rasmussen Page layout: envision Printer: BB Offset, Bjerringbro Danoline, HQ Antoinettevej 3, DK-2500 Valby, tel. +45 36-159000 [email protected], Danoline, Sales and Technical Service Kløvermarksvej 6, DK-9500 Hobro tel. +45 96-573000 [email protected],






Sound is here to stay

We can perceive sound even before we are born. And when our time comes, hearing is the last of our senses to leave us. To put it another way, our whole lives are framed by sound. We use it to find our way around, to learn and to express ourselves. We use it to live. This is an extremely important fact for architects, engineers and anyone else whose job it is to create the physical context of people’s everyday lives. For there will always be sound – you could say that it is here to stay. This means that the choice is not between good acoustics and no acoustics – the only alternative to good acoustics is bad acoustics. Bad acoustics are an obstacle to all the impressions and insights that sound can give us. For example, bad acoustics make it more difficult to enjoy music, acquire knowledge and enter into dialogue with other people. Fortunately, it is relatively easy to avoid bad acoustics. At least, it is if the acoustics are taken into account in the planning phase. And this is where “Acoustic design – The sound of architecture” comes in. This book is a basic introduction to sound and acoustics, intended to allow architects and engineers to cooperate on acoustic issues as an integral part of their disciplines, and to encourage them to take account of acoustics whenever they create the spaces in which people work and live. “Acoustic design – The sound of architecture” came about thanks to the efforts of Danoline employees and our external partners. We would particularly like to thank the acousticians Christian Simmons (from Simmons akustik & utveckling), Bo Gärdhagen (from Gärdhagen Akustik AB), Lennart Karlén (from ACAD International) and Niels Jordan (from Jordan Akustik), the architect and acoustician Jens Bak (from AB Studiedesign) and the architect Karsten Bro (from Arkitema) for their contributions.

John Christensen Director, Danoline


CHAP1 A HISTORY OF SOUND simply bounces along the water, which is why sounds can be heard over very long distances out at sea. The same happens on land when dew is falling – the sound experience becomes more intense. You can hear this for yourself in a forest at night, for example, where you would perceive many more sounds after the dew has fallen. That is why soldiers make sure they are particularly quiet at night. The effect of the moisture simply speeds up the sounds and reflects them for longer.

The first human sounds Photo: Imageclub

Sound has played an important part throughout human history. For early humans, the ability to listen was a matter of life and death. Using their sense of hearing was the only way they could perceive potential prey or enemies nearby. Sound was crucial. For example, sound could be used to frighten animals into certain places where it was easier to catch them. Like an early round-up, it is easy to imagine that the echoes from rock faces were used to confuse the quarry and move it in a certain direction. Yet sound behaves differently depending on where you are, because it is acted upon by many different natural phenomena. You can clearly hear this if you stand in front of a rock face, for example, and shout. The rock reflects the sound, which returns as an echo. However, the echo will not be the same for every rock face – for example there might be overhangs and vegetation that filter out certain elements, transforming the sound. If you stand on a hilltop higher than the surrounding trees, you will hear sounds coming from far away.

Humans have known about this relationship between sound and natural phenomena for millennia. Because the earth was sparsely populated, people used their own calls and a range of instruments capable of producing sound. For example, drums and trees were used to send messages to other humans far away. Australia’s indigenous people used an instrument called a bullroarer to produce low frequency (deep) sounds capable of creating such high sound pressures that they could be heard over very long distances. The Australian aboriginals used the low frequencies because they can be carried over longer distances than high frequencies. You can hear this when a

The Australian aboriginals knew that low frequency sound could be used as a way of communicating over long distances.

Photo: Imageclub

The surface of water is good at transporting sound over long distances.

Photo: Beeline

The surface of water is also good at transporting sound over long distances. In principle, the sound

Photo: Imageclub

The terraced seating ensured that the sound was carried upwards instead of being absorbed by the audience in the bottom rows.

fighter flies past. You hear the high frequencies first, which gradually disappear until the only sound left is the low rumble.

in the bottom rows. There was another wall behind the audience to contain the sound, allowing people right at the back to hear what was happening down on the stage.

Acoustic design in ancient times – the Greek amphitheatre

Down to the smallest detail, the Greek amphitheatres were created in order to guarantee the optimum sound experience for the audience. Some academics even claim that the seats in some of the theatres helped the acoustics. They think the seats were raised to create an air space between the seat and the terrace – the air space amplified the sound.

The lofty interior creates a very special sound environment.

The Christian church – a study in sophisticated sound design

Sound also has an important part to play in church. You notice this the moment you walk in: the sound inside a church gives you the feeling that you are walking into a large and high space. Into something that’s bigger than you. This experience is no accident – it is the result of acoustic design. For

Photo: Imageclub

To hear the high frequencies for longer, we need to do something to keep hold of the sound. This was already known to the architects building the amphitheatres in Ancient Greece, which allowed large numbers of people to see and also hear what was happening on stage. On the stage, the actors’ voices were amplified by a rear wall, reflecting the sound and carrying it out to the audience. However, the main acoustic achievement was to construct the theatre with terracing. The terraced seating allowed the audience to sit in rows and follow what was happening on stage without obstructing each other visually and also acoustically. What the terraced seating did was to ensure that the sound was carried upwards instead of being absorbed by the audience


This means that acoustics also had a disciplinary element, focusing attention on the minister, who was able to play on the sound response of the building and the rhetorical effects of voice production – by talking slowly, the minister can leave his voice hanging in the space, without doubt increasing thereby the impact of his message. Hymn singing is another important part of the church-going experience. The arches and the shape of the space are designed so that the various choirs complement each other. And the precise location of the choir is no

In cinemas, the sound from the space and from the audience has to be muffled, leaving a clear sound from the film.

accident. In churches, the choir is carefully placed so that the singing can make the whole space vibrate. You can hear this in Catholic churches – masses are sung in long monotones, making full use of the acoustics. Put simply, the church interior becomes one big musical instrument. In some churches, pots with small openings were even set into the vaulted roof, and ash was added in order to correct problems with the sound.

New acoustic challenge for architects: the cinema

Up to the First World War, acoustic strategies were generally reserved for buildings like churches and theatres. Things changed between the wars, when architects started to apply the principle of acoustics to other types of building like the first cinemas. In cinemas – unlike churches – the space itself should not be a factor, and should have the smallest possible influence on the sound experience. Basically, the sound from the space and from the audience has to be muffled, leaving a clear sound from the film. At the time, no sound absorbing materials had yet been invented, so architects had to find other ways of muffling the sound. What they came up with was angled or jagged walls to scatter the sound, and wooden panels with gaps to absorb the sound.

Biocity, Aalborg, Denmark. Photo: Finn Manford

more than a thousand years, builders have deliberately striven to create churches intended to demonstrate the power and might of God. The church often occupies the highest point in the town for the same reason – it is raised above all other buildings, towering above the town’s more worldly institutions. Inside, the spaces contained soaring arches, which were designed to illustrate man’s strive towards heaven but which had another acoustic effect that was just as important. Raising the ceiling and using hard materials on the floors, walls and ceilings created a building with a long reverberation time. These acoustic signal gravity and solemnity – it is an acoustic environment in which you automatically lower your voice.

Lecture hall in the Viipuri Library in Finland, designed by Alvar Aalto. Note the undulations in the ceiling.

Upholstered chairs and thick carpets also helped to dampen the sound from the room. Although upholstered chairs and thick carpets are still essential features of cinema interiors, there have been many other changes. In early cinemas, the sound travelled through the screen. These days, the sound is split up, with the high frequencies coming from the side walls and the low frequency speakers behind the screen. It is now possible to muffle the sound from the room entirely using diffusing panels to break up the sound.

Photo: Martti Kapanen, Alvar Aalto Museum

The ceiling became the focus of attention in attempts to regulate the acoustic environment. The functionalists, in particular, did a lot of work with the structure of the surfaces, especially in lecture halls, where it was important for the sound to reach the whole audience. The Viipuri Library in Finland, designed by Alvar Aalto, is an example of a lecture hall in which the acoustics are an integral part of the design. The ceiling has undulations, with sound reflectors on some of the curved surfaces and slits to absorb the sound in others. Using these methods, Aalto managed to ensure that the sound reached every part of the room. There is another example of good acoustic design in the old airport terminal in Kastrup, designed by Wilhelm Lauritzen. Taking his inspiration from Alvar Aalto, he decided to install an undulating ceiling in the arrivals hall. But the intention here was to scatter the sound and reduce the echo. He also arranged the shop fronts so they were angled in relation to each other, preventing the sound bouncing off the frontage opposite and resounding within the space for long periods of time. However, despite these advances, in the interwar period and the years following the Second World War there was no coherent strategy on how to design spaces with good acoustics. Fortunately the architects of the period usually

In many ways, the interwar period was a time of great change – people were experimenting in lots of different areas. The field of acoustics was no exception. Experts started to develop measuring instruments that were able to measure the sound in a room, and there were early experiments to find out how to design materials capable of absorbing sound. At the same time, an increasingly rational approach was being used in construction. Savings needed to be made in the cost of materials, and an easy way to reduce cost was to reduce the ceiling height, which by now rarely exceeded five metres.

The old airport terminal in Kastrup, designed by Wilhelm Lauritzen. Another undulating ceiling.


A period of experimentation

Photo: Strüwing


The Novo canteen in Bagsværd, designed by Arne Jacobsen, with angled walls, ceilings and sound baffles.

designed spaces in a way that minimised the worst problems associated with poor acoustics. One example of this is the Novo Nordisk building designed by Arne Jacobsen and built in the mid-50s. The canteen, where large numbers of people gather for lunch every day, was fitted with angled wall surfaces to remedy unwanted echo, and there are slits in the ceiling and absorbers on the walls.

Acoustic design saves on the materials needed to modify the acoustics

These days we have measuring instruments to analyse how much noise there is in a room. We also understand much more about how noise can affect people’s well-being and working conditions – which means that the acoustics of a building have to meet higher and higher standards. Yet in spite of the growing demands placed on acoustics, less importance is placed on the planning of the acoustic environment today than in the past. Instead of integrating the acoustics into the design like Arne Jacobsen or Alvar Aalto, many architects


choose to create their buildings without giving acoustics a second thought. All too often architects build box shapes without knowing how the acoustics behave in quadrilateral rooms. So the acoustics are not even considered until the final phases of construction, when materials have to be added to the ceiling and/or walls in order to modify the acoustics. This solution is often unsatisfactory, especially when the materials are positioned in places that do not suit the architect’s original plans. If, on the other hand, the room is planned so that form, function and sound are all integrated from the start, there is a lot of money to be saved on sound-regulating products, while still achieving optimum sound comfort. And, just as important, the original architectural vision can be realised without being disrupted by these added elements. To help them achieve this goal, architects can use a range of acoustic interventions when they design a room. The rest of this book explains how to use these interventions to create rooms in which good sound is one of the factors contributing to the overall aesthetic experience.


CHAP2 what is sound?

To understand acoustics, we need to understand what sound is. Sound is both highly tangible and highly abstract. Take a perfectly normal conversion between a mother and child in the supermarket for example. The little girl asks for something, the mother says no, and the conversation ends – something that happens every day. But if we go into detail and examine what is needed to allow this conversation to take place, things start getting more complicated.

