Room Acoustics - Steve Kindig

Room Acoustics Steve Steve Kindig — Mar 01, 2008 2008 Article contents

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The Final Component in Your System is... Your Room We generally think of the speakers in our stereo or home theater systems as the final li nk in the audio chain — and the one that makes the biggest difference difference to our ears. But it's really not that simple. There's much more to the sound we hear than just what comes out of our speakers.

In addition to the sound from your speakers, you hear reflected sound from your room's four walls (above left). Your room's ceiling and floor contribute reflected sound, as well (above right). The sound that you hear in any room is a combination of the direct sound that travels straight from your speakers to your ears, and the indirect reflected sound sound — the sound from your speakers that  bounces off the walls, floor, ceiling or furniture before it reaches your ears. Reflected sounds can be both good and bad. The good part is that they make music and speech (like movie dialogue) sound much fuller and louder than they would otherwise. (If you've ever played your speakers outdoors where there are no walls to add reflections, you've probably noticed that they don't sound very good — thin and dull, with very little bass.) Reflected sound can also add a  pleasant spaciousness. The bad part is that these reflections distort sound in a room by making certain notes sound louder  while canceling out others. The result may be midrange and treble that's too bright and harsh or  echoey, echoey, or bass notes that are boomy, boomy, with a muddy "one-note" quality that drowns out deep bass. Because these reflections arrive at your ears at different different times than the sound from your speakers, the three-dimensional "soundstage" created by your speakers and the images of the instruments and

singers may become vague or smeared.

Sound energy is in the form of invisible waves. Since our  hearing can perceive sounds from 20-20,000 Hz, we're talking about wavelengths that range from 11/16" at 20,000 Hz, to over 56 feet at 20 Hz. These same reflections affect the music's timbre, or tonal quality. For example, a flute and an oboe have different timbre; they should sound different even when playing the same note, because each instrument's tones have a different harmonic structure. Reflections can obscure those crucial differences. So, for all of these reasons it should be clear that your room is really the final component in your  system. As with any other component, there are steps you can take to improve your room's  performance. Many people find that after accomplishing as much as they can with speaker   placement, they still aren't happy with t heir system's sound. If you feel that the way your room interacts with your speakers is causing problems, it's time to turn to Science; the science of  acoustics, or more specifically, room acoustics.

The ABCs of room acoustics: Reflection, Absorption, Diffusion As we talk about room acoustics problems and room treatment solutions, the three main concepts we'll keep coming back to are reflection, absorption and diffusion. Some reflected sound is necessary for music and speech to sound natural, but too much can rob your system of sound quality. You can control reflected sound by absorbing or by diffusing (scattering) these reflections. Treating room problems can be simple, like installing drapes over a large expanse of glass, or   placing an area rug on the floor between your speakers and where you sit to l isten. Or you can treat your room's trouble spots with specially designed products that change the way your room responds to sound. But the first step is understanding the basic concepts of room acoustics and the part they  play in your everyday listening.

Reflection First, the good news One of the reasons that the effects of room reflections are so noticeable is that our ears (actually, our  entire auditory system, which also includes the brain) are amazingly sensitive at locating the source of a sound. Even with your eyes closed, you can usually locate the position of someone speaking to you in a room. Your brain uses timing differences between the original and the reflected sound to locate the source. It would be much more difficult in a highly reflective room with uncontrolled echoes. (Or outdoors in an open field, where the only reflective surface is the ground.) But our ears aren't perfect. Sounds that arrive at our ears soon enough after the direct sound are  perceived as being part of the original sound. As the graph at the right shows, early reflections that are not too loud or delayed too long will not only increase the loudness of the sound, but can actually add a pleasant spaciousness. This effect is similar to the way our eyes fuse together the series of still pictures used to create TV or movies into an impression of continuous fluid movement. How quickly each image follows is the key: there must be at least 16 frames a second to avoid noticeable flicker. When it comes to sound, there are two factors: loudness and length of delay. If the reflection is too loud, or if the delay  between the original sound and the reflection lasts too long, you'll generally hear a distinct echo. Part of the reason that the surround speakers in a Dolby® Digital system can create such a  believable impression of spaciousness is that the signal fed to the surround speakers includes a 1520 millisecond delay.  Now, the bad news There are several different ways that room reflections can interfere with your enjoyment of music and movie sound. Some can be treated easily and inexpensively, while others are trickier to deal with. Let's start by talking about the unique set of reflections that develop based on the size, shape and dimensions of your room. Standing waves and room resonance modes Any time you have a pair of parallel reflective surfaces (like room walls, or the floor and ceiling), you're going to experience some degree of a phenomenon known as standing waves. Standing waves distort the bass and lower midrange frequencies from 300 Hz on down.

