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Wave superposition and Doppler effect.

  • Thread starter FerN61
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Homework Statement


A student is at some point between two sound reflecting walls. He's holding a tuning fork and runs towards one of the walls at 5m/s. The pulsating frequency he hears is 8Hz. ¿What's the sound frequency emitted by the tuning fork?
Answer: 137.17 Hz.

I know I'm supposed to use doppler effect but I guess I have to consider wave superposition since there are two walls, but I don't know how to unify both.
 

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  • #2
Doc Al
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I know I'm supposed to use doppler effect but I guess I have to consider wave superposition since there are two walls, but I don't know how to unify both.
In addition to the Doppler effect, you need to understand beat frequency. (Look that up!)
 
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  • #3
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Beats

Ok so, it is a beat frequency so

f2-f1 = 8

f2= f1+8

using doppler's formula

f1+8=f1((Vsound+5)/Vsound)
Assuming Vsound= 343 m/2

f0=548.8

The answer is supposed to be 137.17.... What am I doing wrong?
 
  • #4
Doc Al
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You're on the right track, but there's a bit more to it. The two sounds that are creating the beat frequency are the reflections off the two walls. So you have to find those frequencies. Hint: For each of those reflections you'll need to apply the Doppler formula twice.
 
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