Let’s go back to the conversation between the mother and child. As mentioned above, a wave is formed from the molecules in the air, which start vibrating when the child speaks. The wave creates a pressure of just a few thousandths of the normal atmospheric pressure of 1013 millibar. That is why we use a special scale to measure sound pressure – decibels (dB). dB is the unit that is used to calculate whether there is too much noise in a room.

The moment the child asks for something, she forms a sound by blowing air over her vocal chords. This causes the air molecules to start vibrating and banging against each other, forming waves that leave the child’s mouth. The waves then move from the child’s mouth to the mother’s ear drums and are converted inside the brain so that she can understand what

As we explained above, sound consists of waves – also known as vibrations. This is why we use the unit hertz (Hz) when describing sound, which represents the number of vibrations per second. The length of a sound wave varies according to the nature of the sound. The wavelength for low pitch sounds is long, and shorter for higher pitches.

We learn to recognise our mother’s voice even before we are born.


Describing the event from the point of view of physics, the child is using energy when she speaks, and the mother is also using energy when she converts the waves inside her head. So in effect, sound consists of the transport of heat – although we are talking about very small amounts. If a symphony orchestra plays for a couple of hours, it only generates enough heat to warm up half a cup of coffee by one degree.

Photo: Imageclub

A little girl speaks. Triggering a complex physical process.

Photo: Imageclub

her little girl is saying. A simple conversation is a complex event.


Canon fire, burst ear drum


Powerful fireworks


Rock concert


Pain threshold of ear


Noise limit in places of work


Normal conversation


Quiet room


Soft rustling of leaves on a tree


Accordingly, low pitches have a low frequency and high pitches have a high frequency. The length of an oscillation is the frequency in Hz divided by the speed of the sound wave. General acoustical literature covers the range from 125 Hz or 125 vibrations per second, to 4000 Hz or 4000 vibrations per second. In speech, the most important information is contained in the frequencies between 250 and 3150 Hz – this is also known as the consonant frequency range, and is where the ear is at its most sensitive. People with normal hearing can perceive sounds between 20 Hz and 20,000 Hz. A sound with a frequency of 20 Hz will be experienced more as a pulsating effect throughout the body, rather than as a sound registered by the ear. Sounds below 20 Hz are called infrasound, and sounds above 20,000 Hz are called ultrasound. People cannot hear ultrasound, but dogs and certain other animals can. In fact, there are special dog whistles that create sounds higher than 30,000 Hz.

Sound is the first thing we perceive – and the last

Sound is also capable of travelling through materials. That is how babies can experience sound while still in the womb. We learn to recognise our mother’s voice before we are born, helping to prepare us for the world of speech waiting for us after our birth. We perceive sound even while sleeping. And hearing is usually the last sense to leave us before we die. A great deal of information is carried by sound.

All perception of sound is subjective. Some people like classical music; others like heavy metal.

Photo: Imageclub


Voice communication is essential for humans, and understanding what is being said involves much more than the meaning of the words. The tone of voice and rhetoric used are also important elements in understanding. Rooms with poor acoustics muffle important information, making it difficult to decode what is being said. If a room is used for teaching or group work, it is important for it to have a good response or sound reflection – so what is being said is actually what is being heard. At the same time, the sound must not be drowned by an echo or a long reverberation time. To stop this happening, soundabsorbing objects can be placed in the room, or absorbers can be installed. If there are not enough absorbers in the room, the sound may become bothersome and be perceived as noise. In other words, if speech, with a normal sound pressure of 60 dB, is combined with background noise exceeding 40 dB, the sound of the voice is masked to such an extent that it becomes difficult to understand.

Good sound – a good question

What is a good sound? Ask ten different people and you’ll get ten different answers. For some people, classical music is a calming and positive experience, others prefer a heavy metal concert – and


others still would rather not listen to music at all. The perception of what makes a good sound depends very much on the environment we grow up in. Take people’s perception of road noise, for example. For people living in London, road noise gives a sense of security. It is what they are used to hearing. They find silence unnerving. For people brought up in a remote farm in Sweden, the perception is usually the other way around: silence brings a feeling of well-being. The perception of sound depends on a large number of other factors: physiological factors, subjective taste and acquired/ cultural perceptions of sound. Different professional groups have different ideas of what constitutes positive and negative sound. A person working in marketing will often be happy in a

sound-intensive environment, whereas kindergarten staff prefer to muffle sounds as much as possible. But the sound should always match the environment. It strikes us as fake if there is a mismatch between sound and function. Designers are well aware of this in the car industry for example, where they deliberately try to create sounds that people associate with a good and solid product. The door has to sound right when it closes. The same is true in construction – if the sound is wrong in an environment, the effect can be jarring, for example if a large open room sounds the same as a sitting room. That is why it is important to know the function of the room when planning the acoustics. Only then is it possible to create a harmonious room, which supports the activities taking place.

Are kindergartens noisy? The answer depends on whether you ask the children or the grown-ups.


Photo: Imageclub

The same is true of silence.

Daycare Centre Islemark, Rødovre, Denmark. Photo: Rune Johansen

Different people perceive road noise differently.

Photo: Imageclub

CHAP2 what is sound?



If you want to modify the sound in a room, there are basically six different things you can do with it. You can:

• Direct the sound in a particular direction • Limit the sound • Emphasise the sound • Spread the sound • Dampen the sound • “Colour” the sound

In the open air, sound moves around unhindered at a rate of around 340 m/sec. But as soon as the sound encounters a barrier, it changes. If you go from open land to the middle of a quadrangle for example, you will notice the sound changing

»Sound waves« along a breakwater.


character. The reason is that the buildings act as a barrier to the sound waves, sending them back where they came from. The type of barrier encountered by the sound is also important, because the material composition and surface structure of the barrier affect the sound. Also important is the distance to the barrier and the angle at which the sound hits it. If you are standing in a farmyard surrounded by buildings with uneven facades, the character of the sound is different from a location where the surrounding buildings have large areas of glass in their facades. The type of surface therefore influences which sounds are reflected and which sounds disappear or are dampened/absorbed.

Sound moves like a billiard ball

In rooms like lecture halls and conference rooms, it is important to be able to direct the sound in certain directions. To direct sound, we need to know how sound behaves in a room. Sound moves in a spherical way – when you speak, sound waves are emitted in all directions, forming a ball shape moving away from the sound source. A good way to

Photo: Martin Tørsleff

Once you become familiar with the basic acoustic phenomena, it is relatively straightforward to create acoustics that matches the function of a particular room. You will also be able to avoid structures that will inevitably create poor acoustics. This chapter is intended as a basic introduction to what determines the character and strength of a sound.

Photo: Imageclub

Sound moves like a billiard ball.

One last illustration of the movement of waves is to think of a billiard ball. If you send a billiard ball straight into the cushion, it will come straight back again. But if the ball hits the edge of a pocket or strikes the cushion at an oblique angle, the direction of the ball changes and ball has a longer

distance to travel after it is deflected – this causes the ball to slow down. The same happens with sound waves. If you keep in mind these three easy ways of picturing the sound movement patterns, it is relatively easy to control the sound in a room.

Sound and air impedance – or how to sound like a Smurf

As we mentioned above, sound is influenced by

Out in the open air, sound can move unhindered.

Photo: Beeline

visualise sound movement is to imagine a stone being thrown in the water: the point where the stone touches the water is the sound source, and the ripples in the water are the sound waves. Now picture the ripples moving not just in two dimensions but in all three dimensions. If the ripples hit an obstacle as they move away from the sound source, the backwash will reflect the contours of the obstacle. Another way to picture the pattern of sound movements is to think of waves hitting a breakwater. If the long axis of the breakwater is at an oblique angle, the waves run alongside the breakwater, and they are reflected back if the breakwater is at right angles to the waves. But if there is an embankment of large stones in front of the breakwater, the waves are absorbed. The reason is that some of the water flows between the stones, which interrupt the water on its way back, dissipating the energy of the waves.


Veikkola School, Finland. Photo: Finn Manford


In large sports halls, it is sometimes difficult to attract the attention of team mates.


many different factors. One of these factors is the impedance of the air itself. It is the air impedance that causes the sound to diminish as the distance to the sound source increases – if you stand right next to the sound source, the sound seems more intense than when you move away. Over longer distances, the air impedance even causes the sound to be noticeably delayed. For example if you see a man in the distance hammering a post into the ground, you can sometimes see the hammer hit the post before hearing the sound. The reason is that the sound takes longer to arrive than it takes to strike the post. Sometimes, the sound never arrives. In large sports halls it can be difficult for a team member to attract the attention of one of his team mates if they are at different ends of the hall, because the sound can easily die away before it gets there.

like Donald Duck. Heavy gases have the opposite effect. Their increased air impedance causes the sound to lose its energy quickly.

Air impedance occurs because the air molecules push against each other. It is not a fixed quantity – for example a light gas like helium has lower air impedance than the air around us. This means that the sound moves over a longer distance and at higher speed. You can hear this for yourself if you breathe in the helium from a balloon. The light air from the balloon makes the sound travel faster, so you sound

Winds and ventilation, on the other hand, are important to the sound level, since sound is transported by air molecules. The molecules are slowed down by a strong headwind, so it can be difficult to attract the attention of people with the wind in their back, but relatively easy if the wind is blowing the sound towards them.

Air humidity as an amplifier

As we mentioned above, sound is also affected by air humidity. The reason for this is that the humidity reduces the air impedance, making it easier for the sound waves to travel through the air. For example, we would perceive noise levels to be higher in an abattoir, with high air humidity, than in a joinery, where the air is dry. The temperature of the air does not itself influence the sound, but a high temperature is sometimes associated with high humidity.

It may at first sight seem illogical to consider the relationship between sound and wind when designing rooms inside buildings, but mechanical ventilation means it is a good idea to take this factor into account. In mechanical ventilation, the intake side is the noisier, so it should be placed a long distance from where people will be talking to each other.

To recap, the distance from a sound source to a specific barrier has an effect on the character of the sound. For example, the distance determines whether the sound is reflected to create an echo. The echo will be most pronounced if the barrier consists of a hard vertical wall and the sound covers a total distance of 17 m or more.

There is no echo here. But the same footstep 10 m from a wide wall in an empty car park would create an echo.

Say you are on a walk one quiet Sunday afternoon and you approach the wall of a large windowless building, for example a shopping centre. Try scraping the soles of your shoes over the road surface – you will be able to hear the echo. If you now experiment with shorter or longer distances you will find that the echo is weak from 0 to 8 m, strong from 8 to 20 m, and you need to scrape more vigorously to create an echo at distances over 20 m.

Photo: Imageclub

Gymnasiums and sports halls usually have a lot of echo.

Pohjoispuiso School, Finland. Photo: Finn Manford

the other, and strong from 8 to 20 m, when the brain can easily tell the two impulses apart. When the distance exceeds 20 m, the sound needs to be louder in order to reach the wall and be reflected back. The intention of this explanation is to show how important distances are in determining how much echo there is in a room. Gymnasiums and sports halls usually have a lot of echo. The reason is that reducing the echoes is rarely a priority, even though it is possible to do so. In cinemas, on the other hand, we are very careful to remove echoes either by slanting the walls, a technique frequently seen in older cinemas, or by covering the walls with sound absorbing and diffusing surfaces. This is the standard method of creating good sound in a cinema today.

The echo is caused by the sound taking time to reach the wall and travel back again. In technical terms, if the sound takes 50 milliseconds or longer to return at a speed of 340–360 m/sec., the brain will perceive the reflected sound as an entirely new sound – the sound we call the echo. That is why the echo is weak from 0 to 8 m, when the brain is barely able to distinguish one sound impulse from

The wind is important to the sound level.