Standing waves are created when sound is reflected back and forth between any two  parallel surfaces in your room. They affect frequencies below 300 Hz.

A room's primary or "axial" resonance modes are based on the room's three main axes: length, width, and height. These resonance modes create bass peaks and dips of up to 10 dB throughout the room. One way to understand the effects of standing waves in a room is to think of how a microwave oven works. The high-frequency microwaves generated to heat the food on your plate are reflected over  and over inside the oven compartment. As these reflections collide, some are reinforced while others are cancelled, creating areas of varying microwave intensity. This translates into definite hot spots and cold spots in your plate of food, from steaming to lukewarm to cool. The sound from your speakers acts in much the same way. It is reflected back and forth, over and over between the parallel surfaces in your room: the side walls, the front and rear walls, and the floor and ceiling. This creates areas of differing sound pressure or loudness: the "hot" and "cold" spots. You can easily hear these standing waves if you play some music with a lot of bass, like pipe organ music or reggae, and take a walk around your room, listening at different spots: the middle of the room, near the walls, and in the corners. You'll probably notice that the bass sounds stronger near  the walls and especially in the corners, where standing waves tend to collect. These are specific types of standing waves which are called room resonance modes. Sizing up your room It's actually pretty easy to calculate the axial resonance modes for your room. Knowing the frequencies of these axial modes will provide valuable information about how your system and room are interacting, specifically on bass notes in the under-300 Hz range. First, get a tape measure and measure the length, width and height of your room. As an example, we'll use these typical room dimensions: 21 feet long x 12 feet wide x 8 feet high. The formula for finding axial room resonance modes:

In the example above, we've calculated our sample room's main resonance mode for length. The room's length is 21 feet, so plugging in 21 for our distance variable in the equation, we get a resonance frequency of 27 Hz.

Our sample room has a length of 21 feet, so plugging 21 into the formula gives us our axial resonance mode for length.

Resonance modes occur when the distance between the room's walls equals half the wavelength of the sound, and at multiples of half a wavelength. Notice that there are always sound pressure (volume level) peaks at the walls.

The circled frequencies will be reinforced by the room. Frequencies appearing in more than one column will receive added emphasis, causing even more sound coloration. In this example, you can see trouble spots at 141 Hz, 188 Hz, and 282 Hz.

