THE MASTER HANDBOOK OF ACOUSTICS F. Alton Everest FOURTH EDITION McGraw-Hill New York San Francisco Washington, D.C. Auckland Bogotá Caracas Lisbon London Madrid Mexico City Milan Montreal New Delhi San Juan Singapore Sydney Tokyo Toronto
viii CONTENTS Using Decibels 33 Example: Sound-pressure level 34 Example: Loudspeaker SPL 34 Example: Microphone specifications 35 Example: Line amplifier 35 Example: General-purpose amplifier 35 Example: Concert hall 35 Example: Combining decibels 36 Ratios and Octaves 37 Measuring Sound-Pressure Level 39 Chapter 3 The Ear and the Perception of Sound 41 Sensitivity of the Ear 41 A Primer of Ear Anatomy 42 The pinna: Directional encoder of sound 43 Directional cues: An experiment 44 The ear canal 44 The middle ear 45 The inner ear 48 Stereocilia 49 Loudness vs. Frequency 50 Loudness Control 51 Area of Audibility 53 Loudness vs. Sound-Pressure Level 54 Loudness and Bandwidth 56 Loudness of Impulses 59 Audibility of Loudness Changes 61 Pitch vs. Frequency 61 An experiment 63 Timbre vs. Spectrum 63 Localization of Sound Sources 64 Binaural Localization 67 Aural harmonics: Experiment #1 68 Aural harmonics: Experiment #2 69 The missing fundamental 69 The Ear as an Analyzer 70 The Ear as a Measuring Instrument 70 An auditory analyzer: An experiment 71
CONTENTS ix Meters vs. the Ear 72 The Precedence Effect 73 Perception of Reflected Sound 75 Occupational and Recreational Deafness 76 Summary 79Chapter 4 Sound Waves in the Free Field 83 Free Sound Field: Definition 83 Sound Divergence 84 Examples: Free-field sound divergence 84 Inverse square in enclosed spaces 87 Hemispherical propagation 88Chapter 5 Speech, Music, and Noise 89 The Voice System 89 Artificial larynx 89 Sound spectrograph 90 Sound sources for speech 92 Vocal tract molding of speech 92 Formation of voiced sounds 94 Formation of unvoiced sounds 95 Putting it all together 95 Synthesized speech 96 Digital speech synthesis 97 Directionality of speech 98 Music 99 Wind instruments 101 Nonharmonic overtones 101 Dynamic range of speech and music 101 Power in Speech and Music 103 Frequency Range of Speech and Music 104 Future Dynamic-Range Requirements 104 Auditory Area 104 Noise 107 Noise—The good kind 108 Random noise 109 White and pink noise 111 Signal Distortion 112 Harmonic Distortion 114
x CONTENTS Chapter 6 Analog and Digital Signal Processing 119 Resonance 120 Filters 122 Active filters 123 Analog vs. digital filters 124 Digitization 125 Quantization 126 Digital filters 126 Application of Digital Signal Processing (DSP) 105 Application of DSP to Room Equalization 106 Chapter 7 Reverberation 129 Reverberation and Normal Modes 130 Growth of Sound in a Room 132 Decay of Sound in a Room 134 Idealized Growth and Decay of Sound 134 Reverberation Time 135 Measuring Reverberation Time 137 Impulse Sound Sources 137 Steady-State Sources 138 Equipment 138 Measurement Procedure 140 Analysis of decay traces 140 Mode Decay Variations 142 Writing speed 143 Frequency effect 144 Reverberation Time Variation with Position 145 Acoustically Coupled Spaces 146 Electroacoustically Coupled Spaces 146 Decay rate 147 Eliminating decay fluctuations 147 Influence of Reverberation on Speech 148 Influence of Reverberation on Music 149 Optimum Reverberation Time 150 Bass rise of reverberation time 152 Living room reverberation time 154
CONTENTS xi Artificial Reverberation: The Past 155 Artificial Reverberation: The Future 156 Arrival Time Gap 157 The Sabine Equation 159 Reverberation calculation: Example 1 160 Reverberation calculation: Example 2 162 Reverberant Field 162Chapter 8 Control of Interfering Noise 165 Noise Sources and Some Solutions 166 Airborne noise 167 Noise carried by structure 167 Noise transmitted by diaphragm action 168 Sound-insulating walls 168 Porous materials 169 Sound Transmission Classification (STC) 170 Comparison of Wall Structures 171 Double Windows 173 Sound-Insulating Doors 175 Noise and room resonances 176 Active noise control 177Chapter 9 Absorption of Sound 179 Dissipation of Sound Energy 179 Evaluation of Sound Absorption 181 Reverberation Chamber Method 182 Impedance Tube Method 182 Tone-Burst Method 185 Mounting of Absorbents 186 Mid/High Frequency Absorption by Porosity 187 Glass fiber: Building insulation 189 Glass fiber: Boards 190 Acoustical tile 190 Effect of Thickness of Absorbent 190 Effect of Airspace behind Absorbent 191 Effect of Density of Absorbent 192
