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Unformatted text preview: In the last couple of lectures we discussed waves on a string these are transverse waves, in which the molecules in the string oscillate up and down as the wave moves horizontally Sound is also a wave but its a longitudinal wave, in which molecules move back and forth along the direction of wave motion: Lecture 20: Sound Waves High pressure Small displacement Low pressure Large displacement High pressure Small displacement We can describe the sound wave either in terms of the pressure at a given point: or in terms of how much the air molecules have moved from their normal positions: The pressure and position amplitudes are related by p x , t ( 29 = +  ( 29 y x , t ( 29 =  + 2 =  ( 29 p max = BkA Note that here the displacement variable y is in the same direction as x Speed of Sound We found earlier that the speed of waves on a string is The velocity of any mechanical wave has a similar form What is the elastic property of a gas? Its the fractional change in volume for a given change in pressure v = Elastic property of string Inertial property of string B =  / Bulk modulus Now we need to find an inertial property for a gas This is the density So the speed of sound in a gas is: The same expression also works to find the speed of sound in a liquid The expression for a solid is similar: v = About 330m/s for air at 1atm and 20 o C v = / = Youngs modulus About 6000m/s for iron Intensity Weve already discussed the power transmitted by a wave now well see how much power is transmitted by a sound wave...
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 Fall '08
 STAFF
 Thermodynamics

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