Acoustics

is usually very broadly defined as "the science of sound."

Hall Acoustics The shaping and equipping of an enclosed space to obtain the best possible conditions for faithful hearing of wanted sound and the direction and the reduction of unwanted sound.

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Fog Art Museum

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The Fog Art Museum hall of Harvard University,

USA when built was found to be so defective

acoustically. The lectures given in it were not

intelligible to audience.

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In 1911 Prof. Wallace C. Sabine, Professor of

Physics, Harvard University, first of all

scientifically tackled the problem of

satisfactory speech and music in a hall..

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Basic Requirements of Acoustically Basic Requirements of Acoustically Good HallGood Hall

There should be adequate loudness in

every part of the room, especially in

remote seats.

The total quality of the speech and

music must be unchanged.

For the sake of clarity, successive

syllables must be clear and distinct.

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Basic Requirements of Acoustically Basic Requirements of Acoustically Good HallGood Hall

The sound energy should be uniformly

distributed within the room.

The boundaries should be sufficiently

sound proof to exclude extraneous noise.

There should be no Echelon effectno Echelon effect.

There should be no resonance no resonance within

the building.

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Basic Requirements of Acoustically Basic Requirements of Acoustically Good HallGood Hall

The hall must be full of audience.

The reverberation timereverberation time should be quite

proper (optimum).

1 sec – 2 sec for music & 0.5 sec – 1 sec for speech.

• The persistence of sound in a room due to

multiple reflections, even when the source

stops, is known as reverberation.

• The time interval between reflections is usually

so short that distinct echoes are not heard.

ReverberationReverberation

• Reverberation is a basic acoustic property of a room.

Parallel reflective surfaces generates unwanted reverberation

• It can enrich speech and music in all areas -- or it can generate higher noise levels throughout a room.

• The duration to which sound persists is called as Reverberation Time.

• The reverberation time for a room is the time required for the intensity to drop to one millionth (10-6) of its initial value.

• The reverberation time for a room is the time required for the intensity to drop by 60 decibels.

If the initial intensity is Ii then, final intensity is If

= 10-6 IidBi = 10 log (Ii / I)

dBf = 10 log (If / I)

dBi – dBf = 10 log (Ii / If)

= 10 log 10 6

= 10 x 6

= 60 dB

Room Acoustics

Reverberation

Absorption Coefficient

• The absorption coefficient ‘a’ of material

is defined as the ratio of the sound energy

absorbed by the surface to that of the

total sound energy incident on the

surface.

Unit for absorption coefficient is Open Window Unit (OWU).

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Sabines’ Formula

Prof. Wallace C. Sabine studied reverberation

time & concluded;

The reverberation time is,

1.Directly proportional to the volume of auditorium.

2.Inversely proportional to the areas of sound

absorbing surfaces.

3.Inversely proportional to the total absorption.

4.Depends on the frequency of sound waves.

Since, absorption coefficient increase with

frequency for most of the materials.

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where, K is the proportionality constant K = 0.165, when the dimensions are in metric units..

where, A = ∑as = a1S1 + a2S2 + a3S3 + ………+ anSn

where, a1, a2, a3,…………….., an are the absorption coefficients of the materials in the hall.

where, S1, S2, S3,…………….., Sn are the surface areas of the absorbing materials in the hall.

OR

OR

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Factors affecting Architectural Hall & their Remedies:

• The persistence of sound in a room due to

multiple reflections, even when the source

stops, is known as reverberation.

(a)Reverberation(a)Reverberation

• The duration to which sound persists is called as Reverberation Time.

1. By providing windows & ventilators.

2. Decorating the walls & windows by pictures & maps.

3. Using heavy curtains with folds.

4. Having full capacity of audience.

5. By covering the floor with carpets.

6. By lining walls with absorbing materials

Viz. felt, fiber board, glass wool.

(a)Remedy for reverberation(a)Remedy for reverberation

• Sufficient loudness at every point in the hall is

an important factor for satisfactory hearing.

(b)Adequate loudness(b)Adequate loudness

RemedyRemedy

1. Using large sounding boards behind the speaker &

facing the audience.

2. Low ceilings to reflect sound energy.

3. Use of equipments like loudspeakers.

Reflection

• If there are focusing surfaces on the walls or

ceilings of the hall, they produce concentration

of sound into particular region.

(c)Focusing due to walls & ceilings(c)Focusing due to walls & ceilings

RemedyRemedy

1. There should be no curved surfaces. If present, it

should be covered with absorbent material.

2. Ceiling should be low.

3. A parabolic reflected surface arrangement with the

speaker at the focus.

Sound reflection

• An echo is heard when direct & reflected sound

waves coming from the same source, reach the

listener with the time interval of 1/10 th sec or

more.

(d)Echoes(d)Echoes

(d(d) Remedy for echoes) Remedy for echoes

1. Echoes may be avoided by covering the long

distance walls and high ceiling with absorbent

material.

(e(e) Echelon effect) Echelon effect

• A set of railings or any regular spacing of

reflected surfaces may produce a musical note

due to the regular succession of echoes of the

original sound to the listener.

RemedyRemedy

1. This type of surfaces should

be avoided or covered with

proper sound absorbing

material.

(f(f) Resonance) Resonance

• Some times the window-panes, sections of the

window portions and walls lacking in rigidity

are throw into forced vibrations and create

other sounds.

RemedyRemedy

1. Such resonant vibrations

should be suitably damped

(g(g) Noise) Noise

• Any unwanted sound is Noise.

Three types of noises:Three types of noises:

1. Air-borne noise

2. Structure borne noise

3. Inside noise

1. Air-borne noise

• The noise which commonly reaches the hall

from outside through open windows, doors and

ventilators, is known as air-borne noise.

• Remedy for air-borne noise

1. By allotting proper places for doors & windows.

2. Arrangement of perfectly shutting doors.

3. Use of heavy glass in windows & ventilators.

4. By avoiding opening for pipes & ventilators.

Site Selection for Sound Control

Orientation

Acoustical Zoning

2. Structure borne noise

• The noises which are conveyed through the

structure of the building are known as

structural noises.

• E.g. footsteps, drilling operating machinery,

moving of furniture, etc.

Structure borne sound Steel Frame

Plumbing Pipe

• Remedy for structure borne noise

1. Using double walls with air space between them.

2. Mechanical equipments viz. refrigerators, lifts,

fans produce vibration in structure. These

vibrations can be checked by insulating the

equipment properly.

3. By insulating the machinery.

Isolation Of Equipment

Low-end RTUs (roof top units) are typically loud.

Isolation of Equipment

3. Inside noise• The noise which are produced inside

the hall or room in big offices are called

as inside noise.

They are produced due to machinery like A.C., type

writers, etc. in the hall.

Remedy for inside noise

1.Noise free air conditioners are to be used.

2.The machinery like typewriters should be placed

on the sound absorbing materials.

3.The floor should be covered with carpet.

4.Any engine should be fitted on the floor with a

layer of wool or felt between them.

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