13 Waves 4

# 13 Waves 4 - Intensity of sound 1 I = s0P0 2 P I= 2 v 2 0 I...

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Intensity of sound 0 0 2 1 P s I Δ = ω – average intensity s 0 – the amplitude of displacement of the air Δ P the amplitude of pressure variations Expressions making a bit more sense: v P I ρ 2 2 0 Δ = Quadratic in the amplitude of pressure variations – proportional to potential energy of the gas deformations . v K v v s I = = = 2 v 2 1 2 osc 2 0 2 ρω Quadratic in the amplitude of velocity of the oscillations of the gas. Proportional to kinetic energy of the gas motion and the wave speed . μm/s 45 v m/s 343 , W/m 10 osc 2 6 = = = v I I

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Measuring sound intensity in decibel, dB = 0 log 10 I I β 2 12 0 W/m 10 = I Number of dB , decibel, dimensionless parameter - reference value, corresponding to threshold of sensitivity of a normal ear at about optimal frequency
Measuring sound intensity in decibel, dB = 0 log 10 I I β 2 12 0 W/m 10 = I Number of dB , decibel, dimensionless para m eter - reference value, corresponding to threshold of sensitivity of a normal ear at about optimal frequency An example: TV is turned down from 75 dB to 60 dB. How does its sound intensity change? From 2 5 2 5 . 7 12 5 . 7 0 1 W/m 10 1 . 3 W/m 10 10 10 = = = I I To 2 6 2 6 12 6 0 2 W/m 10 W/m 10 10 10 = = = I I Goes down by a factor of 31 10 5 . 1 = In general: sound level is going up/down by x dB from its initial level, , - sound intensity is multiplied by 10 / 10 x ± 10 / 0 10 = I I 10 / 10 / 10 / 0 10 ) ( 10 10 ) ( x x I I x I = = +

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Full or partial reflection always occurs at a boundary the wave cannot cross For example, with a medium, which does not support the waves of that kind. Reflection depends a lot on the boundary conditions: a clamped string vs. a
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## This note was uploaded on 12/13/2011 for the course PHYS 2C PHYS 2C taught by Professor Groisman during the Spring '11 term at UCSD.

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13 Waves 4 - Intensity of sound 1 I = s0P0 2 P I= 2 v 2 0 I...

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