room acoustics-1

8/7/2019 room acoustics-1

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What is room acoustics?

Room Acoustics is concerned with the behavior ofsound within an enclosed space

with a view to obtaining theoptimum acoustics effect on theoccupants.


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What are the requirements for good room acoustics?

1. An adequate amount of sound must reach all parts ofthe room. Most attention in this respect needs to begiven to those seats furthest from the source.

2. An even distribution of sound should be achievedthroughout the room. Irrespective of distance from thesource.

3. Other noise which might tent to mask the requiredsound must be reduced to an acceptable level in allparts of the room.


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5. Acoustical defect to be avoided include:-Long delayed echoes-Flutter echoes

-Sound shadows-Distortion-Sound concentration

4. The rate of decay of sound within the room(reverberation time) should be the optimum for therequired use of the room. This is to ensure clarity forspeech and fullness for music.


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The Sound reverberation:

Reverberation is the persistence of sound in anenclosed due to repeated reflections atthe boundaries

Reverberation time (t60) is the time required for themean squared sound pressure of agiven frequency in an enclosure,initially in a steady state to decreaseafter the source is stopped, to a one-millionth of its initial value.

Ex. The time for 60 dB decay


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Optimum reverberation @ 500 to 1000 Hz

Room function Reverberationtime (s)

Recording and broadcast studios 0.45 0.55

Elementary classrooms 0.6 0.8

Playhouses, intimate drama

production

0.9 1.1

Lecture and conference room 0.9 1.1

Cinema 0.8 1.2

Small theaters 1.2 1.4

High school auditoriums 1.5 1.6

General purpose auditoriums 1.5 -1.6

churches 1.4 3.4


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Optimum volume per person for various types of hall

Types of hall Optimum

volume/person (m3)

Concert halls 7.1

Italian-type opera houses 4.2 5.1

Churches 7.1 - 9.9

Cinemas 3.1

Rooms for speech 2.8


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V = r v l ( )r = f r s c

= f r rc stra

= f r c ir

Comput tionof r r tion times

1. Stephens nd te formul(For ideal rever eration time omputation)

t = r ( . 123V + o.1070) seconds


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V room volume (m3)

r total absorption units (m2)

(for a room : the sum of all

absorption of the ceiling, walls,floor, furnishing and occupants)

V room volume (ft3)

r total absorption units (ft2)

2. abines formula(for actual reverberation time with average absorption

less then or equal to 0.2)

Metric abine:

t60 0.16 seconds

a

Customary abine:

t60 0.049 seconds

a


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3. Norris Eyrings formula( for actual reverberation time with average

absorption coefficient greater then o.2)

t60 = 0.049V seconds

- S ln (1- )

t60 0.161 seconds- ln (1- )

S total surface area (m2

or ft2

)average absorption

coefficient of the reflecting

surface


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A furthercorrectionmayneed to eadded forhi her

frequency toallow forairabsorption.

t 0= ___0.16V seconds- ln (1- ) xV

or valuesof less thanabout 0.2but frequencyabove1000 Hz thenamodified formof abines formula isconsider

t60= 0.16V secondsa xV

x = soundabsorptionperunit volumeofair (m2 /m3)


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x per m3 at a temperature of 20C

frequency Relative Humidity (RH)

30% x 10-3 40% x 10-3 50% x 10-3 60% x 10-3 80% x 10-3

1000 Hz 3.28 3.28 3.28 3.28 3.28

2000 Hz 11.48 8.2 8.2 6.56 6.56

3000 Hz 39.36 29.52 22.96 19.68 16.4


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Fitzroy equation- for non uniform distribution of absoption


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The coefficient of absorptionThe coefficient of absorption- Is the ratio of the absorbed sound intensity to the

incident sound intensity.

= Ia (unitless)Ii

= 1 for a perfect absorbent material

Ia = Ii It

It = reflected sound intensity


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Average absorption coefficient ()

= 1

+ 2

+ 3.+

n

n

total absorption (a)

a A (m2 or ft2)

A surface area of the absorbent structure (m2 or ft2)


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The Measurement of Reverberation time

a. Using a beat frequency oscillator or white noisegenerator as a sound source

b. Using a pistol as a sound source

Generally, the methods of measuring reverberation timeconsists of a microphone connected to a frequencyanalyzer connected in turn to a level recorder.

The level recorder still need to have a logarithmicpotentiometer in the circuit to convert the pressuremeasurement into dB.


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Problem 1:A church has an interval volume of 90.05

ft3 (2.550 m3). When it contains 2000customary sabines of absorption (186 metric

sabines), what will be its reverberation time inseconds?

Ans : 2.25 s


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Problem 2

A lecture room 16m long, 12.5 m wide and 5 mhigh has a reverberation time of 0.75s. Calculate the

average absorption coefficient of the surfaces using theNorris-Eyring,s formula.

ans. 0.658

room acoustics-1

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