# Physics - Waves

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A speaker which produces a frequency of 400 Hz is moving away from an observer and towards a wall at 5 m/s when it is 23 degrees Celsius. What is the frequency of the sound that travels back to the observer? What is the frequency of the sound that travels forward and bounces off the wall, then gets to the observer?

• Physics - Waves -

and your thinking is?

• Physics - Waves -

My thinking is:

f2=f1(V/V+Vs)

f2=Heard frequency=394.3 Hz
f1=Source frequency=400Hz
V=velocity of sound in medium=332+0.6(23)=345.8 m/s
Vs=velocity of source=5 m/s

I'm not sure about the second question, though.

• Physics - Waves -

The wall will send back exactly the frequency it hears, because it is stationary.
It will hear a higher frequency than is radiated because the source is moving toward the wall.

• Physics - Waves -

So the answer to the first question is 394.3 Hz and the answer to the second question is 400 Hz?

• Physics - Waves -

Yes, on the first. But the second, the speaker is moving toward the wall, so it is an upward doppler pitch, and that exact frequency is reflected.

• Physics - Waves -

So the answer is 400(345.8/340.8) = 405.87 Hz?

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