xii CONTENTS Open-Cell Foams 192 Drapes as Sound Absorbers 193 Carpet as Sound Absorber 196 Effect of carpet type on absorbance 199 Effect of carpet underlay on absorbance 200 Carpet absorption coefficients 200 Sound Absorption by People 200 Absorption of Sound in Air 203 Low-Frequency Absorption by Resonance 203 Diaphragmatic Absorbers 205 Polycylindrical Absorbers 209 Poly Construction 212 Membrane Absorbers 213 Helmholtz Resonators 215 Perforated Panel Absorbers 218 Slat Absorbers 224 Placement of Materials 225 Reverberation Time of Helmholtz Resonators 225 Taming room modes 226 Increasing Reverberation Time 229 Modules 229 Chapter 10 Reflection of Sound 235 Reflections from Flat Surfaces 235 Doubling of Pressure at Reflection 237 Reflections from Convex Surfaces 237 Reflections from Concave Surfaces 237 Reflections from Parabolic Surfaces 238 Reflections inside a Cylinder 240 Standing Waves 240 Reflection of Sound from Impedance Irregularities 240 The Corner Reflector 243 Echo-Sounding 243 Perceptive Effects of Reflections 244
CONTENTS xiiiChapter 11 Diffraction of Sound 245 Rectilinear Propagation 245 Diffraction and Wavelength 246 Diffraction of Sound by Large and Small Apertures 247 Diffraction of Sound by Obstacles 248 Diffraction of Sound by a Slit 249 Diffraction by the Zone Plate 250 Diffraction around the Human Head 251 Diffraction by Loudspeaker Cabinet Edges 253 Diffraction by Various Objects 254Chapter 12 Refraction of Sound 257 Refraction of Sound 258 Refraction of sound in solids 258 Refraction of sound in the atmosphere 260 Refraction of sound in the ocean 263 Refraction of sound in enclosed spaces 265Chapter 13 Diffusion of Sound 267 The Perfectly Diffuse Sound Field 267 Evaluating Diffusion in a Room 268 Steady-state measurements 268 Decay Beats 269 Exponential Decay 270 Spatial Uniformity of Reverberation Time 271 Decay Shapes 275 Microphone Directivity 275 Room Shape 275 Splaying Room Surfaces 281 Nonrectangular rooms 281 Geometrical Irregularities 282 Absorbent in Patches 282 Concave Surfaces 286 Convex Surfaces: The Poly 286 Plane Surfaces 287
xiv CONTENTS Chapter 14 The Schroeder Diffusor 289 Schroeder’s First Acoustic Diffusor 290 Maximum-Length Sequences 292 Reflection Phase-Grating Diffusors 292 Quadratic-Residue Diffusors 293 Primitive-Root Diffusors 296 Quadratic-Residue Applications 298 Performance of Diffraction-Grating Diffusors 298 Expansion of the QRD line 304 Solving flutter problems 304 Application of fractals 306 Diffusion in three dimensions 308 Acoustic concrete blocks 309 Measuring diffusion efficiency 311 Comparison of Gratings with Conventional Approaches 312 Chapter 15 Modal Resonances in Enclosed Spaces 317 Resonance in a Pipe 318 Bathroom Acoustics 319 Reflections Indoors 320 Two-Wall Resonance 322 Waves vs. Rays 322 Frequency Regions 323 Dividing the Audio Spectrum 325 Wave Acoustics 326 Mode calculations—An example 328 Experimental Verification 331 Mode Identification 331 Mode Decay 333 Mode Bandwidth 334 Mode Pressure Plots 339 Modal Density 341 Mode Spacing and Coloration 342 Experiments with Colorations 344 Simplified Axial Mode Analysis 346
CONTENTS xv The Bonello Criterion 348 Controlling Problem Modes 348 Mode Summary 350Chapter 16 Reflections in Enclosed Spaces 353 Law of the First Wavefront 353 Mean Free Path 354 The effect of single reflections 355 Perception of sound reflections 355 Perception of spaciousness 357 Image changes 357 Discrete echoes 357 Effect of angle of incidence on audibility of reflection 357 Effect of signal type of audibility of reflection 358 Effect of spectrum on audibility of reflection 358 Using reflection data 359 Large Spaces 359 Echoes 359 Spaciousness 360Chapter 17 Comb-Filter Effects 363 What Is a Comb Filter? 363 Superposition of Sound 364 Tonal Signals and Comb Filters 365 Combing of music and speech signals 367 Combing of direct and reflected sound 368 Comb Filters and Critical Bands 371 Comb Filters in Stereo Listening 374 Coloration and Spaciousness 374 Combing in Stereo Microphone Pickups 375 Audibility of Comb-Filter Effects 375 Comb filters in practice 376 Estimating comb-filter response 380Chapter 18 Quiet Air for the Studio 385 Selection of Noise Criterion 386 Fan Noise 388
xvi CONTENTS ASHRAE 389 Machinery Noise 390 Air Velocity 390 Effect of Terminal Fittings 391 “Natural” Attenuation 391 Duct Lining 392 Plenum Silencers 393 Packaged Attenuators 394 Reactive Silencers 394 Resonator Silencer 395 Duct Location 395 Some Practical Suggestions 395 Chapter 19 Acoustics of the Listening Room 399 The Acoustical Link 399 Peculiarities of Small-Room Acoustics 400 Room size 401 Room proportions 401 Reverberation time 403 The Listening Room: Low Frequencies 403 Control of modal resonances 406 Bass traps for the listening room 406 Modal colorations 408 The Listening Room: The Mid-High Frequencies 409 Identification and treatment of reflection points 411 Lateral reflections: Control of spaciousness 413 Chapter 20 Acoustics of the Small Recording Studio 415 Acoustical Characteristics of a Studio 416 Reverberation 418 Studio Design 419 Studio Volume 419 Room Proportions 421 Reverberation Time 422
