# Moving at the speed of sound

1. Aug 9, 2014

### razidan

Hi everyone,
A couple of question I was thinking about regarding sound:

1) A sound source is moving at the speed of sound, and because sound waves cannot move faster, the waves build up at the front (direction of movement). If there is also an observer at the source (traveling at the speed of sound), what will he hear?

2) A sound wave is sent from a stationary source. Together with sound wave we let an observer travel at speed of sound (let's say that the observers travels exactly on one of the crests of the wave).
what will he hear?

I hope the description is clear.

I have my ideas but I would be happy to hear yours first.
Thanks,
R.

2. Aug 9, 2014

### Meson080

See: Sonic boom-Wikipedia

The article is about the objects moving faster than the speed of sound, it might help you as well.

3. Aug 9, 2014

### razidan

4. Aug 11, 2014

### razidan

Anyone?

5. Aug 11, 2014

### rcgldr

The observer would hear nothing, just sense pressure, if the shock wave was completely steady. There have been tests where an aircraft deliberately flies in the shock wave produced by another aircraft. Any sound produced behind the shock wave would sound fairly normal to an observer behind the shock wave. A shock wave would have variations that could cause any surface impacted by the shock wave to osciallate and produce sound to be heard by the observer.

Since the shock wave is going to have variation, there will be some sound in addition to pressure.

Doppler shift will increase the observered frequency. A continous sound produced over a given period of time produces a wave spans a finite distance, and even at the speed of sound, it still takes a finite amount of time to move across that finite distance.

If the duration of the sound is relatively short (as opposed to continuous),and if the source of the sound is not sufficiently ahead of and close to the super sonic aircraft, then the aircraft passes by before the sound ever reaches the aircraft. Even at sub-sonic speeds, the sound may dissipate so that by the time it catches up to a sub-sonic aircraft, the sound is inaudible.

Last edited: Aug 11, 2014
6. Aug 11, 2014

### razidan

Thank you!

So, what I was thinking is the the observer would basically feel a continuous Supersonic Boom.
Is that the pressure you were refering to?

Thank you again, but what i meant is the that a pulse is sent in the (let's say) positive direction at T=0. at the same time, an observer starts to move at the speed of sound in the same direction.
assuming point observer and instantaneous acceleration, and no damping/dissipation...

what I think is that no sound would be heard. the observer would travel next to a "packet" of air in a certain pressure (the amplitude of the wave) and would not "see" (hear) any changes in that pressure, therefore would not know of the sound wave. does that make sense?

Thanks again,
R.

7. Aug 11, 2014

### billy_joule

That is how I interpret it also.

If there is no relative motion between the wave/s and the observer then no sound will be heard.