Photo: Imageclub

What is an echo?


CHAP3 ACOUSTIC PHENOMENA The shape of the barrier also affects the sound

Photo: Martin Tørsleff

As we explained above, the shape of a barrier influences the way the sound behaves. When it encounters the barrier, the sound can be transformed, reflected or diffused. This depends on the shape.

• If the barrier is shaped like a ball, the sound is diffused.

Different materials affect the sound differently

The character of the sound is also affected by the materials from which a barrier is made. Different materials respond to different sound frequencies: for example, if a lorry or bus drives past a house with large windows, the panes of glass start rattling. The technical explanation for this is that the panes of glass are in resonance – in other words the sound waves are transformed into kinetic energy in the glass, which absorbs some of the incoming sound. As we have discovered, the effect on the materials depends on the frequencies of the sound. When your neighbour plays loud music, it is usually only the bass sounds that penetrate the walls. This is because the high frequencies are not strong enough to pass through heavy materials and start the wall vibrating.


Photo: Martin Tørsleff

Photo: Martin Tørsleff

• Like ripples in the water, sound waves surround and reflect the contours of an object.

• In spaces with two parallel walls or similar surfaces, the sound bounces back and forth between the two surfaces. If the surfaces are placed at an angle instead, the sound pressure can be diffused, preventing the sound from “hanging” in the room.

Only the low notes can do this. As the low notes pass through the wall, causing the wall to vibrate, they lose the energy generated in the room where the music is playing. But when the high notes hit the wall, they are reflected back into the room. This knowledge of the way materials vibrate is used in acoustics to design sound absorbing panels, which can reduce the reverberation time and improve the acoustics in a room. The sound absorbing panels are fixed to walls and/or the ceiling, and when a sound wave hits a panel, it starts vibrating. When the frequency of the sound wave is the same as the resonance frequency of the panel (its ability to oscillate with the sound), a large proportion of the arriving sound waves start the panel vibrating, causing the sound to be absorbed.

Photo: Martin Tørsleff

Photo: Martin Tørsleff

• When waves meet even surfaces at an oblique angle, the waves are reflected and mirrored.

But as the stiffness or mass of the structure increases, the material acts like a trampoline, bouncing the sound back into the room. In other words, the absorption is reduced and the reverberation time is increased. These factors can be used in the design of concert halls, for example, where the music must be clearly audible even at the very back. That is why sound propagation is taken so seriously – sound waves must not cause the panels to start vibrating – they must not be absorbed.

Photo: Martin Tørsleff

• If the barrier is bowl shaped, the sound is reflected to a central point.

Photo: Martin Tørsleff

• If the barrier is flat and at right angles to the sound source, some of the sound returns as an echo.

• When waves meet uneven surfaces at an oblique angle, the waves are diffused and spread in many directions.

Ear trumpets were often made of a cow’s horn, and shaped to produce greater sound pressure at one end, making it easier to hear what was being said. The same thing happens when you cup you hands

Sound makes a greater impression if the sound pressure is increased by concentrating the sound using a funnel shaped structure. The principle is familiar from old fashioned ear trumpets.

Sound is a good thing. But you can have too much of a good thing.

Photo: Imageclub

Concentrating and spreading sound – or louder and quieter sound


If you open the window on a busy road, you will hear the high frequencies.


Photo: Imageclub


behind your ears – you are not only concentrating the sound to hear more clearly, but you are also excluding other bothersome sounds.

Fingernails on a blackboard and an electric bass at full volume – about high and low frequency noise

High frequencies are the first to die away as a result of resistance from the air, whereas low frequencies are carried over long distances. Noise in the high frequency range affects our ears the most. High, squealing sounds are very annoying – just think of the sound of fingernails scraping down a blackboard. To reduce or eliminate high frequency noise, the room design must include structured surfaces and porous materials. The porous materials absorb the high frequencies as a result of friction within the material, while the structured surfaces send the sound waves on a longer journey, where the air impedance finally wears out the sound waves. High frequencies are usually created in the same room as the listener, because they can only penetrate structures with great difficulty. For example, if you open a window onto a busy road, you will be able to hear the high frequencies that are blocked with the window closed. That is why gaps in structures or openings between two rooms should be avoided – they allow high frequen-

It is quiet at the bottom of the wall. As you move higher, the sound from the other side of the wall becomes louder.

cies to pass from one room to another. Whereas high frequencies affect our ear the most, low frequencies affect our environment the most. Low frequency sounds can pass through virtually anything and cause problems – for example heavy road traffic causing windows to rattle, or footsteps in the corridor outside a classroom, disturbing the lesson. This is the kind of background noise we all have to live with one way or another. We are also familiar with low frequency sound from rock concerts, where the powerful bass can create pressure on the chest and make breathing difficult. High frequencies are muffled using porous and structured surfaces, but the way to reduce low frequency sound is to use heavy structures, or structures consisting of several layers of varying thickness. To limit low frequencies within a room, you can use panel materials with a carefully specified weight, rigidity and thickness. A low frequency absorber can also incorporate air gaps with a length greater than the sound waves to be absorbed. The depth and width of the cavity must be calculated carefully for optimum effect.

A fingernail is silent. Unless it is scraping down a blackboard.

Photo: Imageclub

Sound can also be diffused. If you sit behind a wall and slowly stand up, the sound from the other side of the walls gets louder the closer you come to the top. This shows that some of the sound is deflected, and this should be taken into account when screens are used between desks in office landscapes. The screen acts as an absorber, or rather as a filter removing the background sound in the room. However, the screen also has the effect of concentrating the sound and making the sound source clearer. If there is a space between two screens, the sound will behave in the same way as waves passing through a harbour entrance and fanning out on the other side.

Photo: Imageclub

The phenomenon is noticeable when two rooms are connected by a narrow corridor. This arrangement can create acoustical problems, because the corridor concentrates the sound, propagating it from one room to the other.



Sound versus sound – about anti-noise and masking

Anti-noise is another relatively advanced way of suppressing sound. With anti-noise, sound is emitted at the same frequency and strength as the original sound. The new sound forms anti-waves, which cancel out part of the original sound. This reduces the general sound pressure. HiFi systems used to describe the speakers as being in antiphase: this means that a mask was superimposed onto the music, and if the speaker leads were switched, the speakers came into phase, making the music signal clearer. There are some types of absorber that use the principle of anti-vibrations to eliminate the sound pressure. They are called resonator absorbers. Resonator absorbers are panels with perforations, designed to work with a particular frequency range according to the location and the size and density of the perforations. Absorbers of this type act as a panel membrane (they absorb the low frequencies), a resonator

If you slow down, the car sounds different.


Another way to dampen sound is to mask it. This method involves playing a soft hiss in the room to mask part of the sound. It is useful in open plan offices, for example, where lots of telephone calls are going on at the same time, or other distracting activities are taking place. The same phenomenon can be experienced in the car – the wind noise and engine noise mask the other sounds within the car. If you switch off the engine but carry on moving, you can hear the sounds you normally do not hear, like the squeaking springs in the rear suspension.


In this chapter we have shown how a knowledge of the properties of sound waves can be used to control and design the sound in a room. Sound waves can be gathered, diffused, angled and transformed in any number of ways, and the materials used are particularly important because their structure and fundamental vibrations help to give sound its character. In other words, sound is more than just sound – it is something that we create ourselves and that we can shape as we want, as long as we know how sound behaves.

Photo: Imageclub

Sound on the move.

Photo: Imageclub

(they absorb sound in the mid range), and as a diffuser (they absorb high frequencies). In terms of sound absorption, perforated absorbers can therefore cover the entire frequency range.


Photo: Imageclub


In a room with hard materials like concrete and glass, the sound has nowhere to go. You can hear it.

When architects select building materials, they are generally conscious of the signals the materials are conveying. Are they going for a futuristic look with zinc panels on the facade, or a more traditional feel with bricks? The same process should be applied to sound. How do the materials affect the acoustics in the room? Do they create a frenetic mood or a more relaxed atmosphere? Good acoustics are essential in making a room pleasant to be in, whereas bad acoustics can frankly make a room unusable. That is why the choice of materials cannot be taken lightly. Sound waves behave differently depending on the materials they encounter, so the choice of materials determines whether the room is perceived as hard or soft, frenetic or calm.

Hard materials reflect the sound

In a room with hard materials like concrete and glass, the sound has nowhere to go. The result is that the sound waves are reflected and sent straight back to the sound source. This is the characteristic echoing sound you hear in the bathroom. The hard


surfaces on the walls, ceiling and floor return the sound, which remains within the room for longer. Or to put it another way: the sound has a long reverberation time. In rooms with a long reverberation time, the sound is dampened only slightly as the distance to the sound source increases. This means that a relatively large number of people can be disturbed by just one or two voices, and it can be difficult to hold a conversation when the sound stays hanging in the room.

Soft materials absorb the sound

If you enter a room with soft or porous materials on the ceiling, floor and walls, you will perceive the sound as muffled and soft. This is because the air molecules hit the material and encounter high resistance, slowing down the molecules. The sound waves are absorbed by the material, the sound is dampened, and the reverberation time is shortened. That is why there are soft chairs and carpets on the floor in cinemas and concert halls, where the sound from the audience needs to be mitigated. The carpets and chairs quite simply absorb a proportion of the sound – primarily the high frequencies.

Uneven surfaces diffuse the sound

The character of the sound can also be modified by using materials that diffuse the sound waves. You can hear the sound changing as it hits the walls. If the surface is textured, the low frequencies will not be affected because of their long wavelength, but the high frequencies will be diffused throughout the room. Depending on the depth of the contouring, the sound absorption can be enhanced: if the depth of the contours is correctly specified in relation to the length of the sound waves, the reflected waves can meet the waves from the source, creating additional absorption. In

Furniture is not just visual – it also has an acoustic effect.

rooms with hard materials, the sound can be broken up by creating protrusions, edges, etc., making the surface three-dimensional. This diffuses the sound into the room and makes it less harsh, while also making consonants easier to understand.

Sound is created by the materials

Materials create the underlying atmosphere in a room. Some materials are good at absorbing low frequencies like road noise or the bass from the sound system downstairs. Others materials are better at absorbing high frequencies like shrieking from a kindergarten. So before selecting materials, it is important to establish exactly what the room will be used for. Function and acoustics must be well matched. Although the underlying atmosphere is established by the materials, absorbers can be used to finetune the room to give it the right character – in much the same way as a musician tunes an instrument. Some absorbers create a hushed atmosphere in a room, others create a brighter sound. It is important to know these characteristics when selecting the materials. It makes a big difference whether the room will be used for teaching, play, meetings or lectures. Each room has its own acoustics.

Private Residence, Holstebro, Denmark. Photo: Henriette Torp

When a new major concert hall is built, chair manufacturers may be asked to document precisely how much sound the chairs absorb, allowing the acoustician to design the right sound environment. During the planning of Danish Radio’s new concert hall in Ørestad, for example, the designers wanted to incorporate the elegant leather chairs from the old concert hall, but were quickly informed by the project’s acoustician that they would not help to create the right acoustic environment. The chairs had to be abandoned. The reason is that chairs must absorb the same sound as a person, so that the hall has the same acoustic profile regardless of attendance. Perfect acoustics requires attention to detail.



