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Unformatted text preview: 2 = 2
4πr02 4πrmoth I0 4πr 22
22 = 2
rmoth r02
I0
4
r22 rr I 0.01×0.1 dB22 =10log10 12 0
+20log10 0 moth =100+20log10 = 13.7 dB
2
2
2 22 10 W/m r22 c: (2 points) How much time elapses between the beginning of the bat’s sound pulse and
the return of the echo? t 2d 2 22 m 65 ms
vs 340 m/s d: (3 points) If the bat is flying toward the moth at a speed of 4 m/s, and the moth is
hovering at rest, what is the frequency of the reflected sound the bat hears? f f0 vs vs vb
340 340 4 f0 1.023 f 0 76, 785 Hz
340 4 340
vs vb vs 2: (5 points) Two speakers emit sound waves in synch with one another. They are
separated by a distance of 2.0 m. The frequency of the sound they produce is 206 Hz.
Consider a point a distance x from speaker A, along the line between them.
2m
x a: (3 points) For what values of x between the speakers will there be constructive
interference between the two? = 340 m/s / 206 Hz = 1.65 m. The path length difference
PL = (2x) – x = 22x
We want this to be an integer multiple of , so 22x = n, which gives
x = 1  n/2
which has values x = 0.175, 1, 1.825 (for n = 1, 0, and 1)….
b: (2 points) For what values of x will there be destructive interference?
PL = 22x = (2n+1)/2
x = 1 – (2n+1)/4
x = 0.5875, 1.4125 (for n = 0 and n = 1)
3: (5 points) Explain why you can see “images” when you look in a mirror but not when
you look in a wall.
To form images, a surface must reflect light, but it must al...
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 Spring '08
 Mckay

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