CONTENTS xvii Diffusion 423 Noise 424 Studio Design Procedure 424 Some Studio Features 424 Elements Common to all Studios 427Chapter 21 Acoustics of the Control Room 429 The Initial Time-Delay Gap 429 The Live End 431 Specular Reflections vs. Diffusion 432 Low-Frequency Resonances in the Control Room 434 Initial Time-Delay Gaps in Practice 436 Managing Reflections 438 The Reflection-Free-Zone Control Room 439 Control-Room Frequency Range 441 Outer Shell of the Control Room 442 Inner Shell of the Control Room 442 Representative Control Rooms 442 Some European Designs 444 Consultants 450Chapter 22 Acoustics for Multitrack Recording 453 Flexibility 545 Advantages of Multitrack 455 Disadvantages of Multitrack 456 Achieving Track Separation 457 Studio Acoustics 458 Distance between artists 458 Microphone management 458 Barriers for separation 459 Electronic separation 459 Electronic instruments and separation 459 The Future of Multichannel 460 Automation 460
xviii CONTENTS Chapter 23 Audio/Video Tech Room and Voice-Over Recording 461 Selection of Space: External Factors 462 Selection of Space: Internal Factors 462 Work Space Treatment 462 Audio/Video Work Place Example 463 Appraisal of Room Resonances 463 Control of room resonances 464 Treatment of work place 465 Calculations 465 The Voice-Over Booth 468 Dead-End Live-End Voice Studio 468 Voice-Over Booths 468 The Quick Sound Field™ 469 Chapter 24 Adjustable Acoustics 473 Draperies 473 Adjustable Panels: Absorption 474 Adjustable Panels: The Abffusor™ 476 Hinged Panels 478 Louvered Panels 479 Variable Resonant Devices 480 Rotating Elements 483 Portable Units: The Tube Trap™ 484 Portable Units: The Korner Killer™ 485 Chapter 25 Acoustical Distortion 489 Acoustic Distortion and the Perception of Sound 489 Sources of Acoustic Distortion 490 Coupling of room modes 490 Speaker-boundary interference response 491 Comb filtering 493 Poor diffusion 498 Conclusion 500 Chapter 26 Room Acoustics Measurement Software 501 The Evolution of Measurement Technologies 502
CONTENTS xix Building a Better Analyzer 504 Time-delay spectrometry (TDS) measurement techniques 504 Maximum-length sequence (MLS) techniques 508 AcoustiSoft’s ETF Program 509 Frequency-response measurements 513 Resonance measurements 517 Fractional-octave measurements 520 Energy-time curve measurements 521 Reverberation time 524 Conclusion 526Chapter 27 Room Optimizer 529 Introduction 529 Modal Response 530 Speaker-Boundary Interference Response 531 Optimization 533 Theory 536 Prediction of room response 536 Optimizing procedure 541 Cost parameter 543 Optimization Procedure 545 Results 549 Stereo pair 549 Stereo pair with two woofers per loudspeaker 550 THX home theater 551 Multichannel music 554 Subwoofer 556 Conclusion 558Chapter 28 Desktop Auralization 565 Introduction 565 The Auralization Process 569 Summary 581 Appendix 585 Glossary 589 Index 599
2 CHAPTER ONE The same situation exists between intensity and loudness. The rela- tionship between the two is not linear. This is considered later in more detail because it is of great importance in high fidelity work. Similarly, the relationship between waveform (or spectrum) and perceived quality (or timbre) is complicated by the functioning of the hearing mechanism. As a complex waveform can be described in terms of a fundamental and a train of harmonics (or partials) of various amplitudes and phases (more on this later), the frequency-pitch inter- action is involved as well as other factors. The Simple Sinusoid The sine wave is a basic waveform closely related to simple harmonic motion. The 5 weight (mass) on the spring shown in Fig. 1-1 is a vibrating system. If the weight is pulled down to the 5 mark and released, 0 W the spring pulls the weight back toward 0. The weight will not, however, stop at zero; its 5 inertia will carry it beyond 0 almost to 5. The weight will continue to vibrate, or oscil- FIGURE 1-1 late, at an amplitude that will slowlyA weight on a spring vibrates at its natural frequency decrease due to frictional losses in the spring,because of the elasticity of the spring and the iner- the air, etc.tia of the weight. The weight in Fig. 1-1 moves in what is called simple harmonic motion. The pis- ton in an automobile engine is connected to the crankshaft by a con- necting rod. The rotation of the crankshaft and the up-and-down motion of the pistons beautifully illustrate the relationship between rotary motion and linear simple harmonic motion. The piston position plotted against time produces a sine wave. It is a very basic type of mechanical motion, and it yields an equally basic waveshape in sound and electronics. If a ballpoint pen is fastened to the pointer of Fig. 1-2, and a strip of paper is moved past it at a uniform speed, the resulting trace is a sine wave. In the arrangement of Fig. 1-1, vibration or oscillation is possible because of the elasticity of the spring and the inertia of the weight.
