THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

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256 11. Acoustics of Enclosed Spaces: Architectural Acoustics11.9 Reverberation as Affected by Sound Absorptionand Humidity in AirWe have previously not considered the effect of absorption of sound and humidity inair on reverberation times. The volume of air contained in very large auditoriumsor a place of worship can absorb an amount of acoustic energy that cannot beneglected as in the case with smaller rooms. If a room is small, the number ofreflections from the boundaries is large and the amount of time the sound wavespends in the room is correspondingly small. In this situation acoustic energyabsorption in the air is generally not important. In very large room volumes thetime a wave spends in the air between reflections becomes greater to the extentthat absorption of energy in air no longer becomes negligible. The reverberationequations must now include the effect of air absorption, particularly at higherfrequencies (>1 kHz).Sound waves lose some energy through viscous effects during the course oftheir propagation through a fluid medium. The intensity of a plane wave lessenswith distance according to the equationI = I 0 e −2βx = I 0 e −mx .Here m = 2β represents the attenuation coefficient of the medium. Some texts useα rather than β to denote the attenuation constant of the medium; we eschew itsuse in order to avoid confusion with α used in this chapter to denote the absorptioncoefficient of a surface. During time interval t, a sound wave travels a distancex = ct, and the preceding equation may be revised to read(β/4V +m)ctI = I0 eThe expression for the reverberation time becomesT = 0.161V(11.18)A + 4mVwhere the constant mis expressed in units of m –1 . The total surface absorption Ais given either by Equation (11.3) or (11.17) depending whether that room fits intothe category of being an acoustically live or dead chamber. As the room volume Vbecomes larger, the second term in the denominator of Equation (11.18) increasesin magnitude, as air absorption becomes more significant, due to increasing pathlengths between the walls. Since m also increases with frequency, air absorptionalso becomes more manifest at higher frequencies (above 1 kHz) than at lowerfrequencies. The values of m are given in Figure 11.7 1 as a function of humidityfor various frequencies at a normal room temperature of 20 ◦ C. More details, alsogiven in tabular form, for a range of air temperatures and humidities are givenin the NASA report (1967), prepared by Cyril M. Harris, listed at the end of thischapter. It is seen from Figure 11.7 that the effect of humidity reaches a maximum1 The plot of Figure 11.7 applies to indoor sound propagation, not to outdoor propagation that includesmeteorological effects not present indoors.
11.9 Reverberation as Affected by Sound Absorption and Humidity in Air 257Figure 11.7. Values of the total attenuation coefficient m versus percent relative humidityfor air at 20 ◦ C and normal atmospheric pressure for frequencies between 2 kHz and12.5 kHz. Values are rendered in both SI units and U.S. Customary System units. (AfterHarris, 1966, 1967.)in each of the given frequencies in the 5–25% relative humidity range and thentrails off at higher humidities.Example Problem 2Find the air absorption at a frequency of 6300 Hz and 25% relative humidity for aroom volume of 20,000 m 3 .Solution and Brief DiscussionFrom Figure 11.7 the value of m is equal to 0.026 m –1 . The air absorption A air =4mV in Equation (11.18) is equal to 4 × (20,000 m 3 ) × 0.026 m –1 = 2080 m 2 .Ifwe consider absorption at 500 Hz the effect of air absorption would be negligiblein comparison.
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THE SCIENCE ANDAPPLICATIONS OFACOUS
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xPrefaceReferencesBacon, Sir Franci
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xiiContents16. Ultrasonics 44317. C
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2 1. A Capsule History of Acoustics
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1. A Capsule History of Acoustics 5
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12 1. A Capsule History of Acoustic
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14 2. Fundamentals of AcousticsFigu
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16 2. Fundamentals of Acoustics2.2
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18 2. Fundamentals of Acousticsof t
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20 2. Fundamentals of AcousticsQ z+
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22 2. Fundamentals of Acousticsin t
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24 2. Fundamentals of AcousticsWe c
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26 2. Fundamentals of Acoustics(2)
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28 2. Fundamentals of AcousticsEqua
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30 2. Fundamentals of Acoustics11.
