BUILDING ACOUSTICS.ppt

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INTRODUCTION

DEFINITION: The word Acoustics is generally derived from a Greek word meaning to hear,refers to generation,detection,transmission,absorption and control of sounds.

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SOUND PRODUCTION AND TRANSMISSION • A sound is produced when a part of the atmosphere is compressed suddenly. • Due to elasticity of air,the particles originally disturbed in turn disturb the neighboring particles • Sound thus travels in the form of waves. • Sound cannot travel in vacuum.

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There are three characteristics of audible sound •

Pitch: The pitch of a sound is the frequency of its vibration .

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Loudness:It is the strength of the sensation received through ear.

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Tone quality:It is the characteristic of the sound which distinguishes it from another sound of same loudness & pitch. 4

Two different units for expressing the energy of sound is employed. • The intensity of sound is expressed in decibel. • Phon is the unit used for measuring the loudness sensation in the ear.

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Acoustical Analysis

1. Determining the use of the structure-the subjective needs. 2. Establishing the desirable acoustical environment in each usable area. 3. Determining the noise and vibration sources. 4. Studying the location and orientation of the structure and its interior spaces with regard to noise and noise sources 6

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Acoustical Design:

1. Designing shapes, volumes, areas and surfaces to accomplish what the analysis indicates. 2. Choosing materials, systems and constructions to achieve desired result.

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Definition: The persistence of sound even after the source of sound is cutoff. The time for which sound persist even after the source is cutoff is called the time of reverberation. Too long a reverberation time renders the sound energy, sustained over unduly long time. It causes the over lapping of consecutive syllabus in speech or music.

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Reverberation time in seconds 0.50-1.50

Acoustics Excellent

1.50-2.00

Good

2.00-3.00

Fairly Good

3.00-5.00

Bad

Above 5.00

Very Bad 9

Sound Absorption

• The best known acoustical materials are acoustical absorbents. • These are light weight, porous, fuzzy types of boards, blankets and panels. • When a sound wave strikes a surface, a part of its energy is absorbed by friction,part of its energy is transmitted and the remaining part is reflected. • Reverberation directly depends on the loss of energy of sound wave due to friction.

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• The property of a surface by which sound energy is converted into another form of energy is known as Absorption. • Absorption coefficient of a surface indicates the degree to which this surface affects the absorption of sound. • It is thus the ratio of energy absorbed by the area to the energy striking the area. SABIN’S EQUATION T=0.16v/A

T=reverberation time in seconds V=volume in m3 A=total absorbing power in m2 11

Following points are to be noted. • It should be economical in construction,maintenance,water proof and fire proof. • In a hall treated with absorbent materials,speech can be heard clearly and music can be fully enjoyed.

• All absorbent materials are found to be soft and porous. • The absorbing capacity of the absorbent materials,depends on the thickness of the material,its density and frequency of sound. • Noise level of the room provided with absorbent materials is considerably reduced. 12

• Suspended absorbers in the form of inverted cones may be provided in the ceiling to make the hall acoustically good.

• Great care should be taken while selecting the covering for an absorbent material. • In a big hall, audience is a major absorbing factor. This is true in high frequency zone. Hence, low frequency absorbent materials should be provided to achieve optimum reverberation time over a wide range of frequency of sound.

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Types of absorbent materials

•Hairfelt: prof. Sabin used this material in his experimental works.The average value of coefficient of absorption of 25mm thick hairfelt is 0.60.

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Acoustic Plaster:

• This is a fibrous plaster . • It includes granulated insulation material mixed with cement.

• For a thickness of 20mm acoustic plaster possess an absorbent coefficient of 0.30 at 500cycles per second

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Acoustical Tiles: • The absorption of the sound is uniform from tile to tile. • They can be fixed easily and most suitable for small area.

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Perforated Plywood: • This material can be used by forming composite panels with minerals wool and cement asbestos or with mineral wool and hardboard.

• It is generally suspended from trusses. • The average value of coefficient of absorption for the former composite panel is 0.95 and the latter is about 0.20.

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Quilts and Mats: • These are prepared from mineral wool and glass wool. •They are fixed in the form of acoustical blankets.

•The absorption coefficient of such mats depends on the thickness, density and frequency of sound.