Materials can be smooth, rough or porous and so you might expect the materials themselves to have special acoustic properties. However, this is not always the case. Take stone, for example – a hard material. In the bathroom, stone is used in the form of tiles, because they are easy to clean and can tolerate water. The tiles create a harsh sound environment, but the harshness disappears when the stone is turned into fibres to create stone wool – a porous material that can be used as an absorber. This shows that the treatment of the material influences the acoustic properties.

Panel materials can also change character

Just as stone can be changed from a dense material to a porous material, the characteristics of the material can be further modified by cutting it into very thin slices to create a panel absorber. In principle, the panel has a good response in the high frequencies, while the low frequencies are capable of causing the panel to start vibrating. If the panel starts vibrating, it converts a proportion of the low

Why do we sing in the bathroom? Because the acoustics lend themselves to it.


frequencies into energy, thereby acting as an absorber. However, if the panel is embossed to give it a rough surface, the effect is different – the sound waves are diffused and high frequency absorption takes place. For extra diffusion, holes can be made all the way through the panel. Another way of altering the properties of the panel is to position it in front of, but not touching, a wall, with an air gap between the panel and the wall. The gap between the wall and the panel creates resonance, making the panel behave like a high frequency absorber. So you can change the panel’s effect on the sound according to the way you treat and use it. You can even attach a fabric or porous mat to the rear of the same panel to cover the holes – this enhances the absorbing effect in the mid to high frequency range. When you plan the acoustics of a room, you can choose to use a single type of absorber on the ceiling and walls. Alternatively, you can combine different types, thereby creating acoustics perfectly adapted to the function of the room. Here is an overview showing how the various types of absorber work:

Photo: Imageclub

The acoustic properties are affected by the treatment of the material

• Mineral wool without a painted surface acts as a high frequency absorber.

• Structured (roughened) or perforated surfaces fixed to a flat base can normally be used to absorb high frequencies.

Mineral wool directly applied to a base.

• Mineral wool with a thin layer of paint acts as a high and mid frequency absorber. • Mineral wool with a thick layer of paint acts as a low frequency absorber.

Thin panel absorbers on a hollow base.

• Perforated absorber panels on a hollow base are capable of low frequency absorption or mid frequency absorption, in addition to sound diffusion.

Surface treated mineral wool directly applied to a base. Perforated panel absorbers on a hollow base.

• Thin hard panels are usually suitable for use as low frequency absorbers.

• Perforated panel absorbers with a felt or acoustic cloth backing, applied to a hollow base are capable of low frequency absorption, mid frequency absorption, high frequency absorption and sound diffusion.

Perforated panel absorbers directly applied to the underlying structure.

The properties of the absorbers can be adjusted and fine-tuned in many other ways too, to give the space the desired character and “tone colour”.

Perforated panel absorbers on mineral wool.


The properties of the absorbers can be adjusted and fine-tuned in many other ways too, to give the space the desired character and “tone colour”.


Private Residence Arendal, Norway. Photo: Karl Ture Sagen, Reklamefotograferne



CHAP5 SOUND IS SOMETHING WE CAN BUILD The sound environment is influenced by more than just the choice of materials. The arrangement of structures is an important element in the acoustic environment, and the smallest of changes can have a dramatic effect on the acoustics.


The characteristics of sound absorbers are entirely dependent on how they are installed. If the absorbers are installed with a cavity behind them, the characteristics will not be the same as if they are fixed directly to a ceiling or wall. In other words, the cavity is an acoustic factor by itself, and if the depth of the cavity is changed, the absorber’s characteristics also change. In the case of sound transparent elements like perforated panels or porous panels, a cavity of 50 mm means that sound absorption will cover the high frequencies. If the depth of the cavity is increased to, say, 200 mm, the absorption of the same product will cover a wider range, absorbing more of the low frequencies and mid frequencies. Increasing the depth of the cavity will further increase absorption in the low frequencies. A change in the structure therefore brings about a change in the character or “tone colour” of an absorber.

Absorption in the low frequencies

In acoustic design, it is often difficult to achieve adequate absorption in the low frequencies. The reason is that unlike the high frequencies, the low frequencies can only be absorbed to a very limited extent by diffusion and absorption from furniture and other fixtures. The ceiling structures often occupy so little space that there is no room for a cavity behind the ceiling absorbers – precisely what would be needed for low frequencies. Low frequencies have such a long wavelength that there are not many materials and structures capable of absorbing them without a deep cavity.


However, there are special ways to construct sound absorbers to give them the necessary absorption in the low frequencies:

• Perforated panels attached to a narrow cavity can be fitted with a porous backing. This increases the density of the structure, making it more difficult for the low frequencies to penetrate them. • The spacing between the holes in the perforated panels can be increased to obtain a greater mass of the panel, for greater absorption in the low frequencies. • The density and thickness of mineral wool panels can be increased by applying an extra coat of paint. • Hard panel materials can incorporate air gaps that are longer than the wavelength at the frequency to be dampened. For example, to dampen sound with a frequency of 125 Hz, the gap must be longer than 2.72 m, and the width and depth must also be carefully calculated. •H  ollow structures covered with thin panel materials that are flexible in response to a particular frequency are sound absorbing in the low frequency range. However, hollow structures should only be used on ceilings and walls because although a hollow floor will absorb the low frequencies, they also produce “drumming sounds” when people walk on them or move chairs about.

Sawtooth designs can diffuse or dampen the sound

A solid structure may cause problems with echo if the material is too hard, with a fast sound response, and if it is installed flat. One solution to this problem is to fit the material in a sawtooth pattern instead of flat. Installing the material in this way significantly alters the acoustic environment: the flat structure carries the sound and produces an echo; the sawtooth

Whereas a hard material in a sawtooth arrangement spreads the sound, a porous material in a sawtooth arrangement provides strong sound absorption. This effect has been put to use in many laboratories incorporating a listening room, for example. The rooms are lined with porous materials fitted in a sawtooth arrangement, allowing research-

The sawtooth ceiling diffuses the sound throughout the room.

ers to concentrate without being disturbed by whatever else is happening in the lab.

Convex and concave surfaces

To create a room with a strong sound intensity, it is a good idea to use concave surfaces. Concave surfaces concentrate the sound at a single point, thereby amplifying the sound. This effect is used in buildings like churches. Convex structures, on the other hand, spread the sound. That is why you often see convex shapes on the ceiling and walls of concert halls, where the

OBH Gruppen, Odense, Denmark. Photo: Finn Manford

arrangement diffuses the sound into the room, making it more suitable for dialogue. Diffusing the sound in this way creates a more harmonious effect, so this is a good way of making sound absorbing surfaces more effective.


Academy of music, Helsinki, Finland. Photo: Finn Manford


In concert halls, the right sound is everything.

Wall niches are another good way of spreading the sound. Niches can also be used to direct the sound to a particular point, and this can be effective in supporting speech, for example in auditoriums, where the lecturer’s voice must be directed to the part of the room where the students are sitting. In this situation, early reflections can ensure that the sound reaches the rows at the very back. An early reflection means that the sound hitting a surface is instantaneously returned to the listener. This is important for speech intelligibility, because most consonants – crucial to the understanding of speech – are in the high frequency range, which means they are among the first sounds to be absorbed or to die out over longer distances. Protrusions designed as barriers on walls or ceilings are good at catching what we call stray sounds. Stray sounds are sounds that bounce around when they are reflected from surfaces. They can be a


problem in open plan offices, where people enjoy the visual contact but would like to be able to work undisturbed. But in rooms designed for speech, like seminar rooms, barriers create delayed echoes that can be very confusing.

Protrusions are good at catching stray sounds.

Sir William Perkins School. Photo: Finn Manford

sound has to be spread to the whole audience.


Private Residence, Holstebro, Denmark. Photo: Henriette Torp

CHAP6 different rooms, different sounds

In a normal room, the sound is louder than outside because it is reflected by the walls.

What goes around, comes around – sound is louder inside than out

A sound is the sum of its surroundings. The sound will behave one way in this landscape, and differently in another. For example, if there is no vegetation, the cliff walls will produce an echo.


Photo: Copyright

Sound behaves differently inside and outside. If you call out in the middle of a field, the sound seems to disappear. It travels into the horizon, where it is swallowed up, never to return. But if you stand in a canyon surrounded by high cliffs, the rock reflects the sound, which you hear again as an echo. If the journey made by the sound is long enough, the brain registers the returned sound as a new sound, which we call an echo.

In a normal room, the sound is louder than outside because there is only a short distance to the walls and ceiling, which reflect the sound. The brain has already combined the spoken sound and the reflected sound before it has a chance to register that they are in fact two separate sounds. That is why the sound seems to be louder inside than out – you hear double the sound inside, so to speak. And inside, the structure of the space has a big part to play in how the sound behaves. The space can create a particular atmosphere – if you go to a sports arena to watch a game of basketball, for example, you are immediately swept along by the atmosphere in the crowd. The large space with parallel surfaces virtually gives free rein to the sound. Although a certain amount of noise can be a good thing in a large sports hall, the same cannot be said of similar sized rooms in an educational establishment. Noise can be disruptive to the

Pohjoispuiso School, Finland. Photo: Finn Manford

In a sports hall, the large space with parallel surfaces gives free rein to the sound.

teaching going on in adjacent rooms. That is why the acoustics must not only take account of the function of the room itself, but also what is happening in its immediate surroundings. Unless these factors are considered right from the start, it may be necessary to retrofit absorbers in places that were not designed for them.

has nowhere to go. This makes it difficult to concentrate or hold a conversation.

The larger the room, the longer the reverberation time

The acoustics of a room are also influenced by the adjacent rooms. A music room next to this classroom is not a particularly good idea.

Kunskabens Hus, Båstad, Sweden. Photo: Martin Tørsleff

In principle, it is safe to assume that the larger the room, the longer the reverberation time. A long reverberation time can be disruptive in spaces intended for dialogue or work, because the sound

Care must be taken when designing open plan offices to keep the reverberation time and the general noise level to a minimum.

KV Ekenäs, Kista, Sweden. Photo: Martin Tørsleff

In a large room it is generally a good idea to avoid too many parallel surfaces. Parallel surfaces bounce the sound back and forth within the space, primarily between the ceiling and floor and from wall to wall, creating a longer reverberation time. The problem can be averted by changing the shape of the room so there are fewer parallel surfaces or by adding protrusions, or by fitting sound diffusers to the walls and ceiling, or even to the floor in the form of furnishings. These methods spread the sound, shortening the reverberation time.


Changing the angle of the glass front reflects the sound up to the ceiling.


Adnec Exhibition Centre, Abu Dhabi, UAE. Photo: Birgitte Godsk

CHAP6 different rooms, different sounds

Reducing noise with the right surface angle

One way to reduce this noise is to fit angled or sound diffusing elements to the opposite wall – or simply to alter the angle of the glass by just six degrees. If the surface is angled to the ceiling, the sound is reflected towards the ceiling instead. Absorbers can also be installed in the ceiling to trap some of the sound, reducing the reverberation time and the general noise level. Another option is to erect screens to curb some of the sound. For example, if people need to make lots of phone calls as part of their work, a screen next to them can prevent the sound from the calls spreading to the rest of the room. Sound barriers in the ceiling can make partition screens even more effective.

Stockholm Public Library has the same acoustics as a church. Welcome to the people’s temple.

