p235w11_practice_exam_3_2_sol

# I return 2 p 4rmoth 4r 22 22 2 4r02 4rmoth i0

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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 = (2-x) – x = 2-2x We want this to be an integer multiple of , so 2-2x = 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 = 2-2x = (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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