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Small Room AcousticsSmall Room Acoustics

Ben KokBen KokNelissen ingenieursbureauNelissen ingenieursbureaub.kok@nelissenbv.nlb.kok@nelissenbv.nl

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IntroductionIntroduction

• What is a sound wave?What is a sound wave?

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(peff)

Verdichting Verdunning

Sound waveSound wave

f

c

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Sound waveSound wave

f

c

(peff)

Verdichting Verdunning

F = 100 Hz → F = 100 Hz → λλ = 3,4 m = 3,4 m

F = 1 kHz → F = 1 kHz → λλ = 0,34 = 0,34 mm

F = 10 kHz → F = 10 kHz → λλ = 0,034 = 0,034 mm

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pressurezone

eigenmodes diffraction specular reflections

f1.0.0 f c 4×f c

When do we consider a room When do we consider a room ‘Small’?‘Small’?

A

VT

V

Tfc

6

2000

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Small RoomsSmall Rooms

• Living RoomLiving Room• Recording StudioRecording Studio• Control RoomControl Room • CarCar• Telephone BoothTelephone Booth• HeadphoneHeadphone

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length = l

Low frequencies in an Low frequencies in an enclosureenclosure

c

l

cf

20

02 f

.etc

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Low frequencies in an Low frequencies in an enclosureenclosure

222

,, 2

z

x

y

x

x

xnnn l

n

l

n

l

ncf

zyx

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Low frequencies in an Low frequencies in an enclosureenclosure““Golden” ratios:Golden” ratios:

ASHREAASHREA1:1.17:1.471:1.17:1.47

ASHREAASHREA1:1.45:2.101:1.45:2.10

BoltBolt1:1.28:1.541:1.28:1.54

IACIAC 1:1.26:1.601:1.26:1.60

SeppmeyerSeppmeyer1:1.14:1.391:1.14:1.39

KokKok 1:1.12:1.411:1.12:1.41

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Room mode analysisRoom mode analysisEigenmode verdeling in rechthoekige ruimte

date 1,2589255,165 4,103 3,657 25-apr-08 1,778279

X Y Z Volume7 4,1 2,7 m 77 m 3̂Nx Ny Nz Fres Density 1/40 0 0 0,0 0,21 0 0 24,5 0,2 3,500 1 0 41,8 0,6 2,051 1 0 48,5 1,7 1,772 0 0 49,0 21,7 1,750 0 1 63,5 1,0 1,352 1 0 64,4 16,4 1,331 0 1 68,1 4,3 1,263 0 0 73,5 3,1 1,170 1 1 76,1 6,9 1,131 1 1 79,9 4,8 1,072 0 1 80,2 58,3 1,070 2 0 83,7 5,6 1,033 1 0 84,6 21,5 1,011 2 0 87,2 7,8 0,982 1 1 90,5 6,3 0,952 2 0 97,0 3,5 0,883 0 1 97,1 117,7 0,884 0 0 98,0 26,5 0,880 2 1 105,0 3,5 0,823 1 1 105,8 33,7 0,814 1 0 106,6 31,3 0,801 2 1 107,9 19,1 0,803 2 0 111,4 7,4 0,772 2 1 115,9 5,9 0,744 0 1 116,8 30,8 0,73

Optimum Eigenmode distribution

0,1

1,0

10,0

100,0

10,0 100,0 1000,0

Frequency

Density

Room OptimumSort

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absorptionabsorption

•porous materialporous material• resonatorsresonators•sandwichsandwich constructionsconstructions

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porous materialporous material

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porous materialporous material40 mm

40 mm

40 mm 150 mm

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porous materialporous material

0

0,2

0,4

0,6

0,8

1

1,2

125 250 500 1000 2000

200 mm spouw

50 mm spouw

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porous materialporous material

Hzfmd

df

c

c

10010,0

10

Hzfmd c 5020,0

Rule of thumb:Rule of thumb:

Hzfmd c 20005,0

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resonatorresonator

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diafragmatic resonatordiafragmatic resonator

mdfc

60

Hzf

md

kgm

c 100

12,0

3

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diafragmatic resonatordiafragmatic resonator

flexible panelsflexible panels

•thinthin

•largelarge

•high internal losshigh internal loss

excited over the fullexcited over the full areaarea

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length = l

volume = v mass = m area = s

Helmholtz resonatorHelmholtz resonator

Vl

scfc 2

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diffusiondiffusion

spatial spatial andand temporal distribution of temporal distribution of energyenergy

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Specular reflectionSpecular reflection

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Diffuse reflection 1Diffuse reflection 1

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Diffuse reflection 2Diffuse reflection 2

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diffuser examplediffuser example

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diffusiondiffusion

repetition of the same sequence repetition of the same sequence should be avoided!!should be avoided!!

> grating lobes!> grating lobes!

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phase modulated array of phase modulated array of diffusersdiffusers

0 90 270 180 90 0 270 180

270 90 0 180 270 90

0 90 180 270 0 90 270 180

Side Wall LeftRear Front

Front WallLeft Right

Side Wall RightFront Rear

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installation exampleinstallation example

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microphone/ loudspeaker microphone/ loudspeaker placementplacement• early reflections (1early reflections (1st st 10 10

ms) cause colorationms) cause coloration• specular reflections specular reflections

(narrow peak) cause (narrow peak) cause comb filteringcomb filtering

• diffuse reflections add diffuse reflections add ‘warmth’‘warmth’

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microphone/ loudspeaker microphone/ loudspeaker placementplacement• distant micing/ listening allows distant micing/ listening allows

sound to developsound to develop• close micing/ listening minimizes close micing/ listening minimizes

room influenceroom influence• a small room has no statistic a small room has no statistic

sound field, strong variation sound field, strong variation between positions can existbetween positions can exist

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microphone/ loudspeaker microphone/ loudspeaker placementplacement• room modes have a significant room modes have a significant

influence on low frequency coupling; influence on low frequency coupling; loudspeaker position can have a strong loudspeaker position can have a strong influence on low frequency responseinfluence on low frequency response

• distributed sub-woofers can create a distributed sub-woofers can create a smoother low frequency responsesmoother low frequency response

• a small room has no statistic sound a small room has no statistic sound field, strong variation between field, strong variation between positions can existpositions can exist

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conclusionconclusion

• a small room behaves different a small room behaves different because it’s dimensions are similar because it’s dimensions are similar to the wavelength of the to the wavelength of the (re)produced sound(re)produced sound

• source and listening positions source and listening positions often are fixed -> the design can often are fixed -> the design can accommodate for thisaccommodate for this

• changing positions can have a changing positions can have a dramatic influencedramatic influence

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QUESTIOQUESTIONSNS