FUNDAMENTALS OF SOUND 3 Paper motion W Time FIGURE 1-2A ballpoint pen fastened to the vibrating weight traces a sine wave on a paper stripmoving at uniform speed. This shows the basic relationship between simple harmonicmotion and the sine wave.Elasticity and inertia are two things all media must possess to be capa-ble of conducting sound.Sine-Wave LanguageThe sine wave is a specific kind of alternating signal and is describedby its own set of specific terms. Viewed on an oscilloscope, the easiestvalue to read is the peak-to-peak value (of voltage, current, soundpressure, or whatever the sine wave represents), the meaning of whichis obvious in Fig. 1-3. If the wave is symmetrical, the peak-to-peakvalue is twice the peak value. The common ac voltmeter is, in reality, a dc instrument fitted witha rectifier that changes the alternating sine current to pulsating unidi-rectional current. The dc meter then responds to the average value asindicated in Fig. 1-3. Such meters are, however, almost universely cal-ibrated in terms of rms (described in the next paragraph). For pure sinewaves, this is quite an acceptable fiction, but for nonsinusoidal wave-shapes the reading will be in error. An alternating current of one ampere rms (or effective) is exactlyequivalent in heating power to 1 ampere of direct current as it flowsthrough a resistance of known value. After all, alternating current canheat up a resistor or do work no matter which direction it flows, it isjust a matter of evaluating it. In the right-hand positive loop of Fig. 1-3the ordinates (height of lines to the curve) are read off for each marked
4 CHAPTER ONE Amplitude relationships for sinusoids Amplitude Peak RMS Average Peak to Time peak Peak RMS (0.707) (Peak) √2 (Average) (1.1) (Average) 2√2 Peak (√2 ) (RMS) ( 1.414) (RMS) ( 2 ) (Average) (1.57) (Average) FIGURE 1-3 Amplitude relationships for sinusoids, which apply to sine waves of electrical voltage or current, as well as to acoustical parameters such as sound pressure. Another term which is widely used in the audio field is crest factor, or peak divided by rms. increment of time. Then (a) each of these ordinate values is squared, (b) the squared values are added together, (c) the average is found, and (d) the square root is taken of the average (or mean). Taking the square root of this average gives the root-mean-square or rms value of the positive loop of Fig. 1-3. The same can be done for the negative loop (squaring a negative ordinate gives a positive value), but simply doubling the pos- itive loop of a symmetrical wave is easier. In this way the root-mean- square or “heating power” value of any alternating or periodic waves can be determined whether the wave is for voltage, current, or sound pressure. Such computations will help you understand the meaning of rms, but fortunately reading meters is far easier. Figure 1-3 is a useful summary of relationships pertaining only to the sine wave.
FUNDAMENTALS OF SOUND 5Propagation of SoundIf an air particle is displaced from its original position, elastic forces ofthe air tend to restore it to its original position. Because of the inertiaof the particle, it overshoots the resting position, bringing into playelastic forces in the opposite direction, and so on. Sound is readily conducted in gases, liquids, and solids such as air,water, steel, concrete, etc., which are all elastic media. As a child, per-haps you heard two sounds of a rock striking a railroad rail in the dis-tance, one sound coming through the air and one through the rail. Thesound through the rail arrives first because the speed of sound in thedense steel is greater than that of tenuous air. Sound has been detectedafter it has traveled thousands of miles through the ocean. Without a medium, sound cannot be propagated. In the laboratory,an electric buzzer is suspended in a heavy glass bell jar. As the button ispushed, the sound of the buzzer is readily heard through the glass. Asthe air is pumped out of the bell jar, the sound becomes fainter andfainter until it is no longer audible. The sound-conducting medium, air,has been removed between the source and the ear. Because air is such acommon agent for the conduction of sound, it is easy to forget that othergases as well as solids and liquids are also conductors of sound. Outerspace is an almost perfect vacuum; no sound can be conducted exceptin the tiny island of air (oxygen) within a spaceship or a spacesuit.The Dance of the ParticlesWaves created by the wind travel across a field of grain, yet the indi-vidual stalks remain firmly rooted as the wave travels on. In a similarmanner, particles of air propagating a sound wave do not move far fromtheir undisplaced positions as shown in Fig. 1-4. The disturbance trav-els on, but the propagating particles do their little dance close to home. There are three distinct forms of particle motion. If a stone isdropped on a calm water surface, concentric waves travel out from thepoint of impact, and the water particles trace circular orbits (for deepwater, at least) as in Fig. 1-5(A). Another type of wave motion is illus-trated by a violin string, Fig. 1-5(B). The tiny elements of the stringmove transversely, or at right angles to the direction of travel of thewaves along the string. For sound traveling in a gaseous medium suchas air, the particles move in the direction the sound is traveling. Theseare called longitudinal waves, Fig. 1-5C.
6 CHAPTER ONE Vibration of air particle Maximum Equilibrium Maximum displacement position displacement Displacement FIGURE 1-4 Time An air particle is made to vibrate about its equilibrium position by the energy of a pass- ing sound wave because of the interaction of the elastic forces of the air and the iner- tia of the air particle. Direction of Particle wave travel motion Water A surface Violin B string Sound C in air FIGURE 1-5 Particles involved in the propagation of sound waves can dance in circular, transverse, or longitudinal motions.
FUNDAMENTALS OF SOUND 7How a Sound Wave Is PropagatedHow are air particles jiggling back and forth able to carry beautifulmusic from the loudspeaker to our ears at the speed of a rifle bullet?The little dots of Fig. 1-6 represent air molecules. There are more thana million molecules in a cubic inch of air; hence this sketch is greatlyexaggerated. The molecules crowded together represent areas of com-pression in which the air pressure is slightly greater than the prevail-ing atmospheric pressure. The sparse areas represent rarefactions inwhich the pressure is slightly less than atmospheric. The small arrowsindicate that, on the average, the molecules are moving to the right ofthe compression crests and to the left in the rarefaction troughsbetween the crests. Any given molecule will move a certain distance tothe right and then the same distance to the left of its undisplaced posi-tion as the sound wave progresses uniformly to the right. C R C R C R C A B C Compression (region of high pressure) R Rarefaction (region of low pressure) Direction of sound wave FIGURE 1-6In (A) the wave causes the air particles to be pressed together in some regions andspread out in others. An instant later (B) the wave has moved slightly to the right.