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• The sound which is produced, should be evenly spread over the whole area covered by the audience. • The initial sound should be clear and distinct to avoid the possibility of disordered speech. • For the hall to be used for the music, the initial sound should reach the audience with the same intensity. • All undesired sound should be reduced. 19

Factors to be considered in the design of auditorium. 1. Volume: The hall should have enough volume,keeping in view the intensities of sounds likely to be developed in the hall.  For halls to be used only for music,the volume should be large.

2. Shape: It involves the geometrical aspects of the hall.  The paths followed by reflected sound waves are traced and concentrations of sound waves,if any are noted.

 Concave walls tend to concentrate sound waves and hence not good for acoustic purposes. 20

 Plain walls are better.  Convex walls are excellent and are used to reduce the echoes to the minimum extent. 3. Sound Absorption:  Adequate absorbing surfaces should be provided in the room to control reverberation.  Careful study of the hall should be made before suggesting the type of absorbent material and its location in the hall.

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Defects and their remedies. 1.

Reverberation:

 This is the most common defect in many auditoriums.  The sound once created prolongs for a longer duration resulting in confusion with the sound created.  The remedy of this defect is to correct the time of reverberation by suitably installing absorbing materials. 2. Formation of echoes:  When the reflecting surfaces are situated at a distance greater than 15m.  This defect can be removed by selecting proper shape and by providing rough and porous interior surfaces to disperse the energy of echoes. 22

 Sound foci:

 In concave reflecting interior surfaces,certain spots are formed where reflected sound waves meet and create the sound of large intensity.  This can be eliminated by avoiding curvilinear interiors by providing absorbent materials on focusing areas. 4. Dead spots:

 Due to high concentration of reflected sound at sound foci,there is deficiency of the reflected sound at some other points.  The sound level at the dead spots are generally inadequate for satisfactory hearing.  By the installation of suitable diffuses to have even 23 distribution of sound.

5. Loudness:  Due to lack of reflecting surfaces near the source of sound and excessive absorption of sound.

 Can be corrected by arranging hard reflecting surfaces near the source of sound and adjusting the absorption of the hall to provide optimum time of reverberation. 6. Exterior Noise:  It is due to the poor insulation and hence noise enters through loose doors and windows.  Remedy is to provide suitable insulation to the various components of the auditorium. 24

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MEADOWS SCHOOL CENTRE FOR PERFORMING ARTS LAS VEGAS

NEVADA

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ACOUSTICS OF STUDIOS Following are the points for its efficient working:  Noise level in the studio should be brought down to 20-30 db.  The echoes should be completely eliminated.  The finish on the outside surfaces of the building should be of sound reflecting material.  Partition wall and exterior walls of the studio should be sufficiently rigid to resist vibrations and to prevent resonance.

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 Provision of windows should be minimum to prevent the transfer of noise.  Air conditioning machinery and other equipments should be suitably insulated so that vibrations are not carried to the working chambers.  If more than one studio in the building,it is preferable to locate all of them on the same floor.  Heavy curtains may be used with advantage to control the time of reverberation in the studio.

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CASE STUDIES Location: M/S Rangarao and Sons, Aggarbatti manufacturer, Vani vilas Road, Mysore. Engineer: Mathru Engineering Enterprises (MEE). Project: Acoustics for a meeting hall. Description: A hall measuring 5m x 15 m having windows on one side and entry doors on the remaining two sides. The height of the roof is 3m. Defect : The noise used to enter the room through the room which was facing the main road. Because of the restricted roof height there used to be echo. 36

Acoustical Treatment: The windows were provided with double glass shutters both inside and outside and the roof was provided with the false ceiling. For this purpose 20 mm, 2 x 4 ft straw boards were used. Along with false ceiling, side curtains along with carpets were also used. This hall is providing this intended function till date.

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It is going to be a long time before anyone invents a way to transfer an electronic or digital signal straight into the brain,by passing the ear.  Until then,at some stage sound must pass through the air.  Architects and engineers have a responsibility to design functional and safe environments.  Acoustics is important from open offices to worship centers.  In order to effectively address these issues acoustics should be considered in the design phase. 41

References: Rangawala (1998),”Advance building construction”. Deshpandey R.S(1988),”Building construction”. www.aco.co.in/aco.html

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