Sound and supersonic motion - a question from Feynman's lectures

[dEdt]

Sound and supersonic motion -- a question from Feynman's lectures

In chapter 51, volume 1 of the FLP, Feynman writes

Incidentally,...it turns out, very interestingly, that once [an] object is moving faster than the speed of sound, it will make sound...Any object moving through a medium faster than the speed at which the medium carries waves will generate waves on each side, automatically, just from the motion itself.

Unfortunately he didn't give a justification for this claim. I was hoping someone in Physics Forums could fill this gap in.

 

[russ_watters]

I don't like it at all. Sound is just disturbances in the medium and a moving object most certainly disturbs the medium at subsonic speed.

 

[dEdt]

Presumably he had in mind sound waves. (This is clear from the context the quote appears in). Subsonic objects definitely disturb the air around them but they don't create sound waves like supersonic objects.

 

[russ_watters]

Sound waves ARE disturbances in the air. The sonic boom shock wave is just the piling-up of all the disturbances on top of each other.

 

[boneh3ad]

He is alluding to the presence of shock waves, which are essentially a form of sound waves that result simply from an object moving through a gas at supersonic speeds.

 

[russ_watters]

Yes, but he seems to be implying that objects moving at subsonic speed don't also "generate waves on each side".

 

[dEdt]

Ok so the only difference between subsonic and supersonic motion is that the small air disturbances created by the motion reinforce each other to form a shock wave in supersonic flight, but don't in subsonic flight?