Photo: Olle Norberg

Care must be taken when designing large spaces, like open plan offices, to keep the reverberation time and the general noise level to a minimum. For example, there is a tendency in commercial buildings to use large glass facades – a hard material that reflects all sound at the same angle it arrives. The result is a long reverberation time and a noise level so high that it affects people’s ability to concentrate.

Public Library, built in the 1920s: the ceiling is domed in order to create a solemn and exalted atmosphere as you move around the world of books and learning. These days, talking is allowed in libraries, and the atmosphere in general is more informal than it was 80 years ago. This exposes the weaknesses of the structure – the sound stays suspended in the space for a long time, making it difficult to hold a conversation.

The importance of the ceiling in acoustics

Whereas hard parallel surfaces send the sound straight back to the source, vaulted ceilings focus the sound at a particular point. A focused sound gives the space an almost sacred character.

A concave shape in the ceiling carries the sound around, which you can experience for yourself in the Whispering Gallery in St Paul’s Cathedral in London. You can stand in the dome and whisper to someone beyond the normal hearing range. The concave shape sends the sound around the dome, unlike a convex shape, which would spread the sound in every direction.

A good example of this is the acoustics in Stockholm

St Paul’s Cathedral in London with its famous Whispering Gallery.

Photo: imageclub

The shape of the ceiling is therefore very important to the acoustics in a room. An incorrectly shaped ceiling can be very disruptive to the activity going on in the room.

The ratio between the length and width of the room also affects the sound

As discussed earlier, the size of the room is an important factor in determining the reverberation time. The size of the room is also a significant ele-


CHAP6 different rooms, different sounds ment in the character of the sound.


In narrow rooms with high ceilings, the absorbers are fixed to the walls.

A small, non-rectangular room will provide ample high frequency absorption, but the low frequencies will be unpleasant, requiring the use of elements like panel absorbers.

In small rooms with low ceilings, the absorbers are fitted to the ceiling surface.

Large, moderately furnished rooms need broadband absorbers throughout the ceiling surface. In large rooms, additional diffusion or barriers should be installed in the ceiling, because that is where the problem of sound propagation is at its greatest. In rooms intended for dialogue, the goal is not so much to dampen noise as to spread the sound without reflecting it back. The shape of the room also influences how the sound behaves in general. For example it is more difficult to predict how sound will behave in a nonrectangular room than in a rectangular room. Some non-rectangular rooms enhance the sound quality, while others can create unintended mirror points.


The room height also determines the quantity of sound absorbing material that must be used. In a room with a low ceiling, the sound waves hit the ceiling before the walls. That is why the absorbers must be fitted to the ceiling. But in a narrow room, the sound hits the walls before the ceiling, so the absorbers go on the wall. If the room is both wide and high, the absorbers must be fitted above the normal storey height (2.5 m) for the most benefit, and generally speaking, a larger number of absorbers or sound diffusers should be fitted to the walls.


Combined sound absorber and sound diffuser.


Private Residence, Korsvang, Assens, Denmark. Photo: Finn Manford

In large rooms with high ceilings, sound absorbers are fitted to the ceiling and sound diffusers/sound absorbers are fitted to the walls.

Furthermore, if two rooms are connected, the sound in one room can influence the sound in the other. For this reason there should be an acoustic transition between the two rooms, so that the sound is not perceived as being very different from one room to the other. If the difference is too great, it can feel as if you have cotton wool in your ears as you move between rooms.


CHAP7 the sound of a sofa

However, the effect of the absorbers on the sound can be enhanced if the room is designed to incorporate materials that also diffuse the sound. Placing three dimensional structures on the walls, ceiling or floor diffuses the sound so it covers a wider area, allowing the absorbers to work more effectively because they can trap the sound over a wider range.

Furniture can diffuse and absorb sound

In furnished rooms, this diffusing effect is produced by the furniture – the sound encounters the furniture and spreads in different directions. The amount of sound diffused depends on the amount of furniture

Furniture can diffuse and absorb sound.


in the room and how it is arranged. In principle, it is possible to calculate the precise sound diffusion on the basis of the floor area occupied by the furniture. It is usually enough simply to consider how much furniture there will be when the room is fully furnished. More care needs to be taken with the type of furniture in the room. Upholstered furniture is capable of absorbing sound as well as diffusing it, especially in the high frequencies, but hard furniture only diffuses the sound. That is why churches usually have wooden pews – the special sacred atmosphere is enhanced by the long reverberation time. The atmosphere would be destroyed if upholstered seating were used in churches. Room dividers are another kind of furniture with a design that differs according to the acoustic needs. They have different acoustic properties depending on their design, orientation and material composition. For example, although room dividers can be used to diffuse the sound, they can also be used to screen off colleagues with jobs that produce a lot of sound.

Private Residence, Korsvang, Assens, Denmark. Photo: Finn Manford

There is a big difference between the way sound behaves in an empty room and in a furnished room. In an empty room there are no furnishings to diffuse the sound in the room, so the sound waves mainly move in a plane, bouncing back and forth between opposite parallel walls or between the floor and ceiling if they are parallel to each other. In situations like this, absorbers are not particularly effective if they are only fitted to one surface.


CHAP8 STRATEGIC SOUND DESIGN Rooms with noisy activities must have a short reverberation time

The preceding chapters explain how sound is affected by wind, air humidity, materials, structures, furnishings and room size. This knowledge can be used in architecture to create the right acoustics and build spaces that people feel comfortable in. The key is to think creatively and get architecture and acoustics to interact – just like the Ancient Greeks, who used their expertise in acoustics to create the unparalleled amphitheatres we admire to this day.

When designing a room, it is important to consider what the room will be used for. If there is a lot of noise from machinery or telephones, it is good to have a short reverberation time so as to prevent the sound staying in the space for a long time. To achieve a short reverberation time, absorbers can be fitted to the ceiling and the walls. Fitting absorbers to the ceiling alone can reveal a pre-existing echo that was previously masked by the general noise. This happens a lot in rooms with walls made of hard materials that are parallel to each other. For this reason, it is a good idea to fit absorbers to the walls or furnish the room in a way that diffuses the sound. Another option is to erect a barrier around the actual sound source, stopping the sound at source.

The reverberation time is an important tool

It is not difficult to achieve an acoustic balance by taking account of the properties of the materials and the shape of the room, and by fitting the right number of sound regulating elements in the right places. The reverberation time is a useful tool in calculating the amount of absorbing material to use and the absorber type that is most appropriate. The reverberation time gives an indication of any need to regulate the sound. It can be calculated relatively easily by dividing the volume of the room by the amount of absorption achieved by one square metre of the absorbing material installed (known as the Sabine formula).

A long reverberation time is acceptable in rooms intended for one way communication

If a room with a short reverberation time is used for orations like a sermon, the sound is usually very “dry” in the sense that it is quickly cut short. Rooms with a dry sound sometimes make it difficult to give the voice – and therefore the oration – the necessary fullness and roundness. A long reverberation time, on the other hand, gives the words a rounder tone or quality, and means that the speaker can leave pauses between the words for rhetorical effect. This is especially important in churches, where the minister can choose to dwell on certain words to lend more weight to the message. A long reverberation time adds emphasis. This makes a long reverberation time a good thing in rooms used for one-way communication, for example churches and lecture halls. One way of creating a long reverberation time is to use flat surfaces and a room with a large volume.

In a church, the long reverberation time emphasises the solemn atmosphere.


Photo: Imageclub

Rooms for two-way communication need good speech intelligibility and ease of listening

It is equally important to avoid a dry sound in rooms where communication takes place over short distances or where human interaction occurs. It can otherwise be difficult to pick up what is being said. In particular, it is important for the consonants to

Råde Continuation School, Norway. Photo: Finn Manford

If pupils need to work hard to hear what their teacher is saying, they will find it difficult to concentrate and learning will be the first thing to suffer.

In teaching situations it may also be beneficial to have relatively long reverberation times, even though the rhetorical demands are not the same as in a lecture hall. In a classroom there is more dialogue between the teacher and pupils, and the speech is faster and more colloquial than a lecture. This means the reverberation time should not be as long as in a lecture hall. Although the sound should dissipate more quickly, the consonants must be clear so that it is easy to understand what is being said. If pupils need to work hard to hear what their teacher is saying, they will find it difficult to concentrate and learning will be the first thing to suffer. It is therefore extremely important to adjust the individual frequencies: if absorption is too effective in the high frequencies, the room will devour the consonants, dramatically reducing speech intelligibility.

Avoid large rooms in day nurseries

In day nurseries, the challenges are very different. There are high levels of physical activity and most of the contact is one-to-one – and there is hardly ever a need to gather everyone in one place. It is a good idea to keep rooms small or medium size – apart from tending to be less noisy, small children can find

In a day nursery, the furnishings can contribute to sound absorption and sound diffusion.

Vestsida Daycare Centre, Norway. Photo: Finn Manford

carry clearly, because the intelligibility of a word largely depends on the consonants. At the same time, it is important to dampen the background noise. The best way to solve the problem is to plan for an early reflection of the sound, followed by suppression. To achieve this, ceilings and walls can be designed to both reflect and absorb the sound. This ensures that the unwanted noise is removed while the dialogue in the room is still picked out.


In open plan offices, extensive sound absorption and sound diffusion are essential.


Rudolf Lolk Architects Practice, Denmark. Photo: Finn Manford


Open plan offices – a major acoustic challenge

In rooms where physical activity takes place alongside concentrated work, for example open plan offices, extensive sound absorption and sound diffusion are essential. There is often a lot of activity in offices: employees get up from their place for a meeting or a cup of coffee while their colleagues next to them carry on working. So to make sure that everyone can function in the room, very extensive noise suppression measures must be put in place. For example, absorbers can be supplemented with sound barriers in the ceiling, and screens can be erected inside the room to keep noise to a minimum. The furnishings of an open plan office must take account of the need for quiet, while also allowing small groups to work together. This can be achieved by positioning meeting rooms as sound locks within the office or by creating informal meeting areas, for example by the coffee machine and away from the desks. If a number of different functions need to be housed in the same building or on the same floor, sound-intensive activities should be separated from sound-sensitive activities – for example it is rarely a good idea to have a call centre in the same room as

If the sound difference between the two rooms is too great, it sometimes feels as if you have cotton wool in your ears as you enter.

the accounts department.

Sound and function must match

When planning the layout of several rooms at a time, it is a good idea to use the zoning principle to make the transition between the rooms as smooth as possible. A building can accommodate a wide range of activities ranging from silent to noisy, so it is important to create acoustics in which the sound matches the function in each of the rooms. If sound, function and environment do not go together, the room will somehow feel fake. Something to aim for is to make the sound conditions reflect the character and size of the rooms so clearly that even a blindfolded person could hear what kind of room it is.

The relationship between rooms affects the sound

The relationship between the individual rooms is also important. If a room is linked to other rooms by a narrow corridor, the sound from that room can spread to the others. This sound movement is vertical as well as horizontal, and the corridor concentrates the sound in the same way as an ear trumpet. This sound lock principle can be used to make two connected rooms sound the same. Absorbers can be installed in strategic places in the sound lock, on the basis of calculations to determine which surfaces reflect most sound. Even if two adjacent rooms are not connected by an open passage, it is still important to consider the way they relate to each other in terms of sound.

Photo: Imageclub

their way around these rooms more easily and they feel more secure there. Day nurseries will still be relatively noisy places, so it will always be beneficial to fit additional sound absorbers and sound diffusers to the ceiling and the walls. The room can also be furnished to aid sound absorption and sound diffusion and thereby prevent the sound propagating and becoming a problem.


