1AES tutorial, 2008 Small Room Acoustics Ben Kok Nelissen ingenieursbureau [email protected]

AES tutorial, 2008AES tutorial, 2008 11

Small Room AcousticsSmall Room Acoustics

Ben KokBen KokNelissen ingenieursbureauNelissen [email protected]@nelissenbv.nl


IntroductionIntroduction

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


(peff)

Verdichting Verdunning

Sound waveSound wave

f

c


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


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


Small RoomsSmall Rooms

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


length = l

Low frequencies in an Low frequencies in an enclosureenclosure

c

l

cf

20

02 f

.etc


Low frequencies in an Low frequencies in an enclosureenclosure

222

,, 2

z

x

y

x

x

xnnn l

n

l

n

l

ncf

zyx


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


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


absorptionabsorption

•porous materialporous material• resonatorsresonators•sandwichsandwich constructionsconstructions


porous materialporous material


porous materialporous material40 mm

40 mm

40 mm 150 mm



0

0,2

0,4

0,6

0,8

1

1,2

125 250 500 1000 2000

200 mm spouw

50 mm spouw



Hzfmd

df

c

c

10010,0

10

Hzfmd c 5020,0

Rule of thumb:Rule of thumb:

Hzfmd c 20005,0


resonatorresonator


diafragmatic resonatordiafragmatic resonator

mdfc

60

Hzf

md

kgm

c 100

12,0

3


diafragmatic resonatordiafragmatic resonator

flexible panelsflexible panels

•thinthin

•largelarge

•high internal losshigh internal loss

excited over the fullexcited over the full areaarea


length = l

volume = v mass = m area = s

Helmholtz resonatorHelmholtz resonator

Vl

scfc 2


diffusiondiffusion

spatial spatial andand temporal distribution of temporal distribution of energyenergy


Specular reflectionSpecular reflection


Diffuse reflection 1Diffuse reflection 1


Diffuse reflection 2Diffuse reflection 2


diffuser examplediffuser example


diffusiondiffusion

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

> grating lobes!> grating lobes!


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


installation exampleinstallation example


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’


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


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


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


QUESTIOQUESTIONSNS

Documents

1AES tutorial, 2008 Small Room Acoustics Ben Kok Nelissen ingenieursbureau [email protected]