# Homework Help: Sound doppler

1. Mar 20, 2014

### barryj

This is not homework but rather a concept question.

If there was a siren on the front of an airplane that is traveling greater than the speed of sound, would there still be a sound emitted?

If the airplane was traveling toward me what would be the frequency? Would I heart it?

Where would the Doppler equations break down?

2. Mar 20, 2014

### BOYLANATOR

A sound is still emitted yes. For a stationary source the wavefronts are circles (in 2D). As the source approaches the speed of sound the circles stretch out behind the source and bunch up in front. At the speed of sound the waves bunch up at the tip of the airplane while at the back they stretch out further. At greater speeds, all the waves propagate backwards in the airplanes rest frame (but still radially in the observers rest frame).

You could hear the sound of the airplanes approach but not until after it has passed (i.e the plane would beat the sound). If the plane travelled towards you travelling at twice the speed of sound you would hear the song playing exactly backwards.

The Doppler equations don't break down, they predict negative frequencies when the plane travels towards you, so signals emitted later in time, arrive earlier in time.

3. Mar 20, 2014

### barryj

"you would hear the song playing exactly backwards." This was exactly what I was wondering. If a person was speaking out the front of the plane, the words would come in reversed but then one could not understand a word played out backwards. Interesting concept.

4. Mar 20, 2014

### BOYLANATOR

Yeah it's very interesting, I suppose from this example you can understand a little better why travelling faster than the speed of light can be thought of as travelling backwards in time as well.

5. Mar 20, 2014

### barryj

These effects could be the basis for a good si-fi movie?????

6. Mar 20, 2014

### BOYLANATOR

I'm not a Star Trek fan but I think this effect is what makes Warp Drive work haha