# Homework Help: The velocity of sound

1. Oct 22, 2016

### Hamal_Arietis

1. The problem statement, all variables and given/known data
Will the velocity of sound change if the source moves?

2. Relevant equations
The velocity of velocity of sound in air equals:
$$V=\sqrt{\frac{\gamma RT}{M}}=const$$

3. The attempt at a solution
I think it doesnt change. But it seems to conflict with the equation :
$$\vec{v_{air}}=\vec{v_s}+\vec{V}$$

2. Oct 22, 2016

### haruspex

Please explain what each velocity represents in that last equation.

3. Oct 22, 2016

### Hamal_Arietis

In room reference frame
$v_{air}$ is the velocity of sound that observer receives. $v_s$ is velocity of source and $V$ is the velocity of sound if source doesnt move.

4. Oct 22, 2016

### haruspex

I don't understand.
In the room reference frame, the source, receiver and air may all have a velocity, and the sound has a velocity relative to the air.
Let's call these vs, vr, va, and V respectively, all measured in the same direction.

What does your equation look like in those terms? You seem to be saying that the sound arrives at the receiver with speed V+vs-vr, but that is not true. No matter how the source moves, the velocity of the sound waves relative to the room is V+va, and relative to the receiver is V+va-vr. The frequency is another matter.

5. Oct 22, 2016

### Hamal_Arietis

Sorry .I seem air and receiver dont move. And source comes near observer. So the velocity of sound that obsever receives v=vs+ V

6. Oct 22, 2016

### haruspex

No, as I posted, the velocity of the source does not affect the velocity of the sound waves. It does affect the frequency.

7. Oct 22, 2016

### Hamal_Arietis

Thanks a lot for helping. I understanded.
Answering this question helps me prove the Dopper effect.