CHAP8 STRATEGIC SOUND DESIGN The sound in the rooms must be harmonised so that there is a pleasant transition from one room to the other. Harmonising sound in this way consists of adjusting the reverberation time and the materials used in the two rooms to minimise the difference perceived when walking from a small room to a large room. If the difference between the two rooms is too great, it sometimes feels as if you have cotton wool in your ears as you enter.

Materials influence the sound

Like the structure, the material composition helps to determine the “tone colour” of the room. In a room with hard materials, the sound is perceived clear and sharp, whereas soft materials produce a softer atmosphere and a mushy sound in the room. The positioning of materials also partly determines the sound experience – the more protrusions there are, the more they can “colour” the sound in the room. Of course, the choice of materials depends on much more than their acoustic function. For example in classrooms, it is a good idea to use materials that are able to withstand rough treatment, or that are easy to care for without impairing the acoustic properties. Otherwise there is a risk that the room will quickly look tatty and that the absorbers will no longer work as intended because they have been incorrectly maintained. Possible vapours from the materials must also be taken into account, especially if the building has large areas of glass, which can heat up the room and the materials. The materials should therefore be selected on the basis of general considerations about the function of the room.

Good acoustics does not have to be difficult

When planning the sound in a room it is a good idea to think of walls, ceilings and floors as mirrors and then imagine the sound waves moving around in the room. The acoustic-modifying materials will be most effective at the places that are first encountered by the sound waves. The changes needed to create a pleasant acoustic environment are normally quite modest. Simply changing the angles between the floor and ceiling or between the walls can diffuse the sound and improve the acoustics:



1. Room shape As described above, the shape of the room has a big impact on the perception of the acoustic environment. But certain other factors are crucial in determining the overall effect: • Parallel walls should be fitted with sound diffusing elements if a short reverberation time is wanted and if sound absorption mainly occurs in the ceiling.

2UMMEDPARALLELLEVGGELOFTERDOMINERESAF Rooms with parallel walls/ceilings are dominated by LYDB’LGERSOMSPEJLESMELLEMDEPARALLELLEFLADER sound waves$ERFORSKALEFFEKTENAFDEFLADERELIMINERESENTEN that are reflected between the parallel VEDLYDABSORBENTEROGELLERLYDDIFFUSORER surfaces. The effect of the surfaces therefore has to be eliminated using sound absorbers and/or sound diffusers.

• Domed ceilings or round rooms can produce an amplifying effect, so they should be avoided in everyday situations.


Round rooms or domes send the sound waves back to TILDETSCENTEROGOPFATTESSOMLYDENPUMPERMELLEM VGGENEOGDETSCENTER the centre, and the perception is that the sound is pul3ÍDANNEKONSTRUKTIONERERSVREATSTYREAKUSTISK sating between the walls and the centre. These structuOGB’RDERFORKUNANVENDESHVORENOVERRASKENDE res are difficult to controlEFFEKT’NSKES acoustically, so they should only be used for effect.

• Surfaces need to be angled by at least 6 degrees to prevent echoes.

• In large rooms with low ceilings, absorbers should be fitted all over the ceiling surface. To minimise sound propagation, sound barriers can be installed in the ceiling.


AnglingFORHINDRESSTÍENDEB’LGER$ETHARTIDLIGEREVRET walls and ceilings by more than 6 degrees prevents persistent waves. ANVENDTIBIOGRAFER Cinemas used to apply this approach.

• In rooms up to 2.8 m in height, absorbers are fitted to the whole of the ceiling surface – alternatively 75% of the ceiling plus 15% of the walls can be covered.

)SMÍLAVTLOFTEDERUMPLACERESABSORBENTERNE In small rooms with low ceilings, the absorbers PRIMRTILOFTFLADEN are mainly fitted to the ceiling surface.

• If the room measures more than 8.5 m and the reverberation time is short, echoes will be audible unless absorbers or sound diffusers are fitted to the walls.


Fitting effective absorption to the ceiling in rooms over 8.5 m in AFSL’RESEKKOETMELLEMVGGENE$ERB’RPLACERESENEFFEKTIVLYDDIFFUSIONPÍVGGENE nEVTKANLYDDIFFUSIONEROPNÍESVEDATVINKLEVGGENEGRADERELLERUDF’RESKRÍSTILLEDE width/length (50 ms – 17 m²) only exposes the echo between VGELEMENTERSOMOGSÍHARGRADERSVINKLING the walls. The walls should have effective sound diffusion – sound diffusion may be achieved by angling the walls by 6 degrees or fitting wall elements at an angle of 6 degrees.

"AFFELKONSTRUKTIONEREREFFEKTIVETILATFORHINDRE Baffles are good at preventing sound propagation in LYDUDBREDELSEIST’RRERUMOGEREFFEKTIVESAMMEN largeMEDFEKSSKRMVGGEISTORRUMSKONTORER rooms, and are effective when combined with screens in open plan offices, for example. But baffles in rooms designed for speech can create problematic echoes.

• In rooms between 2.8 and 3.2 m in height, the amount of absorbers should correspond to 115% of the floor surface area, i.e. 100% on the ceiling and 15% on the walls. • In rooms between 3.2 and 3.8 m in height, the amount of absorbers should correspond to 120% of the floor surface area. • In rooms between 3.2 and 4 m in height, the amount of absorbers should correspond to 125% of the floor surface area. • For acoustic reasons, room heights over 4 m should be avoided.

Rooms2UMH’JDEROVERMMSKALTILF’RESEKSTRA more than 2800 mm in height must have extra ABSORBENTERPÍVGGENE$ERKANTILF’RESDENI absorbers on the walls. The quantity is given above – SKEMAETANF’RTEMNGDEnOPTILMETER up to+ANSKEMAETANVENDESnOVERMETERRUMH’JDE 4 m. In rooms over 4 m in height, the result can be FOREKOMMERRESULTATETUSIKKERT6EDSTORRUMSKONTORER uncertain. In open plan offices, etc., the stated coOLSKALMNGDENOVERHOLDESSAMTDENEKSTRAMNGDE verageAFLYDDIFFUSERENDEMATERIALERSOMANBEFALESVED of absorbers must be fitted, as well as the quNORMALRUMH’JDE antity of sound diffusing materials recommended for normal room heights.



• In narrow rooms with high ceilings, the absorbers are mainly fixed to the walls.


In narrow rooms with high ceilings, the absorbers are normally fixed to the walls.

• Stairwells are fitted with absorbers or diffusers in places where the sound is expected to be carried or reflected – usually the wall, under the landing and the banisters.


In high rooms, absorbers or diffusers are fitted underneath and in front of balconies, and on the walls connecting balconies.

• Solid structures reflect much of the sound, which means their sound absorption capacity is limited. However, this is a good thing in rooms where acoustic music is played and one way communication takes place. • Panels on a hollow base provide sound absorption in the bass range but not in the high frequency range. • False floors provide sound absorption in the bass range, but can produce drumming sounds when people walk on them. • In low rooms, adding mineral wool to the cavity behind a perforated panel increases absorption in the low frequencies.


• In rooms with curved ceilings, absorbers are fitted over the ceiling surface and sound barriers are installed to prevent sound transmission along the ceiling surface.


In barrel vaults, barriers should be installed to prevent sound propagation.

• Domed or concave ceilings concentrate sound so they may have an amplifying effect. That is why they should be avoided in everyday situations. In rooms with domed ceilings, absorbers are fitted to the whole ceiling surface, and the geometrical centre of the dome should be well above head height.

"UEDEFLADERSENDERLYDENTILBAGEMODDETSCENTER Curved surfaces send the sound back to their centre, LIGESOMLYDENKANL’BEPÍLANGSAFBUENOGDYKKENED and sound can travel along the curve, re-emerging a LANGTFRALYDKILDEN"UEDEKONSTRUKTIONEROGKUPLEN longOPFATTESSOMLYDFORSTRKENDEKONSTRUKTIONEROG way from the sound source. Curved structures and domesANVENDESDERFORIKIRKENOL are perceived to be amplifying structures, so they are used in churches, etc.

• Convex surfaces spread the sound and reflect the sound over distance.

+ONKAVEKONSTRUKTIONERVIRKERLYDSPREDENDE Convex structures diffuse the sound. Structures of this type +ONSTRUKTIONERAFDENNEARTANVENDESIFOREDRAGSSALE are used in lecture halls or concert halls, where sound ELLERIKONCERTSALE HVORLYDSPREDNINGENKANFORSTRKES VEDATBYGGEKONSTRUKTIONERMED’GETTYNGDE diffusion can be enhanced by creating heavier structures.

3. Absorbers The general sound environment of the room is influenced by all the materials in the room – negatively by some, positively by others. Absorbers can improve the factors that stand in the way of a perfect sound environment. • To reduce the general sound level, broadband absorbers should be strategically deployed on the largest surfaces. • Bass absorbers are also effective even if they are not strategically placed. • Absorbers providing high absorption should be used to prevent sound propagation – in other words for noise reduction. • Absorbers with reflection as well as absorption properties should be used where speech intelligibility and ease of listening are essential, and where there will also be an element of interaction. • Highly reflective materials should be used where speech intelligibility and ease of listening are important, and where the sound needs to travel over a long distance. • High frequency absorbers are used in large rooms with hard surfaces, or in unfurnished rooms. • Mid frequency absorbers are used in moderately furnished rooms. • Low frequency absorbers are used in small, densely furnished rooms. • Low ceiling heights with a porous or perforated absorber provide good high frequency absorption, but poor absorption in the low frequencies. • Upholstered furniture, carpets, mattresses and people are all good high frequency absorbers. 4. Reverberation time The anticipated reverberation time of a room can be calculated by comparing the absorption properties of the individual materials with their surface area. • In an empty room, the reverberation time can be calculated using the Sabine formula without the addition of sound diffusion. • In room with sparse furnishings providing some

)RUMMEDENBREDDELNGDEPÍOVER METERUDG’RM’BLERINGOGREOLERLANGSVGGENEEN In rooms more than 8.5 m in width/length, furniture and EFFEKTIVLYDDIFFUSIONSOM’GEREFFEKTENAFLOFTABSORBENTER bookcases against the walls deliver effective sound diffusion, enhancing the effect of ceiling absorbers.

diffusion, the overall absorption is higher. This means that a factor of 0.15 can be added to the original absorption. • In a moderately furnished room, a factor of 0.2 can be added to the original absorption. • In a densely furnished room, a factor of 0.3 can be added to the original absorption. • But the calculation of reverberation time does not indicate where the absorbers should be fitted or whether insufficient diffusion may result in unwanted echoes, which is usually the case with short reverberation times of 0.4 seconds and lower. That is why in this situation additional absorbers or diffusers are specified for the wall surfaces. • Once the room has sufficient diffusion, there is a minimal difference between the measured reverberation times when comparing absorbers with a high absorption value and others with a lower absorption value. 5. Sound diffusion Sound diffusion occurs if a material has a hard, structured surface or if structures with angular, three dimensional edges are used, or with the help of furniture. A sound is diffused if the sound waves travel in many different angles, making echoes impossible. Another very important effect is that sound absorbing materials become more effective when they are used with sound diffusing materials, because the absorption can act over a larger surface area. At first sight it can seem quite complicated to calculate the necessary quantity of diffusing materials in a room, but the instructions below contain some general guidelines for achieving the optimum acoustic effect.