8 CHAPTER ONE Why does the sound wave move to the right? The answer is revealed by a closer look at the arrows of Fig. 1-6. The molecules tend to bunch up where two arrows are pointing toward each other, and this occurs a bit to the right of each compression. When the arrows point away from each other the density of molecules will decrease. Thus, the movement of the higher pressure crest and the lower pressure trough accounts for the small progression of the wave to the right. As mentioned previously, the pressure at the crests is higher than the prevailing atmospheric barometric pressure and the troughs lower than the atmospheric pressure, as shown in the sine wave of Fig. 1-7. These fluctuations of pressure are very small indeed. The faintest sound the ear can hear (20 Pascal) is some 5,000 million times smaller than atmospheric pressure. Normal speech and music signals are represented by correspondingly small ripples superim- posed on the atmospheric pressure. A Compression Pressure Atmospheric Rarefaction pressure B Time FIGURE 1-7 (A) An instantaneous view of the compressed and rarefied regions of a sound wave in air. (B) The compressed regions are very slightly above and the rarefied regions very slightly below atmospheric pressure. Pressure variations of sound waves are thus super- imposed on prevailing barometric pressure.
FUNDAMENTALS OF SOUND 9Sound in Free SpaceThe intensity of sound decreases as the distance to the source isincreased. In free space, far from the influence of surrounding objects,sound from a point source is propagated uniformly in all directions.The intensity of sound decreases as shown in Fig. 1-8. The same soundpower flows out through A1, A2, A3, and A4, but the areas increase asthe square of the radius, r. This means that the sound power per unitarea (intensity) decreases as the square of the radius. Doubling the dis-tance reduces the intensity to one-fourth the initial value, tripling thedistance yields 1 9, and increasing the distance four times yields 1 16 of A4 A3 A2 A1 r 2r 3r 4r FIGURE 1-8In the solid angle shown, the same sound energy is distributed over spherical surfacesof increasing area as r is increased. The intensity of the sound is inversely proportionalto the square of the distance from the point source.
10 CHAPTER ONE the initial intensity. The inverse square law states that the intensity of sound in a free field is inversely proportional to the square of the dis- tance from the source. This law provides the basis of estimating the sound level in many practical circumstances and is discussed in a later chapter. Wavelength and Frequency A simple sine wave is illustrated in Fig. 1-9. The wavelength is the dis- tance a wave travels in the time it takes to complete one cycle. A wave- length can be measured between successive peaks or between any two corresponding points on the cycle. This holds for periodic waves other than the sine wave as well. The frequency is the number of cycles per second (or hertz). Frequency and wavelength are related as follows: Speed of sound (ft/sec) Wavelength (ft) (1-1) Frequency (hertz) which can be written as: Speed of sound Frequency (1-2) Wavelength The speed of sound in air is about 1,130 feet per second (770 miles per hour) at normal temperature and pressure. For sound traveling in air, Equation 1-1 becomes: Wavelength Peak Amplitude Time FIGURE 1-9 Wavelength is the distance a wave travels in the time it takes to complete one cycle. It can also be expressed as the distance from one point on a periodic wave to the corre- sponding point on the next cycle of the wave.
FUNDAMENTALS OF SOUND 11 1,130 Wavelength (1-3) Frequency This relationship is used frequently in following sections. Figure1-10 gives two graphical approaches for an easy solution to Equation 1-3. Wavelength Inches Feet 12 8 6 4 3 2 1 50 40 20 10 5 2 1 A 20 50 100 200 500 1 k 2 kHz 5 kHz 10 kHz 20 kHz Frequency - Hz 100 50 30 10 5 Wavelength, ft. 3 B 1 0.5 0.3 0.1 10 30 50 100 300 500 1 2 3 45 10 Hz kHz FIGURE 1-10(A) Convenient scales for rough determination of wavelength of sound in air fromknown frequency, or vice versa. (B) A chart for easy determination of the wavelength inair of sound waves of different frequencies. (Both based on speed of sound of 1,139 ftper second.)
12 CHAPTER ONE Complex Waves Speech and music waveshapes depart radically from the simple sine form. A very interesting fact, however, is that no matter how complex the wave, as long as it is periodic, it can be reduced to sine components. The obverse of this is that, theoretically, any complex periodic wave can be synthesized from sine waves of different frequencies, different ampli- tudes, and different time relationships (phase). A friend of Napoleon, named Joseph Fourier, was the first to develop this surprising idea. This idea can be viewed as either a simplification or complication of the situ- ation. Certainly it is a great simplification in regard to concept, but some- times complex in its application to specific speech or musical sounds. As we are interested primarily in the basic concept, let us see how even a very complex wave can be reduced to simple sinusoidal components. Harmonics A simple sine wave of a given amplitude and frequency, f1, is shown in Fig. 1-11A. Figure 1-11B shows another sine wave half the amplitude and twice the frequency (f2). Combining A and B at each point in time the waveshape of Fig. 1-11C is obtained. In Fig. 1-11D, another sine wave half the amplitude of A and three times its frequency (f3) is shown. Adding this to the f1 f2 wave of C, Fig. 1-11E is obtained. The simple sine wave of Fig. 1-11A has been progressively distorted as other sine waves have been added to it. Whether these are acoustic waves or electronic signals, the process can be reversed. The distorted wave of Fig. 1-11E can be dis- assembled, as it were, to the simple f1, f2, and f3 sine components by either acoustical or electronic filters. For example, passing the wave of Fig. 1-11E through a filter permitting only f1 and rejecting f2 and f3, the origi- nal f1 sine wave of Fig. 1-11A emerges in pristine condition. Applying names, the sine wave with the lowest frequency (f1) of Fig. 1-11A is called the fundamental, the one with twice the frequency (f2) of Fig. 1-11B is called the second harmonic, and the one three times the frequency (f3) of Fig. 1-11D is the third harmonic. The fourth harmonic, the fifth harmonic, etc., are four and five times the fre- quency of the fundamental, and so on. Phase In Fig. 1-11, all three components, f1, f2, and f3, start from zero together. This is called an in-phase condition. In some cases, the time relation-
FUNDAMENTALS OF SOUND 13 f1 Amplitude 0 A Time f2 0 B f1 f2 0 C f3 0 D f1 f2 f3 0 E FIGURE 1-11A study in the combination of sine waves. (A) The fundamental of frequency f1. (B) Asecond harmonic of frequency f2 = 2 f1 and half the amplitude of f1. (C) The sum of f1and f2 obtained by adding ordinates point by point. (D) A third harmonic of frequencyf3 = 3 f1 and half the amplitude of f1. (E) The waveshape resulting from the additionof f1, f2, and f3. All three components are “in phase,” that is, they all start from zeroat the same instant.
