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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 ω 2 v ρ SA 2 = 1 2 ω 2 v μ A 2 Sound waves in air (bulk density) Sound waves in rod or wave in rope .. etc (linear density) ω=2πf 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 π 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 β- = = 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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Lect_18_SoundWaves-Interference-Doppler - Lecture 18 Sound...

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