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chapter14.key - Sound Waves Chapter 14 Sound is...

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Chapter 14 Sound 1 Sound Waves Sound is longitudinal pressure (compression) waves Range of hearing: 20 Hz to 20,000 Hz FREQUENCY DEMO 2 Speed of Sound Liquids and Gases: B is bulk modulus, ! is mass/volume Solids: Y is Young’s modulus 331 m/s is v at 0° C; T is the absolute temperature. v = B ! v = Y ! v air = (331 m s ) T 273 K 3 Example 14.1 John Brown hits a steel railroad rail with a hammer. Betsy Brown, standing one mile down the track, hears the bang through the cool 32 ° F air while her twin sister Boopsie is lying next to her and hears the bang through the steel by placing her ear on the track. DATA: Y steel =2.0x10 11 Pa, ! steel =7850 kg/m 3 What is the time difference between the moments when Betsy and Boopsie hear the bang? 4.54 s 4 Intensity of Sound Waves SI units are W/m 2 Power Area Intensity is proportional to square of amplitude (pressure modulation) I = ! E A ! t = P A 5 Intensity Range for Human Hearing Threshold of Hearing 10 -12 W/m 2 " P ~ 10 -10 atm ! Threshold of Pain 1.0 W/m 2 6
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Decibel Scale Sensation is logarithmic I 0 is threshold of hearing (0 dB) Threshold of Pain is therefore 120 dB ! = 10 log 10 I I o I = I 0 10 ! /10 7 Intensity vs. Intensity Level INTENSITY is P/A, W/m 2 INTENSITY LEVEL is in decibels (dimensionless) 8 Sound Level Demo 9 Example 14.2 A noisy machine in a factory produces a sound with a level of 80 dB. How many machines can the factory house without exceeding the 100-dB limit? a) 12.5 machines b) 20 machines c) 100 machines 10 Spherical Waves Energy propagates equally in all directions # I = P 4 ! r 2 I 1 I 2 = r 2 2 r 1 2 11 Example 14.3 (skip) A train sounds its horn as it approaches an intersection. The horn can just be heard at a level of 50 dB by an observer 10 km away. Treating the horn as a point source and neglect any absorption of sound by the air or ground, a) What is the average power generated by the horn?
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