• Sound diffusers are effective on walls facing a high glass facade, for example. • Textured and structured surfaces and thick perforated panels are good sound diffusers.

spread beyond the private sphere. The downside to this is reduced speech intelligibility. In conference rooms and other rooms intended for voice communication, good sound propagation and good speech intelligibility are important – this means minimal sound level reduction.


DIMENSIONELLEFORMEREREFFEKTIVESOMLYDDIFFUSORER Three dimensional shapes are effective sound difOGKANANVENDESOVERFORSTRKTLYDREFLEKTERENDE fusers and can be used opposite highly sound reKONSTRUKTIONERnFEKSSTOREGLASFACADER flective structures like large glass facades. Three  DIMENSIONELLEKONSTRUKTIONEREROGSÍTYKKEPLADER dimensional MEDENH’JPERFORERINGSGRAD structures also include thick panels with dense perforations.

• Examples of other sound diffusers: polygonal elements like boards placed edgeways, profiled ceilings, various profiled shapes, old-fashioned wide door frames, stucco and similar fixtures. • Special sound diffusers can also be designed as sound absorbers by calculating their depth and width in relation to the wavelength to be absorbed. • Niches in the walls and ceiling also have a sound diffusing effect. • Angled wall elements (minimum 6 degrees) or convex elements are sound diffusers. • Furniture, room dividers, etc., are good sound diffusers. • In large rooms with a high ceiling, bookcases and sound absorbers on the walls are an effective combination. 6. Lowering the sound level It is essential to lower the sound level quickly in certain rooms, while in others, doing so causes problems. People need to be able to concentrate in an open plan office, so the propagation of sound must be kept to a minimum. The level of privacy must be high enough that people nearby cannot understand calls and be disturbed by them, in other words that the sound does not


The privacy index in open plan offices can be boosted by combining room dividers with barriers in the ceiling and diffusers on the walls.

7. Privacy • Use strategically placed screens to prevent sound reflecting from areas where privacy is required. • Partitions and room dividers must be placed as close to the ceiling/floor/wall as possible. • Partitions should form shielding angles. • The natural deflection of sound waves means that partitions less than 1.5 m in height are ineffective. • The higher the partition, the more effective the absorbers and sound barriers in the ceiling will be. • Good speech intelligibility is the enemy of good privacy. 8. Speech intelligibility Create an effective early reflection followed by good sound absorption. • To achieve an effective early reflection, use absorbers providing absorption, diffusion and reflection. • Over longer distances, where greater reflection and diffusion are important, curved or undulating surfaces can be used to reflect and diffuse the sound. • Small curved niches, pointing in the direction of the listeners, enhance speech intelligibility.



It is important for the materials to provide good absorption in the low frequencies, so that extraneous sounds are absorbed instead of disturbing the lesson. At the same time, there should, if possible, be good reflection in the high frequencies – this is the consonant frequency range that is so crucial to good speech intelligibility. The absorbers should also be able to withstand wear and tear and must be easy to care for.


Teaching used to take place from the front of the class, with pupils copying down what the teacher wrote on the blackboard. The teacher had to be clearly audible, but chatter from the pupils had to be dampened. There were some fixed rules about where the absorbing and reflecting materials were to be placed – for example, sound reflective materials over the teacher’s desk and sound absorbing materials over the pupils. These days there is more interaction between teachers and pupils. This means that the sound regulating products need to both spread and reflect the sound, while also having sound absorbing properties. In other words, the acoustic design is much more complex. In a modern classroom, the reverberation time must be short. As a result, quiet sounds may be difficult to hear, and the teacher may not be able to use rhetorical techniques in the lesson. The ceiling must be actively used in order to support the dialogue between teacher and pupil. Absorbers on the walls are also useful in creating a short reverberation time and good sound distribution in the room. The number of absorbers required depends on the height of the room. The higher the ceiling, the more absorbers are needed. The sound can also be spread by positioning furniture and fittings in strategic places, for example bookcases along the walls. If the room is lit by overhead lighting units, it is also a good idea to fit absorbers inside the units.


Teaching used to take place from the front of the class. Now there is much more dialogue, and this calls for different acoustics in the classroom.

Other requirements and recommendations • The acoustic products must not release dust and particles into the surroundings. • Hazardous materials must not be allowed to accumulate in the products. • Wall and ceiling materials must be light, and light-diffusing to prevent reflections. • It must be possible to clean, wash and repaint absorbers on ceilings and walls without altering the acoustic properties. This is especially relevant for absorbers that are permanently attached and difficult to replace. • Absorbers on ceilings and walls must be impact resistant. It may be necessary to fit surfaces with additional impact resistance to the lower parts of the wall. • Floor-mounted material must be easy to maintain and not too hard. • Rubber disks under the chairs reduce scraping noises while also protecting the floor.

Open plan solutions

In some schools, teaching takes place in large open rooms – this presents many, often contradictory, acoustic challenges. The pupils need to be able to work in their groups undisturbed. But the teacher also needs to be able to speak to all groups at once, or to individual pupils. This requires what the experts call a high Speech Transmission Index (STI) and a high level of privacy. The problem is that a high STI creates poor privacy. In rooms with very good pri-

Photo: Imageclub

If the teacher’s role is more about supporting the individual groups, the acoustic needs are different. In this case, the emphasis should be on reducing sound propagation in the room. This can be done using screens, for example, but other elements in the room can also make a contribution, say if the ceiling has a three dimensional shape. This creates sound barriers that limit the spread of sound in the room.

vacy, the teacher may have to use a microphone to reach all pupils, which ruins any rhetorical effects the teacher might want to use.

The many challenges facing open plan solutions in schools

Acousticians willing to recommend open plan solutions in schools are few and far between. As we have seen, open plan arrangements have many problems, and it is difficult to identify straightforward and effective acoustic solutions. If too much attention is paid to reverberation time alone, it is not possible to create the best environment for speech communication. There is much more to it than that. But if we start by looking at the reverberation time, a lot of absorbers will be needed to create a short reverberation time in a large open plan classroom. It is not enough to place absorbers on the ceiling and walls – additional sound barriers are needed in the form of movable partitions and/or room dividers. Partitions and room dividers must be installed either touching or as close as possible to a wall, ceiling or floor. This achieves good privacy between different groups. In order for the teacher to work effectively, the partitions should be strategically located to create a central point where the teacher can stand. Everyone in the room must have unimpeded acoustic contact with the centre point, so it may be a good idea to fit sound reflecting materials in the ceiling above the centre point.

Other recommendations Teaching in open plan environments is at its most effective in peace and quiet. There are many conflicting acoustic requirements that need to be met in open plan environments, so acoustic interventions alone may not manage to create ideal teaching conditions. It will also be necessary to develop what we could call an ethos of quiet – in other words, behaviour and behaviour patterns that are compatible with the acoustics of the room. The ethos can be promoted by the furnishings, just as the choice of colours can determine the mood in the room. For example, warm red colours create more intensity in the room, while most blue and green colours create a low intensity. It is also important for an open plan solution not to open onto a thoroughfare, where the footsteps and chatting of passers-by can disrupt the lesson. The sound of footsteps can be very distracting, and the chances of hearing them are higher in an open plan arrangement. Hollow floors should be avoided, the floor covering should be soft and absorbing, and all chair legs should be fitted with soft rubber disks. When open plan solutions are designed, it is not recommended to combine different storeys. If several storeys are combined in an open plan arrangement, the sound can be carried between the floors – despite attempts to preempt this with absorbers on mezzannine decks and deck front faces. The reason is that the connected rooms act as amplifiers for each other.


Dahlske Vid School, Grimstad, Norway. Photo: Karl Ture Sagen, Reklamefotograferne


In the main assembly hall, the general noise level has to be reduced, but the speech response must be good.

Main assembly halls

Assembly halls are often used for group teaching. This means that the teacher must be able to impart information, and it must also be possible for group work to take place undisturbed. This presents some difficult acoustic challenges: the general noise level has to be reduced, but the speech response must be good – if the sound absorbers are too effective, the teacher may not be able to reach the back rows.


Too few sound absorbers, on the other hand, will increase the effect of noise and make concentration difficult. In main assembly halls and similar spaces, it is therefore a good idea to use wall absorbers and some partitions to absorb and spread the sound, with effective absorbers installed in the ceiling. This

creates a good acoustic environment that can be used for group work as well as one-way communication.

direct connection between the rooms.


Gymnastics and other sports make a lot of noise. So in rooms used for sports it is a good idea to dampen some of the sound without taking away the room’s natural sound – a quiet sports hall would sound wrong: calls and loud sounds often have a positive effect on physical exertion. Because the high frequencies disappear as they move through the air, there is no need to dampen high frequencies in gyms. Instead, the entire ceiling should be covered with sound absorbers with effective absorption in the low frequencies, and steps should be taken to prevent echo. Echoes can occur in rooms between 9 and 20 m in width – most sport halls, in other words. Problems with echo can be avoided by installing sound diffusers on the walls.

It is generally a bad idea for a hall to span multiple floors, because this makes it particularly difficult to control the acoustics. Folding walls and cover plates are widely used to subdivide large spaces into smaller rooms, but they rarely produce the desired acoustics. The problem is that the sound of footsteps is carried by the floor structure when the walls are closed. And when the walls are open, the plates covering the tracks may cause annoying delayed echoes.

Music rooms

See the section about concert halls later in this chapter.

Music room as sound source

Acoustic music needs a response from the room, so it is highly dependent on the room shape.

Academy of music, Stockholm, Sweden. Photo: Martin Tørsleff

Music has long wavelengths, especially in the bass range, which means it can easily travel through the walls into adjacent rooms. To prevent this happening, mass must be added to the walls. If the walls consist of plasterboard, for example, several layers can be placed on top of each other. The floor structures must also be interrupted so there is no

It isn’t always just a question of suppressing the sound as much as possible. For example, calls and loud sounds often have a positive effect on physical exertion.

Veikkola School, Finland. Photo: Finn Manford

Absorbers in sports halls must be highly impact resistant. That is why the absorbers used should pass the so-called ball safety test. Ball-safe ceilings are tested according to a special standard in which the ceilings are hit with a pumped up handball. In Germany they have gone a step further, testing wall absorbers even more stringently than ceiling absorbers to see if the absorbers could withstand being installed behind a handball goal. But the toughest test uses a hockey ball, and tests


CHAP9 DIFFERENT ROOMS, DIFFERENT ACOUSTICS the materials intended for installation close to the floor. In halls for badminton and other sports that are usually played with white balls, the absorber colour should be chosen to create a contrast with the ball – such as grey. If white is used, there is a risk that the players will find it difficult to see the ball.

Other recommendations In school rooms, many materials are permanently attached. So it is very important to be able to care for the materials. It must be possible to paint the surfaces without affecting the acoustic properties. The surfaces of the materials must also be able to withstand wear and impact. If the environment is particularly tough, the strength of the material can be increased by reducing the spacing between the battens. Alternatively, products with reinforced surfaces can be used.

Rooms in special schools

In special schools, teaching and play often take place in the same room. This makes it necessary to dampen noise as well as increase speech intelligibility. The best way to do this is to use effective absorbers with diffusing properties throughout the room. Because the room is also intended for play, the products are recommended to be impact resistant and easy to clean.


Noise is often a problem in day nurseries. With so many children all in a small area, it can be difficult to find a quite spot. On top of this, day nurseries are increasingly expected to prepare children for life at school and to provide elementary instruction.

