Noise 02 - Sound Absorption

8/7/2019 Noise 02 - Sound Absorption

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Sound Inside a RoomSound Inside a Room

When a sound source is enclosed in a room,When a sound source is enclosed in a room,

the level of sound inside the room isthe level of sound inside the room ischanged.changed.

The level of the sound (reinforcement ofThe level of the sound (reinforcement of

sound) depends on how much the surfacessound) depends on how much the surfaces

of the room absorb (or reflect) sound.of the room absorb (or reflect) sound.


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As sound travels reaches a surfaceAs sound travels reaches a surfacethere isthere is

reflection, absorption and transmissionreflection, absorption and transmission


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Sound AbsorptionSound Absorption

Sound absorption is quantified bySound absorption is quantified by

absorption coefficient:absorption coefficient:

= (= (EEiiEErr) /) /EEii

The absorption coefficient is the ratio ofThe absorption coefficient is the ratio of

sound energy absorbed to the sound energysound energy absorbed to the sound energy

incident.incident.EEiiIncident Energy.Incident Energy.

EErrReflected EnergyReflected Energy


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Absorption CoefficientAbsorption Coefficient

The sound absorbing efficiency of a materialThe sound absorbing efficiency of a material

is described by itsis described by its sound absorptionsound absorptioncoefficientscoefficients..

It is denoted byIt is denoted by . The sound absorption coefficients areThe sound absorption coefficients are

obtained from testing of the material in theobtained from testing of the material in thelaboratory (either in alaboratory (either in a reverberantreverberant chamber,chamber,

or by theor by the impedance tubeimpedance tube method).method).
http://../Mediacoustic.lnkhttp://../Mediacoustic.lnkhttp://../Mediacoustic.lnk


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Absorption coefficientAbsorption coefficient is a dimensionlessis a dimensionlessquantityquantity ieie. a number from 0 to 1.0.. a number from 0 to 1.0.

The higher the coefficient number, the betterThe higher the coefficient number, the better

is the absorption.is the absorption.

The absorption coefficient varies withThe absorption coefficient varies withfrequency.frequency.


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Noise Reduction Coefficient (NRC)Noise Reduction Coefficient (NRC)

Another term employed to describe theAnother term employed to describe the

absorption properties of a material.absorption properties of a material.

A single number obtained by averaging theA single number obtained by averaging theabsorption coefficient (absorption coefficient () at 250, 500, 1000) at 250, 500, 1000and 2000 Hz.and 2000 Hz.

NRC = (NRC = (200200 ++ 500500 ++ 10001000 ++ 20002000) / 4) / 4


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An AnalogyAn Analogy

If one wishes to contain water (sound)If one wishes to contain water (sound)..


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An AnalogyAn Analogy..

If one wishes to absorb water (sound)If one wishes to absorb water (sound)..


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Typical Data SheetTypical Data Sheet


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Absorption CoefficientsAbsorption Coefficients

Look up the Data sheet.Look up the Data sheet.

Unpainted brickworkUnpainted brickwork..

at 63 Hz =0.02,at 63 Hz =0.02, at 125 Hz =0.02at 125 Hz =0.02.... at 2000 Hz = 0.05, etc. NRC = 0.03.at 2000 Hz = 0.05, etc. NRC = 0.03.

100 mm rockwool blanket100 mm rockwool blanket

at 63 Hz =0.25,at 63 Hz =0.25, at 125 Hz =0.45at 125 Hz =0.45.... at 2000 Hz = 0.85, etc. NRC = 0.81.at 2000 Hz = 0.85, etc. NRC = 0.81.


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and NRC comparisonsand NRC comparisons Compare theCompare the and NRC values of differentand NRC values of different

materials and building construction.materials and building construction.

Impervious materials have lowImpervious materials have low and NRCand NRCvalues.values.

Porous materials have higherPorous materials have higher and NRCand NRCvalues.values.

It is possible to have plywood (and otherIt is possible to have plywood (and otherimpervious) but perforated / openings over theimpervious) but perforated / openings over the

higherhigher materials without substantialmaterials without substantialreduction in thereduction in the and NRC values.and NRC values.


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Room Constant (Room Constant (RRcc

or R)or R)

The sound absorptivity of a space (room) isThe sound absorptivity of a space (room) is

given by thegiven by the Room ConstantRoom Constant..

TheThe Room ConstantRoom Constant depends on the totaldepends on the totalarea and absorption coefficients of thearea and absorption coefficients of the

materials making up the room.materials making up the room.


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Room ConstantRoom Constant RRcc is given byis given byRRcc = S= S meanmean / (1/ (1meanmean ))

SStotal surface area of the exposed surfacetotal surface area of the exposed surface

of the room (mof the room (m22))

meanmean -- the average sound absorptionthe average sound absorptioncoefficient of the room.coefficient of the room.


