Acoustic Absorbers and Diffusers - Theory, Design and Application

428 Pages·2004·12.419 MB·English

by  T. Cox, P. D'Antonio

Acoustic Absorbers and Diffusers
Also available from Taylor & Francis
Understanding Active Noise Cancellation
Colin Hansen
Hb: 0–415–23191–4;Pb: 0–415–23192–2
Taylor & Francis
Room Acoustics
Heinrich Kuttruff
Hb: 0–419–24580–4
Taylor & Francis
Fundamentals of Sound and Vibration
Frank Fahy and John Walker
Hb: 0–419–24180–9;Pb: 0–419–22700–8
Taylor & Francis
Active Control ofNoiseand Vibration
Colin Hansen and Scott Snyder
Hb: 0–419–19390–1
Taylor & Francis
Sound ReinforcementEngineering
Wolfgang Ahnertand Frank Steffen
Hb: 0–415–23870–6
Taylor & Francis
Informationandorderingdetails
Forpriceavailabilityandorderingvisitourwebsitewww.ergonomicsarena.com
Alternativelyourbooksareavailablefromallgoodbookshops.
Acoustic Absorbers and Diffusers
Theory, design and application
Trevor J. Cox
University of Salford, UK
and
Peter D’Antonio
RPG Diffusor Systems Inc., USA
Firstpublished2004
bySponPress
11NewFetterLane,LondonEC4P4EE
SimultaneouslypublishedintheUSAandCanada
bySponPress
29West35thStreet,NewYork,NY10001
This edition published in the Taylor & Francis e-Library, 2005.
“To purchase your own copy of this or any of Taylor & Francis or Routledge’s
collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.”
SponPressisanimprintoftheTaylor&FrancisGroup
(cid:1)2004TrevorCoxandPeterD’Antonio
Allrightsreserved.Nopartofthisbookmaybereprintedorreproduced
orutilisedinanyformorbyanyelectronic,mechanical,orothermeans,
nowknownorhereafterinvented,includingphotocopyingandrecording,
orinanyinformationstorageorretrievalsystem,withoutpermissionin
writingfromthepublishers.
BritishLibraryCataloguinginPublicationData
Acataloguerecordforthisbookisavailable
fromtheBritishLibrary
LibraryofCongressCataloginginPublicationData
Acatalogrecordforthisbookhasbeenrequested
ISBN 0-203-49299-4 Master e-book ISBN
ISBN 0-203-57215-7 (Adobe eReader Format)
ISBN0–415–29649–8(Print edition)
To our families and Manfred Schroeder

Contents
Preface xiv
Acknowledgements xviii
Glossary of frequently used symbols xix
Introduction 1
1 Applications and basic principles of absorbers 6
1.1 Reverberation control 6
1.1.1 A statistical model of reverberation 7
1.2 Noise control in factories and large rooms with
diffuse fields 11
1.3 Modal control in critical listening spaces 12
1.4 Echo control in auditoria and lecture theatres – basic
sound propagation models 13
1.4.1 Sound propagation – a wave approach 15
1.4.2 Impedance, admittance, reflection factor and
absorption 16
1.5 Absorption in sound insulation – transfer matrix
modelling 18
1.5.1 Transfer matrix modelling 19
1.6 Absorption for pipes and ducts – porous absorber
characteristics 20
1.6.1 Characterizing porous absorbers 21
1.7 Summary 22
1.8 References 22
2 Applications and basic principles of diffusers 23
2.1 Echo control in auditoria 23
2.1.1 Example applications 23
2.1.2 Wavefronts and diffuse reflections 26
2.1.3 Coherence and terminology 30
viii Contents
2.2 Reducing colouration in small rooms 32
2.2.1 Sound reproduction 32
2.2.2 Music practice rooms 39
2.3 Controlling modes in reverberation chambers 43
2.4 Improving speech intelligibility in underground
or subway stations 44
2.5 Promoting spaciousness in auditoria 44
2.6 Reducing effects of early arriving reflections
in large spaces 45
2.7 Diffusers for uniform coverage with overhead
stage canopies 46
2.8 Diffusers for rear and side of stage enclosures 48
2.9 Diffusers to reduce focussing effects of concave surfaces 52
2.10 Diffusion and road side barriers 53
2.11 Diffusion and street canyons 55
2.12 Conclusions 55
2.13 References 56
3 Measurement of absorber properties 58
3.1 Impedance tube or standing wave tube measurement 58
3.1.1 Standing wave method 61
3.1.2 Transfer function method 62
3.2 Two microphone free field measurement 64
3.3 Multi-microphone techniques for non-isotropic,
non-planar surfaces 65
3.3.1 Multi-microphone free field measurement
for periodic surfaces 66
3.4 Reverberation chamber method 68
3.4.1 Measurement of seating absorption 71
3.5 In situ measurement of absorptive properties 74
3.6 Internal properties of absorbents 78
3.6.1 Measurement of flow resistivity 78
3.6.2 Measurement of flow impedance 81
3.6.3 Direct measurement of wavenumber 82
3.6.4 Indirect measurement of wavenumber and
characteristic impedance 82
3.6.5 Measurement of porosity 83
3.7 Summary 85
3.8 References 85
4 Measurement and characterization of diffuse reflections
or scattering 87
4.1 Measurement of scattered polar responses 88
4.1.1 Near and far fields 95
Contents ix
4.1.2 Sample considerations 102
4.1.3 The total field and comb filtering 103
4.2 Diffusion and scattering coefficients, a general discussion 104
4.3 The need for coefficients 105
4.3.1 Diffuser manufacturer and application 105
4.3.2 Geometric room acoustic models 106
4.4 The diffusion coefficient 107
4.4.1 Principle 107
4.4.2 Obtaining polar responses 109
4.4.3 Discussion 110
4.4.4 Diffusion coefficient table 111
4.5 The scattering coefficient 112
4.5.1 Principle 112
4.5.2 Rationale and procedure 113
4.5.3 Sample considerations 116
4.5.4 Anisotropic surfaces 116
4.5.5 Predicting the scattering coefficient 118
4.6 From polar responses to scattering coefficients,
the correlation scattering coefficient 120
4.6.1 Scattering coefficient table 123
4.7 Contrasting diffusion and scattering coefficient:
a summary 124
4.8 Other methods for characterizing diffuse reflections 124
4.8.1 Measuring scattering coefficients by solving
the inverse problem 124
4.8.2 Room diffuseness 125
4.9 Summary 126
4.10 References 127
5 Porous absorption 129
5.1 Absorption mechanisms and characteristics 129
5.1.1 Covers 131
5.2 Material types 132
5.2.1 Mineral wool and foam 132
5.2.2 Recycled materials 133
5.2.3 Curtains (drapes) 134
5.2.4 Carpets 135
5.2.5 Absorbent plaster 135
5.2.6 Coustone or quietstone 137
5.3 Basic material properties 137
5.3.1 Flow resistivity 138
5.3.2 Porosity 140
5.4 Modelling propagation within porous absorbents 141
5.4.1 Macroscopic empirical models such as
Delany and Bazley 141