14 CHAPTER ONE ships between harmonics or between harmonics and the fundamental are quite different from this. Remember how one revolution of the crankshaft of the automobile engine (360°) was equated with one cycle of simple harmonic motion of the piston? The up-and-down travel of the piston spread out in time traces a sine wave such as that in Fig. 1-12. One complete sine-wave cycle represents 360° of rotation. If another sine wave of identical frequency is delayed 90°, its time relationship to the first one is a quarter wave late (time increasing to the right). A half- wave delay would be 180°, etc. For the 360° delay, the wave at the bot- 90¡ 180¡ 270¡ 360¡ Time FIGURE 1-12 Illustration of the phase relationships between waves with the same amplitude and fre- quency. A rotation of 360 degrees is analogous to one complete sine cycle.
FUNDAMENTALS OF SOUND 15tom of Fig. 1-12 falls in step with the top one, reaching positive peaks andnegative peaks simultaneously and producing the in-phase condition. In Fig. 1-11, all three components of the complex wave of Fig. 1-11Eare in phase. That is, the f1 fundamental, the f2 second harmonic, andthe f3 third harmonic all start at zero at the same time. What happens ifthe harmonics are out of phase with the fundamental? Figure 1-13 illus-trates this case. The second harmonic f2 is now advanced 90°, and thethird harmonic f3 is retarded 90°. By combining f1, f2, and f3 for eachinstant of time, with due regard to positive and negative signs, the con-torted wave of Fig. 1-13E is obtained. The only difference between Figs. 1-11E and 1-13E is that a phaseshift has been introduced between harmonics f2 and f3, and the funda-mental f1. That is all that is needed to produce drastic changes in theresulting waveshape. Curiously, even though the shape of the wave isdramatically changed by shifting the time relationships of the compo-nents, the ear is relatively insensitive to such changes. In other words,waves E of Figs. 1-11 and 1-13 would sound very much alike to us. A common error is confusing polarity with phase. Phase is the timerelationship between two signals while polarity is the / or the /relationship of a given pair of signal leads.PartialsA musician is inclined to use the term partial instead of harmonic, butit is best that a distinction be made between the two terms because thepartials of many musical instruments are not harmonically related tothe fundamental. That is, partials might not be exact multiples of thefundamental frequency, yet richness of tone can still be imparted bysuch deviations from the true harmonic relationship. For example, thepartials of bells, chimes, and piano tones are often in a nonharmonicrelationship to the fundamental.OctavesAudio and electronics engineers and acousticians frequently use theintegral multiple concept of harmonics, closely allied as it is to thephysical aspect of sound. The musician often refers to the octave, alogarithmic concept that is firmly embedded in musical scales and ter-minology because of its relationship to the ear’s characteristics. Audiopeople are also involved with the human ear, hence their common useof logarithmic scales for frequency, logarithmic measuring units, and
16 CHAPTER ONE f1 Amplitude 0 A Time f2 0 B f1 f2 0 C f3 0 D f1 f2 f3 0 E FIGURE 1-13 A study of the combination of sine waves that are not in phase. (A) The fundamen- tal of frequency f1. (B) The second harmonic f2 with twice the frequency and half the amplitude of f1 advanced 90 degrees with respect to f1. (C) The combination of f1 and f2 obtained by adding ordinates point by point. (D) The third harmonic f3 with phase 90 degrees behind f1, and with half the amplitude of f1. (E) The sum of f1, f2, and f3. Compare this waveshape with that of Fig. 1-11(E). The difference in waveshapes is due entirely to the shifting of the phase of the harmonics with respect to the fundamental.
FUNDAMENTALS OF SOUND 17 Harmonics Fundamental 2nd 3rd 4th 5th 6th 7th 8th 100 Hz 200 300 400 500 600 700 800. . . Octaves Fundamental 100 Hz 200 400 800 Octave Octave Octave FIGURE 1-14Comparison of harmonics and octaves. Harmonics are linearly related; octaves are loga-rithmically related.various devices based on octaves, which are more fully discussedlater. Harmonics and octaves are compared in Fig. 1-14.The Concept of SpectrumChapter 3 relates the commonly accepted scope of the audible spec-trum, 20 Hz to 20 kHz, to specific characteristics of the human ear.Here, in the context of sine waves, harmonics, etc., we need to estab-lish the spectrum concept. The visible spectrum of light has its coun-terpart in sound in the audible spectrum, the range of frequencies thatfall within the perceptual limits of the human ear. We cannot see far-ultraviolet light because the frequency of its electromagnetic energy istoo high for the eye to perceive. Nor can we see the far-infrared lightbecause its frequency is too low. There are likewise sounds of too low(infrasound) and too high frequency (ultrasound) for the ear to hear. Figure 1-15 shows several waveforms that typify the infinite num-ber of different waveforms commonly encountered in audio. Thesewaveforms have been photographed directly from the screen of a cath-ode-ray oscilloscope. To the right of each photograph is the spectrumof that particular signal. The spectrum tells how the energy of the sig-nal is distributed in frequency. In all but the bottom signal of Fig. 1-15,the audible range of the spectrum was searched with a wave analyzerhaving a very sharp filter with a passband only 5 Hz wide. In this way,
18 CHAPTER ONE 10 6 4 2 0 f1 f2 f3 f4 f5 f6 f7 10 6 4 Amplitude-arbitrary units 2 0 f1 f2 f3 f4 f5 f6 f7 10 6 4 2 0 f1 f2 f3 f4 f5 f6 f7 Frequency FIGURE 1-15The spectral energy of a pure sinusoid is contained entirely at a single frequency. The triangular and squarewaves each have a prominent fundamental and numerous harmonics at integral multiples of the fundamentalfrequency. Random noise (white noise) has energy distributed uniformly throughout the spectrum up to somepoint at which energy begins to fall off due to generator limitations. Random noise may be considered a mix-ture of sine waves with continuously shifting frequencies, amplitudes, and phases.