So, the main mode for the length axis of the room falls at 27 Hz (it's actually 26.9, but we're rounding to the nearest whole number). This means that although you'll still be able to hear deep  bass sounds from your speakers below 27 Hz, your room cannot provide any reinforcement of  frequencies much below 27 Hz. In addition to this fundamental mode at 27 Hz, there will be other weaker modes at multiples of the fundamental mode (2x27, 3x27, 4x27, etc...). So, along with the first mode at 27 Hz, there will be other resonance modes at 54 Hz, 81 Hz, 108 Hz, etc....  Now we can use the same formula for the room's width and height. Plugging the 12-foot width into the formula gives us a fundamental mode at 47 Hz, with multiples at 94 Hz, 141 Hz, 188 Hz, etc. Using the formula again, our fundamental 8-foot height mode is at 71 Hz, plus multiples at 141 Hz, 212 Hz, etc. It's a little easier to see what's going on if we arrange our room modes into a table (see right). There's actually more to the story than just the axial modes involving two walls, described above. There are also tangential resonance modes involving four room surfaces, and oblique modes involving all six surfaces. These other room modes don't affect the sound as strongly, but as we've mentioned before, all reflections affect the overall sound. How to deal with room resonance modes So now that you know what room resonance modes are and how they can distort your system's sound, what can you do about them? In many cases, not much. These room modes are based on your room's dimensions, which are difficult to change. (Even bass-loving audiophiles will hesitate to move a wall just to hear more accurate low frequencies.) And room treatment products that are great for controlling treble reflections with short wavelengths don't work at all on long-wavelength  bass reflections. Here are some tips and things to keep in mind concerning room resonance modes: 

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Certain room shapes are fundamentally bad from a room-mode standpoint. A cube is one of  the worst shapes for a room (each resonance mode gets triple emphasis). You'll also hear  more standing wave distortion in rooms with two equal dimensions, or rooms with dimensions that are multiples, ie. 8' x 16' x 24'. If you're building a house or finishing a room, here are some room dimension ratios that are superior soundwise:

Applying the 1 : 1.4 : 1.9 room dimension ratio (see table) to a room with an 8-ft. ceiling yields dimensions of 8'H x 11.2'W x 15.2'L.   

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In general, the smaller the room, the more its resonance modes will color bass response. A high, sloped ceiling tends to scatter ceiling mode effects. Common types of wall construction such as drywall or wood paneling on 2x4s will absorb a significant amount of added bass reflections in the under-125 Hz range (see table below). Try moving the position of your chair or sofa closer to or farther from your speakers to get

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out of a standing wave hot spot. Standing waves are always stronger next to walls. If your chair or sofa has its back against a wall, moving it out away from the wall should reduce standing wave boominess. Room corners are notorious collection points for standing waves. If your room has an 8-foot ceiling, professionally designed bass traps can help reduce or eliminate these standing waves. This is accomplished by soaking up the bass reflections created by the 71 Hz fundamental resonance mode of the 8-foot ceiling.

Flutter echo Probably the most common and immediately noticeable room problem results from having parallel surfaces (walls, floor and ceiling) with a hard finish that reflects sound. The resulting effect is called flutter echo, a ringing reverberation that remains after the direct sound has stopped.

The sound-absorbing effectiveness of some common room surfaces. Fibrous materials like carpet and drapes provide significant absorption above 500 Hz, but have little effect on lower frequencies. Conversely, window glass and drywall can absorb bass frequencies, but are very reflective above 500 Hz. The most successful approaches combine materials like these with professionally-designed room treatment products. If you've ever stood in an empty uncarpeted room or hallway, and clapped your hands, you've heard flutter echo. The original clap sound is reflected back and forth between two surfaces. Because the wavelengths of mid- and high-frequency sounds are so much shorter than those of bass notes, the reflections bounce around very directionally, like reflected light. The resulting sound is this ringing flutter echo rather than the boomy standing waves described previously. Flutter echo affects music by blurring transients (fast musical attacks) and adding an unpleasant harshness to the midrange and treble. Flutter echo and other primarily side wall reflections affect sounds above 500 Hz, and are a major reason why the same pair of speakers will sound different in different rooms. To treat flutter echo you need to control the reflections on one or both of the parallel surfaces. This usually means applying some sort of sound-absorbing or sound-diffusing material to the side walls  between the speakers and your listening position. Carpeting or acoustic ceiling tile will reduce floor/ceiling flutter echo. We'll go into detail about locating and treating your room's points of  reflectivity later. Reflection effects on movie dialogue The movie industry certainly understands how sonically damaging reflections can be. Think about all the reflection-absorbing surfaces in your neighborhood movie theater: heavy drapes all around,

upholstered chairs, and a human audience (that's right, our bodies act as sound absorbers too). Studies have shown that dialogue is more easily understood in rooms using one or more types of  reflection control. Reflections can be controlled in listening rooms and home theater rooms by sound absorption, sound diffusion, or some combination of both.

