Lect_18_SoundWaves-Interference-Doppler

Lect_18_SoundWaves-Interference-Doppler - Lecture 18 Sound...

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Unformatted text preview: Lecture 18 Sound Waves: Intensity, Interference, Beats and Doppler Effect. Speed of sound Speed of soun in air, depends on temperature: v = ( 331 + 0.60T ) m/s where T in C Power Energy for a particle in SHM (attached to a spring k) This energy propagates at speed v. the average energy per unit time that flows in the direction of propagation should be proportional to v Average power P v 2 A 2 E = 1 2 kA 2 = 1 2 m 2 A 2 remember: 2 = k m Average power for harmonic waves: P = 1 2 2 v SA 2 = 1 2 2 v A 2 Sound waves in air (bulk density) Sound waves in rod or wave in rope .. etc (linear density) =2f Intensity (Loudness) Example: A siren emits a sound of power 2W at 100 m from you. How much power reaches your ear (eardrum area = 0.7 cm 2 ) Intensity at distance r from source: area P I = Average power (over time) in wave Area of the surface where this power is distributed Power absorbed by eardrum: P eardrum = I R area of eardrum = 1.6 10 5 W/m 2 0.7 10 4 m 2 = 1.1 nW r 2 Sphere of area 4 r I R = P at source 4 r 2 = 2W 4 100 m 2 = 1.6 10 5 W/m 2 Sound intensity level 12 2 10log with 10 W/m I I I - = = Units: decibels Threshold of human hearing: 10-12 W/m 2 = Normal conversation: 10-6 W/m 2 = 65 decibels Threshold of pain: 1 W/m 2 = 120 decibels Twice the decibels does NOT feel twice as loud! Normal modes (Harmonics or over tones)...
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This note was uploaded on 02/11/2012 for the course PHYSICS 111 taught by Professor B during the Summer '08 term at Iowa State.

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Lect_18_SoundWaves-Interference-Doppler - Lecture 18 Sound...

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