Rooms for special education

Special education usually involves just one teacher and one pupil. A muted sound environment is good because it enhances personal contact during teaching. If the room has a high ceiling and/or is wide, the right atmosphere can be created by fitting a large number of absorbers to the ceiling. If the room is narrow, the best place for the absorbers is on the walls.

The day’s routine in a day nursery consists of indoor and outdoor play, food and rest, alternating between high and low noise levels. The usual attempts to solve the noise problem involve large amounts of sound absorbing materials. But this often causes the children to shout louder because they do not get an immediate response – children these days are used to lots of attention from adults, so naturally they respond with shouts and calls if they are not heard first time. One way of solving this problem is to avoid gathering large groups of children in one place.

A muted sound environment is good for rooms used for special education because it enhances personal contact during teaching.


Photo: Imageclub

Yet the problematic sounds sometimes do not come from the children themselves but from the surroundings – for example mechanical noises from play equipment or from chairs being dragged across the floor. It is good practice to try to dampen the mechanical sounds when designing the acoustic environment – one way to do this would be carpets, and another would be rubber disks under furniture.


The playroom in a day nursery is usually a noisy place with lots of people and lots of play equipment. This room usually has a large volume, and it is

Daycare Centre Islemark, Rødovre, Denmark. Photo: Martin Tørsleff

It is quiet in here now. But noise is often a big problem in day nurseries.


Daycare Centre Islemark, Rødovre, Denmark. Photo: Martin Tørsleff


The playroom in a day nursery is usually a noisy place with lots of people and lots of play equipment.

Bookcases and bouncing mattresses can also help dampen the sound, but the best solution is to divide the playroom into smaller zones to differentiate the intensity of play.

Other recommendations As in schools, many materials are permanently attached. So it is very important to be able to care for the materials. It must be possible to paint over the surfaces without affecting the acoustic properties. Because children tend to explore the world using their fingers, it is important that the materials cannot break apart, and also that there are no small parts that children might swallow. The surface of the materials must also be resistant to wear and tear and be able to withstand bumps and bangs. In places where conditions are particularly tough, the material can be strengthened by reducing the distance to the base or by using products with reinforced surfaces.


OFFICES Reception

The reception is the face the company presents to the outside world. As soon as people enter, they get a sense of the spirit of the place and the atmosphere at work. That is why it is so important for the reception to reflect the image the company wants to present to the outside world – relaxed and informal, or formal and efficient. This atmosphere is created by the materials and the sound in the room. The reception should provide the peace and quiet needed in order to serve clients, but the receptionist also has to be able to hear when someone approaches. To meet these twin requirements, the most effective sound regulating materi-

Receptions should provide the peace and quiet needed in order to serve clients, but the receptionist also has to be able to hear when someone approaches.

Office Building Outokumpu, Finland. Photo Finn Manford

important to have lots of natural light. This equates to a large amount of glass in the room, and this can reflect the sound and make it seem louder. In this type of room it is important to fit as many absorbers as possible to the walls as well as the ceilings.

Office landscapes

Fitting out an office landscape to create a pleasant working environment is a major acoustic challenge. The problem is how to reduce the general noise level while ensuring that there is adequate masking of the sound: reducing the reverberation time immediately reveals lots of sounds that had been masked by the general noise environment. The shorter the reverberation time, the clearer even quiet sounds become – for example the sound of a computer keyboard. Open plan offices are widespread in the US and Canada. They solve the noise problems there by introducing artificial sound designed to counteract the noise. They use electroacoustic systems that produce sounds at the same frequency as the sounds being combated. This cancels out part of

In open plan offices in the US and Canada, they solve noise problems using artificial sound to cancel out the noise.

the incoming sound waves. However, this is not a common solution in Europe. In other cases, the noise problems can be solved by increasing the distance between the workstations to make the sound insignificant and to create more sound intimacy within the workstations. When designing an open plan office, it is important to consider which jobs thrive on interaction, and which are happiest with a high degree of concentration. Sales and marketing jobs enjoy and need interaction, unlike the accounting department, where people need to work more quietly and concentrated. In general, very noisy activities like call centres should be separated from the other offices. Group rooms and one-person offices should be incorporated into open plan office concepts, to enable more concentrated work to take place in small groups or individually.

Atea, Växjö, Sweden. Photo: Martin Tørsleff

als are placed above the reception desk, with less effective materials elsewhere.


In group rooms, the acoustics need be able to cope with hectic as well as more subdued group activity.


Paint Factory, Teknos Oy, Rajamäki, Finland. Photo: Finn Manford


Group rooms are small rooms intended for project work and similar activities. In group rooms, the acoustics need to be able to cope with hectic as well as more subdued group activity. So it is a good idea to know in advance what kind of activity will take place in the room: in small units, the need for privacy is outweighed by the need for good speech intelligibility.

One-person offices

One-person offices are designed for concentration, so they must be shielded from other people’s activities. They are often filled with bookcases and furniture, removing any need for sound regulating materials. In the case of larger offices, however, it may be necessary to adjust the sound, and the sound environment inside and outside the office will have to be harmonised. If the whole building is divided into individual offices, the rooms must be harmo-

In buildings with individual offices, the corridor is the link between the offices and other rooms like the conference rooms or dining halls.

Pfizer, Ballerup, Denmark. Photo: Martin Tørsleff

If the building is divided into individual offices, the rooms must be harmonised with each other in terms of the sound environment.

nised with each other in terms of the sound environment, so that people do not move from a place with a long reverberation time to a room with a short reverberation time or vice versa. If the reverberation times are not in balance, there can be an unpleasant feeling in the ears when moving from one room to another.


In buildings with individual offices, the corridor is the link between the offices and other rooms like the conference rooms or dining halls. As well as connecting the rooms physically, the corridor should also incorporate an acoustic adjustment or zoning between the rooms, to prevent major differences in reverberation time from room to room. Corridors can also be landings or mezzanines covering several floors. Absorbers should be fitted in carefully selected places to achieve the desired zoning between the rooms.

Rådhusparken, Glostrup, Denmark. Photo: Finn Manford

Group rooms

KV Ekenäs, Kista, Sweden. Photo: Martin Tørsleff

In one-person offices there is rarely any need for sound absorbers.


Conference rooms

Conference rooms must be good for one-way communication from a platform, while also allowing dialogue to take place. The design therefore follows the same principles as for classrooms.


In lecture halls, the amount of dialog between the lecturer and audience is usually very limited. This means that the room should be designed for good speech intelligibility from the platform, while sound from the audience needs to be dampened so that the lecturer does not need to strain to be heard above any noise. Sound reflecting materials are useful above and behind the speaker, with sound diffusing materials over the audience. To prevent echoes, sound absorbing and sound diffusing materials are also fitted to the rear wall and the side walls.


The room must have a reverberation time of 0.9 seconds for acoustic music, and a reverberation time of 0.3 seconds for electrically amplified music. In other words, the type of music being played in a music room makes a big difference. Acoustic music needs a response from the room, so it is highly dependent on the room shape. Thus, a long reverberation time creates a better sound – the reason why people enjoy singing in the bathroom is that the reverberation creates a fuller voice. In a very small room, it may be difficult to bring out the lowest notes because of their long wavelength. That


In rooms intended for classical music, the absorbers should have limited absorption and good reflection.

is why a music room needs to be a certain size if very deep instruments will be played in it. In rooms intended for classical music, the absorbers should also have limited absorption and good reflection. Panel structures can sometimes create resonance or “sing along”, so to speak – for example instruments like the piano, double bass and cello have a stronger sound if they are in contact with a hollow floor structure. Sound barriers on the ceiling and walls should also be avoided in rooms intended for music and singing without amplification, as they can produce delayed echoes. With electrically amplified music, the room must affect the sound as little as possible. The room should not play along. The sound must therefore be dampened as much as possible using absorbers on the walls and ceiling. The floor should be fitted with carpets, and if the walls are angled in relation to each other, the sound will be absorbed even more. If acoustic music and electrically amplified music will be played in the same room, reversible acoustic panels can be installed on the walls. Reversible acoustic panels have an acoustically absorbing material on one side and a reflecting material on the other. The reflecting side is used for acoustic music, and the sound absorbing side is used for electrically amplified music.

Academy of music, Helsinki, Finland. Photo: Finn Manford

Conference rooms must be good for one-way communication from a platform, while also allowing dialogue to take place.

Emirates Aviation College, Dubai. Photo: Birgitte Godsk


Seminar Centre, Holbæk, Denmark. Photo: Martin Tørsleff

In lecture halls, the amount of dialogue between the lecturer and the audience is usually very limited. This means that the room must be designed for good speech intelligibility from the platform.


Biocity, Aalborg, Denmark. Photo: Finn Manford


In cinemas, sounds from the audience must be dampened as much as possible.

In cinemas, it must not be possible to hear the room at all – the only audible sound should come from the speakers. To achieve the optimum sound, the room must have many speakers and the reverberation time must be brought as low as possible. When the reverberation time is greatly reduced, other sounds that were concealed before become more prominent. In some situations, even very quiet sounds can create echoes if the sound source has a flat surface, for example when a door closes. To prevent echoes, all surfaces must be angled by at least 6 degrees to each other. If the surfaces in a cinema are parallel and cannot be angled in this way, sound diffusing structures on surfaces are vital.

Other recommendations • Because the absorbers are installed in the field of peripheral vision, the surfaces should be matt and rough textured to prevent glare and light reflection. • The materials should also be dark and lightabsorbing. • The wall materials must be child-proof and impact resistant.



For many people, a hospital stay is a time of uncertainty: they are away from home and do not always know what to expect. There are lots of unfamiliar sounds in a hospital, and this can also be unsettling. So the individual wards must be well insulated from the sounds outside, creating an atmosphere of intimacy and safety on the ward. Large amounts of hard materials tend to be used in hospitals, which need to be counteracted by similar amounts of absorbers.

It is important for hospital wards to be effectively shielded from external sounds, in order to create an atmosphere of intimacy and safety.

Holbæk Hospital, Denmark. Photo: Finn Manford



It is difficult to formulate clear rules for the acoustic environment in restaurants and other eating estab-

In a restaurant where people meet with friends before a night on the town, the sound environment should be livelier than in a quiet romantic restaurant.

In general, though, it is true to say that acoustics have not been taken into account at all in many new restaurants. Large glass facades and hard materials on floors and walls amplify the noise from the other guests, so you virtually have to shout at each other. That is why it is a good idea, even in lively restaurants, to spread and dampen the sound in selected places.

Hotel Scandic Rubinen, Sweden. Photo: Finn Manford

Other recommendations • The materials must be easy to clean. • The materials must only accumulate or give off minimal quantities of dust, to avoid damaging sensitive instruments and impairing hygiene. • In some rooms there is overpressure above the ceiling for reasons of hygiene, requiring additional stability in the ceiling structure. • In operating theatres, porous materials are not recommended because bacteria and other impurities can settle there. • In a hospital environment, bright, warm colours often give a greater feeling of security.

lishments, because not all restaurants are trying to create the same atmosphere. If it is the kind of restaurant where people meet with friends before a night on the town, the sound environment should be much livelier than in a quiet, romantic restaurant. Basically, the sound needs to match the restaurant, so the architect should always find out as much as possible about what the restaurant will be like. Will the customers sit at long refectory tables or in small, cosy alcoves? With refectory tables, noise from the other guests is part of the atmosphere, but alcoves must feel isolated, giving guests a sense of being in their own little world.


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