Absorption The first choice for reflection control The sound produced by your speakers, as well as its reflections from your room's walls, ceiling, floor and furnishings, is actually sound energy, or acoustical energy. These sound waves cause air   particles to vibrate, and when they vibrate against our eardrums, we hear sound. A basic law of physics states that energy can neither be created nor destroyed, but can be converted into another form. If it's impossible to simply destroy all these unwanted sound reflections, how can we control them? This is where the concept of sound absorption enters the picture. If you've ever been inside a recording studio, radio or TV station, concert hall, or music practice room at a school or music store, you've probably seen some t ype of sound-absorbing material, even if you didn't know what it was for. For nearly 60 years, applying absorptive material to walls and other reflective surfaces has been the  primary method for taming unwanted reflections. Dense, porous materials like polyurethane foam and fiberglass have been the most popular choices. These materials absorb sound by converting the acoustical energy (the sound) into heat. This happens when the air particles are driven into motion by the sound waves, then attempt to pa ss through the dense sound-absorbing material, resulting in heat-generating friction. (Don't worry, this energy conversion process generates tiny amounts of heat.) Whether we're talking about common room materials ( see table) or professionally designed room treatment products, a material's ability to absorb sound varies according to the f requency of the sound. As the table shows, soft, fibrous materials like carpet and drapes will absorb most reflected sound above 500 Hz, yet have little or no effect on reflections below 125 Hz.

The illustration above left shows that a 1" thick fiberglass panel  provides excellent absorption of sounds above 500 Hz, but that controlling lower-frequency reflections requires the use of thicker   panels. As an alternative to thicker fiberglass, the illustration above right shows how creating an air space between the panel and wall surface increases low-frequency absorption. This makes sense when you remember the huge differences in the wavelengths of high- and lowfrequency sounds. Fibrous materials, which are so effective at absorbing 1000 Hz sound waves a little over a foot long, can do very little when it comes to 125 Hz wavelengths that are 9 feet long. These long-wavelength reflections simply pass right through these soft materials with almost no resistance. The table on the previous page shows that drywall and window glass provide significant absorption in the 125 Hz range. This conversion of acoustic energy is accomplished in a different way than that of the soft, fibrous materials described previously. When a low-frequency sound wave strikes drywall or a window, those surfaces convert some of the sound energy to motion; they actually flex a tiny amount, thus absorbing some of the acoustic energy.

 Notice the increase in the absorption of reflected sounds —  especially for sounds at or above 1000 Hz (1kHz) — when the fabric is folded into drapes.

Tips on absorptive treatments Although absorptive treatments are very effective at taming flutter echo and mid- and highfrequency reflections in general, they won't cure all room acoustics problems. In fact, using too much absorptive material can itself cause problems.

If your system was in a room with thick carpeting on the floor, acoustic tile on the ceiling, and heavy drapes covering much of the wall surfaces, you would have nearly all of the high-frequency reflections being absorbed and nearly all of the bass sounds being reflected. The sound in this room would be unpleasant: thick and boomy in the bass with little or no sense of spaciousness. An overabsorptive room can make spoken dialogue sound unnaturally dry. At the other extreme, a room with painted drywall on the walls, drywall or plaster on the ceiling, linoleum over concrete on the floor, and no sound absorbing drapes or rugs, would sound extremely  bright, thin and echoey. And as we discussed, too many echoes can negatively affect movie dialogue, making it more difficult to understand. Your goal should be to balance the amount and frequency of the absorption in your room to achieve some bass and high-frequency absorption. Typically, bass absorption is the more difficult to achieve. Meanwhile, here are a few tips and ideas to keep in mind concerning sound absorption: 