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32 3. Sound Wave Propagation and Ch
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3.14 Performance Indices for Enviro
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3.14 Performance Indices for Enviro
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or in terms of complex exponential
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3.17 Sound Intensity 61Here the sur
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3.18 The Monopole Source 63The thre
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3.21 Energy Density 653.20 The Hemi
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References 67medium. The time avera
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Problems for Chapter 3 69(a) the wa
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4Vibrating Strings4.1 IntroductionI
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4.4 General Solution of the Wave Eq
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4.6 Simple Harmonic Solutions of th
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4.7 Standing Waves 77Figure 4.3. Di
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4.8 The Effect of Initial Condition
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4.10 Forced Vibrations in an Infini
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4.11 Strings of Finite Lengths: For
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References 854.12 Real Strings: Fre
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Problems for Chapter 4 878. A devic
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90 5. Vibrating BarsFigure 5.1. A b
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92 5. Vibrating BarsSettingc 2 = E/
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94 5. Vibrating BarsThe allowable f
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96 5. Vibrating Barsthe acceleratio
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98 5. Vibrating BarsFigure 5.5. Loc
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100 5. Vibrating Barsmore difficult
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102 5. Vibrating BarsFigure 5.7. An
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104 5. Vibrating Barsthe light beam
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106 5. Vibrating BarsFigure 5.8. Tr
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108 5. Vibrating BarsFigure 5.10. T
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110 5. Vibrating Barsof 0.005 m dia
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112 6. Membrane and PlatesFigure 6.
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114 6. Membrane and PlatesBecause t
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116 6. Membrane and PlatesThe left-
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118 6. Membrane and PlatesTable 6.1
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120 6. Membrane and PlatesIn real a
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122 6. Membrane and PlatesTable 6.2
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124 6. Membrane and PlatesAt low fr
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126 6. Membrane and PlatesThe elast
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128 6. Membrane and PlatesFor the f
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130 6. Membrane and Plates(b) Deter
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132 7. Pipes, Waveguides, and Reson
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152 8. Acoustic Analogs, Ducts, and
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9Sound-Measuring Instrumentation9.1
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9.3 Microphones 175Figure 9.1. A cu
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SolutionEquation (9.3) is applied t
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9.5 Selection and Positioning of Mi
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9.6 Vector Sound Intensity Probes 1
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9.8 Proper Procedures for Using the
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9.8 Proper Procedures for Using the
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Example Problem 39.10 Dosimeters 18
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9.11 Noise Measurement in Selected
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9.11 Noise Measurement in Selected
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9.12 Real Time Analysis 193analyzer
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9.13 Fast Fourier Transform Analysi
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9.14 Data Windows and Selection of
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9.16 Measurement Error 199whereβ =
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9.18 Measurement of Sound in a Free
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Page 217 and 218: References 209acoustical output and
Page 219 and 220: Problems for Chapter 9 2112. Determ
Page 221 and 222: 214 10. Physiology of Hearing and P
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Page 251 and 252: 11.2 Sound Fields 245Figure 11.1. P
Page 253 and 254: 11.4 Sound Intensity Growth in a Li
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Page 257 and 258: 11.6 Growth of Sound with Absorbent
Page 259 and 260: 11.7 Decay of Sound 253Following Sa
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Page 283 and 284: References 277preferred subsequent
Page 285 and 286: Problems for Chapter 11 279Siebein,
Page 287 and 288: 12Walls, Enclosures, and Barriers12
Page 289 and 290: 12.3 Mass Control Case 283Figure 12
Page 291 and 292: 12.5 Effect of Frequencies on Sound
Page 293 and 294: 12.6 Coincidence Effect and Critica
Page 295 and 296: 12.7 The Double-Panel Partition 289
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Page 299 and 300: 12.8 Measuring Transmission Loss 29
Page 301 and 302: 12.10 Combined Sound Transmission C
Page 303 and 304: 12.11 Noise Insulation Ratings 297F
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12.14 Enclosures 307and similarly f
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12.15 Small Enclosures 309walls. Ac
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12.16 Acoustic Barriers 311Figure 1
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12.16 Acoustic Barriers 313In the c
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Equation (12.67) becomes(11D = λ+
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Problems for Chapter 12 3173. A 0.7
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13Criteria and Regulations forNoise
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13.3 The Occupational Safety and He
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13.4 Perception of Noise 323inspect
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13.6 Indoor Noise Criteria 325accou
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13.6 Indoor Noise Criteria 327Room
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13.7 Equivalent Sound Level, Day-Ni
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13.7 Equivalent Sound Level, Day-Ni
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Composite Noise Rating13.9 Rating o
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13.10 Evaluation of Traffic Noise 3
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Highway Construction Noise13.10 Eva
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13.11 Evaluation of Community Noise
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13.12 Guidelines and Regulations in
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References 353Computer Program, HIC
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Problems for Chapter 13 355Problems
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14Machinery Noise Control14.1 Intro
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14.4 Fan or Blower Noise 359Table 1
