Room acoustics: 4. ed

Preface to the fourth edition ix Preface to the first edition xi Introduction 1 1 Some facts on sound waves, sources and hearing 6 1.1 Basic relations, the wave equation 6 1.2 Plane waves and spherical waves 8 1.3 Energy density and radiation intensity 13 1.4 Signals and systems 15 1.5 Sound pressure level and sound power level 20 1.6 Some properties of human hearing 21 1.7 Sound sources 2 References 30 2 Reflection and scattering 31 2.1Reflection factor, absorption coefficient and wall impedance 32 2.2 Sound reflection at normal incidence 33 2.3 Sound reflection at oblique incidence 36 2.4 A few examples 39 2.5 Random sound incidence 46 2.6 Scattering, diffuse reflection 51 References 58 3 The sound field in a closed space (wave theory) 59 3.1Formal solution of the wave equation 60 3.2 Normal modes in rectangular rooms with rigid boundaries 64 3.3 Non-rigid walls 71 4 3.4 Steady state sound field 75 3.5 Decaying modes, reverberation 82 References 88 4 Geometrical room acoustics 89 4.1Enclosures with plane walls, image sources 90 4.2 The temporal distribution of reflections 97 4.3 The directional distribution of reflections, diffusion 102 4.4 Enclosures with curved walls 105 4.5 Enclosures with diffusely reflecting walls 110 References 114 5 Reverberation and steady state energy density 115 5.1Basic properties and realisation of diffuse sound fields 116 5.2 Mean free path and average rate of reflections 122 5.3 Sound decay and reverberation time 126 5.4 The influence of unequal path lengths 130 5.5 Enclosure driven by a sound source 133 5.6 Enclosures with diffusely reflecting walls 137 5.7 Coupled rooms 142 References 145 6 Sound absorption and sound absorbers 147 6.1The attenuation of sound in air 147 6.2 Unavoidable wall absorption 150 6.3 Sound absorption by vibrating or perforated boundaries 151 6.4 Extended resonance absorbers 154 6.5 Helmholtz resonators 159 6.6 Sound absorption by porous materials 163 6.7 Audience and seat absorption 173 6.8 Miscellaneous objects (freely hanging fabrics, pseudostochastic diffusers, etc.) 180 6.9 Anechoic rooms 184 References 187 7 Characterisation of subjective effects 189 7.1Some general remarks on reflections and echoes 193 7.2 The perceptibility of reflections 196 7.3 Echoes and colouration 199 7.4 Early energy: definition, clarity index, speech transmission index 207 7.5 Reverberation and reverberance 213 vi Contents 7.6 Sound pressure level, strength factor 221 7.7 Spaciousness of sound fields 222 7.8 Assessment of concert hall acoustics 226 References 231 8 Measuring techniques in room acoustics 234 8.1General remarks on instrumentation 235 8.2 Measurement of the impulse response 237 8.3 Correlation measurement 244 8.4 Examination of the time structure of the impulse response 248 8.5 Measurement of reverberation 254 8.6 Sound absorption – tube methods 260 8.7 Sound absorption – reverberation chamber 265 8.8 Diffusion 268 References 276 9 Design considerations and design procedures 277 9.1Direct sound 278 9.2 Examination of the room shape with regard to reflections 280 9.3 Reverberation time 286 9.4 Prediction of noise level 293 9.5 Acoustical scale models 297 9.6 Computer simulation 300 9.7 Auralisation 306 References 308 10 Electroacoustic installations in rooms 310 10.1 Loudspeaker directivity 311 10.2 Design of electroacoustic systems for speech transmission 316 10.3 A few remarks on the selection of loudspeaker positions 319 10.4 Acoustical feedback and its suppression 323 10.5 Reverberation enhancement with external reverberators 330 10.6 Reverberation enhancement by controlled feedback 337 References 342 Index 343