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Before turning to professional room treatment products for absorption, try to get the most out of ordinary room materials ( see table). Large expanses of glass such as picture windows or French doors should be covered with drapes. You don't have to treat every surface in your room. There are a few key spots which, if  treated, will give you maximum sound improvement for your investment. The pad beneath a carpet contributes to its sound-absorbing ability. While your first considerations should be durability and comfort, it's worth knowing that an "open-cell" pad such as foam rubber will absorb more sound than a "closed-cell" pad.

Absorption is an important ingredient of room treatment, and is especially effective at treating side wall reflections. But absorption is not the only answer, and in many situations, it's not the best choice. In a small listening room, overuse of absorptive material for reflection control can result in a room that is too acoustically "dead." Some music lovers think of professional recording studios which have been heavily treated with absorptive materials as an acoustic model, but keep in mind that studios are able to add artificial reverberation through electronic signal processing. Music lacking the richness contributed by the room effect is less involving. Fortunately, there are other ways to control room reflections, and increasingly, audiophiles and musicians are turning to diffusion.

Diffusion Until fairly recently, your acoustical room treatment options were generally limited to reflection and absorption. Diffusion, the scattering or redistribution of acoustical energy, was recognized as being sonically beneficial, but was also difficult to achieve. All that changed about 20 years ago when a company called RPG Diffusor Systems began developing innovative diffusion products based on mathematical number theory. The advantage of diffusion is that because the sound energy is scattered rather than absorbed, that energy isn't lost, thereby maintaining more of a "live" sound in your room. It's difficult to describe this type of diffusion because it is completely rooted in advanced mathematics. But it has created a revolution in sound treatment that touches nearly every aspect of  sound production and reproduction, from world-famous concert halls to top-flight recording studios and broadcast facilities. Tips on diffusive treatments Diffusion products can be used to treat many of the same problems that absorption is used for. Again, diffusion will rid your room of echoey reflections without replacing them with acoustic deadness. Here are some situations where diffusion works particularly well:

A bookcase filled with odd-sized  books makes a very effective sound diffusor.

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If your room already has built-in absorption in the form of carpeting, drapes, or acoustic ceiling tile, diffusion may control side wall reflections better than adding more absorption. You may already have a good natural diffusor in your home without realizing it. A bookcase filled with odd-sized books makes an effective diffusor. In a home theater system using traditional bookshelf speakers for surrounds, place diffusors in the middle of the back wall and aim your surrounds toward the diffusors at a 45° angle (see below). One of the best-sounding setups for music or home theater is to use absorptive material on room surfaces between your listening position and your front speakers, and treat the back  wall with diffusive material to re-distribute the reflections.

If you're using conventional (nondipole) surround speakers in a home theater system, you can achieve much of the diffuse sound of dipole speakers  by treating your rear wall with diffusors and aiming your surrounds at them.

Locating Your Room's Reflective Trouble Spots Congratulations. If you've made it this far, you're past the most technical information. You've learned how room reflections affect the sound of your stereo or home theater system. And you've learned how absorptive and/or diffusive materials can help control those reflections. Now the fun  begins. You're going to learn how to locate your room's reflective trouble spots.

At frequencies above 500Hz, sound waves bounce off  reflective surfaces (like walls) as predictably as light bouncing off a mirror. In the above illustration, sound from a speaker is reflected off a side wall toward the listening  position. Mirror, mirror on the wall... Along with standing waves, the reflections that distort sound the most are the loud reflections that  bounce once off the side walls, ceiling and floor on the way to your listening/viewing spot. These strong reflections are called "early" reflections. Controlling the intensity of early reflections is crucial to achieving optimum sound.

