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

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Chapter 14 Chapter 14 Sound

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Sound Waves Sound Waves Sound is longitudinal pressure (compression) waves Range of hearing: 20 Hz to 20,000 Hz FREQUENCY DEMO
Speed of Sound Speed of Sound Liquids and Gases: B is bulk modulus, ρ is mass/volume Solids: Y is Young’s modulus 331 m/s is v at 0° C; T is the absolute temperature. v = Β ρ v = Ψ v air = (331 μ σ 29 Τ 273 Κ

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Example 14.1 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. What is the time difference between the moments when Betsy and Boopsie hear the bang? 4.54 s
Intensity of Sound Waves Intensity of Sound Waves SI units are W/m 2 Power Area Intensity is proportional to square of amplitude (pressure modulation) I = ∆Ε Α∆τ = Π Α

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Intensity Range for Human Hearing 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
Decibel Scale Decibel Scale Sensation is logarithmic I 0 is threshold of hearing (0 dB) Threshold of Pain is therefore 120 dB β= 10 log 10 I I o I = I 0 10 b /10

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Intensity vs. Intensity Level Intensity vs. Intensity Level INTENSITY is P/A, W/m 2 INTENSITY LEVEL is in decibels (dimensionless)
Sound Level Demo Sound Level Demo

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Example 14.2 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
Spherical Waves Spherical Waves Energy propagates equally in all directions I = Π 4πρ 2 I 1 I 2 = ρ 2 2 1 2

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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? b) What intensity level of the horn’s sound is observed by
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## This note was uploaded on 07/25/2008 for the course PHY 231C taught by Professor Pratt during the Spring '06 term at Michigan State University.

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

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