BUILDING ACOUSTICS.ppt

7/22/2019 BUILDING ACOUSTICS.ppt

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INTRODUCTION

DEFINITION:

The word Acoustics is generally derived from aGreek 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 .

• Loudness:It is the strength of the sensation

received through ear.

• Tone quality:It is the characteristic of the soundwhich distinguishes it from another sound of same

loudness & pitch.



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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



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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 cutoffis 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 timein seconds

Acoustics

0.50-1.50 Excellent

1.50-2.00 Good

2.00-3.00 Fairly Good

3.00-5.00 BadAbove 5.00 Very Bad



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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 towhich 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



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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.



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• Suspended absorbers in the form of inverted cones may

be provided in the ceiling to make the hall acousticallygood.

• 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 hisexperimental 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 withthe same intensity.

• All undesired sound should be reduced.



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Factors to be considered in the design of auditorium.

1. Volume: The hall should have enough volume,keeping inview 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.



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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 theroom 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 ofreverberation 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.



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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

distribution of sound.



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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.



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MEADOWS SCHOOL CENTRE FOR PERFORMING ARTS

LAS VEGAS NEVADA



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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 ofsound reflecting material.

Partition wall and exterior walls of the studio should be

sufficiently rigid to resist vibrations and to prevent resonance.

ACOUSTICS OF STUDIOS



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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 tolocate 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 throughthe room which was facing the main road.

Because of the restricted roof height there used to

be echo.

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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 worshipcenters.

In order to effectively address these issues acoustics

should be considered in the design phase.



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References:

Rangawala (1998),”Advance building construction”. Deshpandey R.S(1988),”Building construction”.

www.aco.co.in/aco.html



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BUILDING ACOUSTICS.ppt

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