Wave superposition and Doppler effect.

[FerN61]

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.

 

[Doc Al]

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!)

 

[FerN61]

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?

 

[Doc Al]

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.