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The piston transmits energy to this element of air in the tube, and the energy is As the sound wave propagates away from the piston, the position of any element of air in front of the piston is given by Equation 17.2. To evaluate the kinetic energy of Imagine that we take a “snapshot” of the wave at t # 0. The kinetic energy of a given element of air at this time is where A is the cross-sectional area of the element and A 'x is its volume. Now, as in As in the case of the string wave in Section 16.5, the total potential energy for one As the sound wave moves through the air, this amount of energy passes by a given point where v is the speed of sound in air. ! # ' E ' t # E ( T # 1 2 ! A()s
max ) 2 ( T # 1 2 ! A()s
max ) 2 ! ( T " # 1 2 ! Av()s
max ) 2 E ( # K ( $ U ( # 1 2 ! A()s
max ) 2 ( # 1 2 ! A()s
max ) 2 ( 1 2 ( ) # 1 4 ! A()s
max ) 2 ( K ( # # dK # # ( 0
1 2 ! A()s
max ) 2 sin 2
kx
dx # 1 2 ! A()s
max ) 2 # ( 0 sin 2
kx
dx # 1 2 ! A 'x()s max ) 2 sin 2 kx ' K # 1 2 ' m(v) 2 # 1 2 ' m(*)s max sin kx) 2 # 1 2 ! A 'x(*)s max sin kx) 2 v(x,
t) # + + t s(x,
t) # + + t [s max cos(kx
* ) t)] # *)s max sin(kx
* ) t) S E C T I O N 17. 3 • Intensity of Periodic Sound Waves 517 We define the intensity I of a wave, or the power per unit area, to be the rate at which the energy being transported by the wave transfers through a unit area A per- (17.5) I
$
! A In the present case, therefore, the intensity is Thus, we see that the intensity of a periodic sound wave is proportional to the square of the displacement amplitude and to the square of the angular frequency (as I # ! A # 1 2 ! v()s max ) 2 Intensity of a sound wave |