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14.4 Fan or Blower Noise 361Figure
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14.6 Pumps and Plumbing Systems 367
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14.6 Pumps and Plumbing Systems 369
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14.8 Gears 371fromwheref BRC = N r
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14.8 Gears 373Figure 14.7. Gear noi
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14.8 Gears 375Figure 14.8. Gear tra
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14.8 Gears 377The subscripts 1 and
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14.10 Ball and Roller Bearings 379l
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14.10 Ball and Roller Bearings 381c
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14.11 Other Mechanical Drive Elemen
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Belt Drivesf CL = n 1N 1 2400 × 12
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14.12 Gas-Jet Noise 387said to be c
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14.12 Gas-Jet Noise 389power discha
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Solution14.12 Gas-Jet Noise 391Usin
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14.13 Gas Jet Noise Control 393Solu
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14.13 Gas Jet Noise Control 395Figu
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14.13 Gas Jet Noise Control 397Figu
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14.14 Mufflers and Silencers 399Fig
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(1 + m)A I 1=A 314.14 Mufflers and
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14.14 Mufflers and Silencers 403Let
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References 405Figure 14.21 illustra
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Problems for Chapter 14 407the nois
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15Underwater Acoustics15.1 Sound Pr
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15.3 Speed of Sound in Seawater 411
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15.4 Velocity Profiles in the Sea 4
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15.4 Velocity Profiles in the Sea 4
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15.5 Underwater Transmission Loss 4
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15.6 Parametric Variation of Absorp
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15.8 Underwater Refraction 421Figur
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15.9 Mixed Layer 423When G is const
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15.11 Sonar Transducers and Their P
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Example Problem 115.12 The Sonar Eq
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Active and Passive Equations15.12 T
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15.13 Noise, Echo, and Reverberatio
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15.13 Noise, Echo, and Reverberatio
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15.14 Transient Form of Sonar Equat
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Example Problem 315.16 Shortcomings
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15.17 Theoretical Target Strength o
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Problems for Chapter 15 441Wilson,
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444 16. Ultrasonicscatching small i
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446 16. UltrasonicsPlanck constant
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448 16. UltrasonicsEquation (16.5)
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450 16. UltrasonicsFigure 16.1. Sou
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452 16. Ultrasonicsthe positive hal
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454 16. Ultrasonicson each and ever
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456 16. Ultrasonicscomplete as a li
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458 16. Ultrasonicsinitial of their
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460 16. Ultrasonicsoptic axis, deno
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462 16. Ultrasonicstransducer, it h
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464 16. UltrasonicsTable 16.1. Valu
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466 16. Ultrasonicsthe mechanical r
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468 16. UltrasonicsThe Physics of M
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470 16. UltrasonicsFigure 16.7. The
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472 16. Ultrasonicscurvilinear arra
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474 16. Ultrasonics(a)amplitudeinpu
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476 16. Ultrasonicsslow to allow th
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478 16. Ultrasonics5. In the cases
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480 17. Commercial and Medical Ultr
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510 18. Music and Musical Instrumen
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Figure 18.7. The frequencies of the
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Figure 18.20. The violin octet deve
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570 19. Sound ReproductionAlbert, a
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572 19. Sound ReproductionD. Magnet
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574 19. Sound Reproductionon, machi
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576 19. Sound ReproductionIt is fai
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578 19. Sound ReproductionINNER SUS
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580 19. Sound Reproductiontuned ape
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582 19. Sound Reproductionuse of MP
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584 19. Sound ReproductionDickason,
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586 20. Vibration and Vibration Con
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618 21. Nonlinear Acousticsby∇ 2
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620 21. Nonlinear Acousticsmay be i
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622 21. Nonlinear Acousticswhereδ
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624 21. Nonlinear AcousticsThe shoc
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626 21. Nonlinear AcousticsBut the
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Appendix APhysical Properties of Ma
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Appendix A. Physical Properties of
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Appendix BBessel FunctionsB.1 The B
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B.8 Tables of Bessel Functions, Zer
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Appendix CUsing Laplace Transforms
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C.3 Solving Differential Equations
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C.4 Equations with Multiple-Order R
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SolutionC.5 Equations with Complex
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C.5 Equations with Complex Roots 64
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SolutionC.5 Equations with Complex
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650 IndexAuditoriums, design of, 26
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652 IndexElectrostatic speakers, 58
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654 IndexKey notation, 518Kinetic e
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656 IndexPascal (unit), 48Percent i
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658 IndexSound intensity growth in
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660 IndexWater-hammer arresters, 37
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