FUNDAMENTALS OF SOUND 19concentrations of energy were located and measured with an elec-tronic voltmeter. For an ideal sine wave, all the energy is concentrated at one fre-quency. The sine wave produced by this particular signal generator is notreally a pure sine wave. No oscillator is perfect and all have some har-monic content, but in scanning the spectrum of this sine wave, the har-monics measured were too low to show on the graph scale of Fig. 1-15. The triangular wave of this signal generator has a major fundamentalcomponent of 10 units magnitude. The wave analyzer detected a signif-icant second harmonic component at f2, twice the frequency of the fun-damental with a magnitude of 0.21 units. The third harmonic showedan amplitude of 1.13 units, the fourth of 0.13 units, etc. The seventh har-monic still had an amplitude of 0.19 units and the fourteenth harmonic(about 15 kHz in this case) an amplitude of only 0.03 units, but still eas-ily detectable. So we see that this triangular wave has both odd and evencomponents of modest amplitude down through the audible spectrum.If you know the amplitude and phases of each of these, the original tri-angular wave shape can be synthesized by combining them. A comparable analysis reveals the spectrum of the square waveshown in Fig. 1-15. It has harmonics of far greater amplitude than thetriangular wave with a distinct tendency toward more prominent oddthan even harmonics. The third harmonic shows an amplitude 34 per-cent of the fundamental! The fifteenth harmonic of the square wave isstill 0.52 units! If the synthesis of a square wave stops with the fif-teenth harmonic, the wave of Fig. 1-16C results. A glance at the spectra of sine, triangular, and square waves revealsenergy concentrated at harmonic frequencies, but nothing between.These are all so-called periodic waves, which repeat themselves cycleafter cycle. The fourth example in Fig. 1-15 is a random noise. Thespectrum of this signal cannot be measured satisfactorily by a waveanalyzer with a 5-Hz passband because the fluctuations are so greatthat it is impossible to get a decent reading on the electronic voltmeter.Analyzed by a wider passband of fixed bandwidth and with the helpof various integrating devices to get a steady indication, the spectralshape shown is obtained. This spectrum tells us that the energy of therandom-noise signal is equally distributed throughout the spectrumuntil the drooping at high frequencies indicates that the upper fre-quency limit of the random noise generator has been reached.
20 CHAPTER ONE There is little visual similarity between the sine and the random-noise signals as revealed by the cathode-ray oscilloscope, A yet there is a hidden relationship. Even ran- dom noise can be considered as being made up of sine-wave components constantly shifting in frequency, amplitude, and phase. If you pass random noise through a B narrow filter and observe the filter output on a cathode-ray oscilloscope, you will see a restless, sinelike wave that constantly shifts in amplitude. Theoretically, an infi- nitely narrow filter would sift out a pure, C but nervous, sine wave. (See chapter 5.) FIGURE 1-16 Electrical, Mechanical, andIn synthesizing the square wave of (A), including Acoustical Analogsonly the fundamental and two harmonics yields (B).An acoustical system such as a loud-Including fifteen components yields (C). It would speaker can be represented in terms of antake many more than fifteen harmonics to smooththe ripples and produce the square corners of (A).equivalent electrical or mechanical sys- tem. The physicist freely uses these equiv- alents to set up his mathematical approach for analyzing a given system. Although such approaches are far out- side the scope of this book, it is useful to develop some appreciation for these methods. For example, the effect of a cabinet on the func- tioning of a loudspeaker is clarified by thinking of the air in the enclosed space as acting like a capacitor in an electrical circuit, absorbing and giving up the energy imparted by the cone movement. Figure 1-17 shows the graphical representation of the three basic elements in electrical, mechanical, and acoustical systems. Inductance in an electrical circuit is equivalent to mass in a mechanical system and inertance in an acoustical system. Capacitance in an electrical cir- cuit is analogous to compliance in a mechanical system and capaci- tance in an acoustical system. Resistance is resistance in all three systems, whether it be the frictional losses offered to air-particle move- ment in glass fiber, frictional losses in a wheel bearing, or resistance to the flow of current in an electrical circuit.
FUNDAMENTALS OF SOUND 21 Electrical Mechanical Acoustical Inductance Mass Inertance Resistance Resistance Resistance Capacitance Compliance Capacitance FIGURE 1-17The three basic elements of electrical systems and their analogs in mechanical andacoustical systems.