So, how do you figure out exactly where these reflections are coming from? An easy, accurate way to locate the precise points of sound reflectivity on your walls, ceiling and floor is to use a mirror. You'll need a friend or family member to act as an assistant. While seated in the listening position have your assistant slide a small mirror (8" x 10" works well) along the left wall at the height of the tweeter. Your assistant should start across from the left speaker and move slowly toward the listening position. As the mirror is moved toward you along the wall, you will at some point see a reflection of the left speaker in the mirror. Mark the spot on the wall where the tweeter reflection appears with a piece of tape. As the mirror continues moving toward your listening position, you will next see a reflection of the right speaker. Mark the location of the right tweeter reflection spot on the wall with another piece of  tape. Now repeat this procedure on the right wall to locate the corresponding two reflective  positions there. Early sound reflections from the points you located are adding significantly to the sound you hear at your listening position. They cause some sounds to be canceled out while others are amplified, resulting in smeared stereo images.

If you can see it, you can hear it. Wherever you see your speaker reflected in the mirror, that's a point of  reflection that should receive absorptive, or in some cases, diffusive acoustic treatment. The solution to this problem is to treat these points of reflectivity with some form of absorptive material. Panels made of 1" fiberglass or foam (polyurethane or melamine) installed on the walls do an excellent job of absorbing these reflections. Be sure to use enough absorptive material so that it extends at least 18" on either side of the marked locations. The material should also be at least as high as the tops of the speakers for best reflection control. If you refer back to the first page of this article, you'll see that sound reflections from your room's ceiling and floor can also contribute to sonic problems. Repeat the wall procedure on the ceiling. Have your assistant move the mirror along the imaginary lines on your ceiling that would connect each of your speakers to your listening position. You should be able to locate one reflective point on each line about midway between the speakers and the listening position. Mark each of these spots with a piece of tape. Apply absorptive material to the cei ling extending at least 12" on either side of  each marked location. The floor between your listening spot and your speakers can also be a source of reflected sound. If  your floor is carpeted, you needn't worry about floor reflections. But if the floor is a hard surface like hardwood, tile, or linoleum, use the mirror technique to find the points of reflectivity. Move the mirror along the imaginary lines connecting each speaker with your listening position. Again, you will be locating one point along each line roughly midway between the speaker and the listening  position. An easy, good-looking way to treat floor reflections is to cover the points of reflectivity with a reasonably thick area rug. It may seem strange, but reflections from the wal l behind your speakers also contribute to the sound you hear at your listening/viewing position (mostly frequencies below 500 Hz). Use the mirror  technique again to find the point of reflectivity for each speaker on this wall and treat it with absorptive material. By using thicker material, or providing for an air space between the material and the wall, you'll get improved low-frequency absorption. The wall behind your listening area may require treatment also, though if it's several feet away, reflections probably aren't a serious problem. This rear wall is the surface that would benefit most from diffusion products or a bookshelf.

Most people are more comfortable thinking about music as notes or  tones, rather than frequencies. Middle "C" on a piano i s 262 Hz. Low "E" on a bass guitar is 41 Hz. Cymbals can go out to 15,000 Hz. Summing up The next time you sit down to watch or listen, think about the ways — good and bad — that your  room may be affecting the overall sound of your system. Although some of the concepts discussed here may be difficult to wrap your brain around, a basic understanding of room acoustics (and speaker placement) can help you maximize the performance of any audio or home theater system.

Also be on the lookout for "acoustics-savvy" products such as powered speakers with built-in bass equalization, and receivers and processors with DSP room correction.

For more room acoustics information: The Master Handbook of Acoustics, Fourth Edition, by F. Alton Everest. A wide-ranging look at room acoustics, both in the home and the studio. Includes detailed explanations and some math, yet is very accessible. Complete with essential information for building or remodeling a listening room. Highly recommended. 592 pages.

The Complete Guide to High-End Audio, Second Edition, by Robert Harley. 558 pages. Special thanks to RPG for their assistance. Their website is www.rpginc.com