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Room ConstantRoom Constant

The average sound absorption coefficient isThe average sound absorption coefficient is

the total absorption of the room divided bythe total absorption of the room divided bythe total surface area of the room.the total surface area of the room.

Calculate by determining and adding up theCalculate by determining and adding up the

respective contribution of each individualrespective contribution of each individualsurfaces.surfaces.

This include the walls (plastered wall,This include the walls (plastered wall,treated wall, windows, doors), the floor andtreated wall, windows, doors), the floor andthe ceiling.the ceiling.


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meanmean == Total Absorption in RoomTotal Absorption in RoomTotal Surface Area of RoomTotal Surface Area of Room

== Sum of Absorption of all SurfacesSum of Absorption of all Surfaces

Total Surface Area of RoomTotal Surface Area of Room

= (S= (S1111 + S+ S2222 ++ ..S..Snnnn )/ (S)/ (S11 + S+ S22 ++SSnn))

where 1, 2, 3where 1, 2, 3n denotes the individual materialn denotes the individual materialsurfaces.surfaces.


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Room or SpaceRoom or Space Typical range ofTypical range ofmeanmeanAcoustic EnvironmentAcoustic Environment

LiveLive 0.020.02 -- 0.070.07

Fairly LiveFairly Live 0.070.07 -- 0.150.15

AverageAverage 0.200.20 -- 0.400.40

Fairly DeadFairly Dead 0.400.40 -- 0.500.50

Dead (very unlikely inDead (very unlikely in 0.500.50 -- 0.800.80industrial environment)industrial environment)


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LiveLive ::All surfaces hard and rigidAll surfaces hard and rigid

Fairly LiveFairly Live ::All surfaces generally hard, but with someAll surfaces generally hard, but with somepanel construction (sheet metal or wood).panel construction (sheet metal or wood).

AverageAverage :: Most surfaces with panel construction.Most surfaces with panel construction.

Significant area have absorptive surfaces.Significant area have absorptive surfaces.

Large openings in walls to exterior.Large openings in walls to exterior.

Fairly DeadFairly Dead :: Majority with absorptive surfaces and theMajority with absorptive surfaces and theremaining with panels.remaining with panels.

DeadDead ::All surfaces absorptive with no reflecting objects.All surfaces absorptive with no reflecting objects.
http://../Mediacoustic.lnk


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Reverberation TimeReverberation Time

The room absorptivity can also be indicated by the reverberationThe room absorptivity can also be indicated by the reverberation timestimes(RT). RT(RT). RT6060 is the time taken for a sound to decay by 60 dB.is the time taken for a sound to decay by 60 dB.


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OPTIMUMOPTIMUM REVERBERATION TIMEREVERBERATION TIME


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Reverberation TimeReverberation Time

Reverberation time can also be estimatedReverberation time can also be estimated

from:from:RT = (0.16V) /RT = (0.16V) / SSmeanmean

RTRT-- reverberation time (sec)reverberation time (sec)VV -- room volume (mroom volume (m33))

SS -- surface area of room (msurface area of room (m22

))meanmeanaverage room absorptionaverage room absorptioncoefficientcoefficient


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ExampleExample meanmean

Room 10 x 8 x 3 m Ht.Room 10 x 8 x 3 m Ht.

Walls plastered areaWalls plastered area SS11 = 90m= 90m22 11 = 0.1= 0.1

Doors areaDoors area SS22 = 8m= 8m22 22 = 0.05= 0.05

Windows areaWindows area SS33 = 10m= 10m

22 33 = 0.15= 0.15

Floor areaFloor area SS44 = 80m= 80m22 44 = 0.2= 0.2

Ceiling areaCeiling area SS55 = 80m= 80m22 5 = 0.5= 0.5

Total absorptionTotal absorption SS = 66.9 m= 66.9 m22 Total areaTotal area SS = 268 m= 268 m22

meanmean == 66.9/268 = 0.2566.9/268 = 0.25


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Example Room ConstantExample Room Constant

Room Constant RRoom Constant Rcc = S= S meanmean / (1/ (1meanmean ))

For example as given hereFor example as given here

RRcc = 268 x 0.25 / (1= 268 x 0.25 / (1--0.25)0.25)= 89 (m= 89 (m22))


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ExampleExample

Example 1Example 1, Notes page 9, Notes page 9

Room Constant = 284 mRoom Constant = 284 m22

RT = 0.16V/ SRT = 0.16V/ Smeanmean = 4.0 seconds= 4.0 seconds
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Noise 02 - Sound Absorption

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