24 CHAPTER TWO When the sound pressure is increased until it sounds twice as loud, the level dial reads 10 units. This completes observation A. For obser- vation B, the source pressure is increased to 10,000 units. To double the loudness, you find that the sound pressure must be increased from 10,000 to 100,000 units. The results of this experiment can now be summarized as follows: Ratio of Two Observations Two Pressures Pressures A 10 – 1 10: 1 B 100,000 – 10,000 10: 1 Observations A and B accomplish the same doubling of per- ceived loudness. In observation A, this was accomplished by an increase in sound pressure of only 9 units, where in observation B it took 90,000 units. Ratios of pressures seem to describe loudness changes better than differences in pressure. Ernst Weber (1834), Gustaf Fechner (1860), Hermann von Helmholtz (1873), and other early researchers pointed out the importance of ratios, which we know apply equally well to sensations of vision, hearing, vibration, or even electric shock. Many years ago, a friend working in a university research labora- tory demonstrated his experiment on the hearing of cats, which in many ways is similar to that of humans. A tone of 250 Hz, radiated from a nearby loudspeaker, was picked up by the ears of an anes- thetized cat, a portion of whose brain was temporarily exposed. A delicate probe picked up the 250-Hz signal at a highly localized spot on the auditory cortex, displaying it on a cathode-ray oscilloscope. When the tone was shifted to 500 Hz, the signal was picked up at another spot on the cortex. Tones of 1,000 and 2,000 Hz were detected at other specific spots. The fascinating point here is that changing the tone an octave resulted in the signal appearing on the auditory cortex at discrete, equally spaced points. Frequencies in the ratio of 2 to 1 (an octave) seem to have a linear positional relationship in the cat’s brain. This indicates a logarithmic response to frequency. Ratios of stimuli come closer to matching up with human perception than do differences of stimuli. This matching is not perfect, but close enough to make a strong case for the use of levels in decibels.
SOUND LEVELS AND THE DECIBEL 25 Ratios of powers or ratios of intensities, or ratios of sound pressure,voltage, current, or anything else are dimensionless. For instance, theratio of 1 watt to 100 watts is 1 watt/100 watts, and the watt unitupstairs and the watt unit downstairs cancel, leaving 1⁄100 = 0.01, a purenumber without dimension. This is important because logarithms canbe taken only of nondimensional numbers.Handling NumbersTable 2-1 illustrates three different ways numbers can be expressed.The decimal and arithmetic forms are familiar in everyday activity.The exponential form, while not as commonly used, has the charm ofsimplifying things once the fear of the unknown or little understood isconquered. In writing one hundred thousand, there is a choicebetween 100,000 watts and 105 watts, but how about a millionth of amillionth of a watt? All those zeros behind the decimal point make itimpractical even to reproduce here, but 10–12 is easy. And the prefixthat means 10–12 is pico; so the power is 1 picowatt (shown later inTable 2-4). Engineering-type calculators take care of the exponentialform in what is called scientific notation, by which very large or verysmall numbers can be entered. Table 2-1. Ways of expressing numbers. Decimal Arithmetic Exponential form form form 100,000 10 × 10 × 10 × 10 × 10 105 10,000 10 × 10 × 10 × 10 104 1,000 10 × 10 × 10 103 100 10 × 10 102 10 10 × 1 101 1 10/10 100 0.1 1/10 10–1 0.01 1/(10 × 10) 10–2 0.001 1/(10 × 10 × 10) 10–3 0.0001 1/(10 × 10 × 10 × 10) 10–4
26 CHAPTER TWO Table 2-1. Ways of expressing numbers (Continued). Decimal Arithmetic Exponential form form form 100,000 (100)(1,000) 102 + 103 = 102+3 = 105 100 10,000/100 104/102 = 104–2 = 102 10 100,000/10,000 105/104 = 105–4 = 10–1 = 10 10 100 = 2 100 1001/2 = 1000.5 4.6416 3 100 1001/2 = 1000.333 31.6228 4 1003 1003/4 = 1000.75 Logarithms Representing 100 as 102 simply means that 10 × 10 = 100 and that 103 means 10 × 10 × 10 = 1,000. But how about 267? That is where loga- rithms come in. It is agreed that 100 equals 102. By definition you can say that the logarithm of 100 to the base 10 = 2, commonly written log10 100 = 2, or simply log 100 = 2, because common logarithms are to the base 10. Now that number 267 needn’t scare us; it is simply expressed as 10 to some other power between 2 and 3. The old fashioned way was to go to a book of log tables, but with a simple hand-held calcula- tor punch in 267, push the “log” button, and 2.4265 appears. Thus, 267 = 102.4265, and log 267 = 2.4265. Logs are so handy because, as Table 2-1 demonstrates, they reduce multiplication to addition, and division to subtraction. This is exactly how the now-extinct slide rule worked, by positioning engraved logarithmic scales. Logs should be the friend of every audio worker because they are the solid foundation of our levels in decibels. A level is a logarithm of a ratio. A level in decibels is ten times the logarithm to the base 10 of the ratio of two power like quantities. Decibels A power level of a power W1 can be expressed in terms of a reference power W2 as follows:
SOUND LEVELS AND THE DECIBEL 27 W1 L1 = log10 bels (2-1) W2Because the decibel, from its very name, is 1⁄10 bel (from Alexander Gra-ham Bell), the level in decibels of a power ratio becomes: W1 L1 = 10 log10 decibels (2-2) W2 Equation 2-2 applies equally to acoustic power, electric power,or any other kind of power. A question often arises when levelsother than power need to be expressed in decibels. For example,acoustic intensity is acoustic power per unit area in a specifieddirection, hence Equation 2-2 is appropriate. Acoustic power isproportional to the square of the acoustic pressure, p, hence thepower level is: p21 Lp = 10 log p22 p1 = 20 log p in decibels (2-3) 2 The tabulation of Table 2-2 will help you decide whether the Equa-tion 2-2 or Equation 2-3 form applies. Table 2-2. Use of 10 log and 20 log. Eq(2-2) Eq(2-3) a b Parameter 10 log10 1 20 log10 1 a2 b2 Acoustic Power X Intensity X Air particle velocity X Pressure X