# Speed of sound super sonic jet

1. Oct 31, 2007

### chrisakel

Quick Question...If a supersonic jet passes directly in front of you and you are infinitely close to the jet (there is no distance between you and the jet theoretically), would you hear the jet as it passes you or after it has already passed you?

I think you would hear it as it passes you...the only reason you would hear it after it had already passed you would be if it was at a certain distance away from you...does this sound right?

2. Oct 31, 2007

### FredGarvin

If you were infinitely close to the jet, then you would hear it as it passes you. The reason the sound is delayed as the aircraft passes you is due to the geometry of the pressure waves created by the aircraft as it is flying. If you are infinitely close, then you would be at the source at the time the sound was generated. As your distance increases so would the delay.

3. Oct 31, 2007

### chrisakel

okay, so you would hear the jet as it passes you, then you would hear the jet coming towards you and then away from you once it has already passed you?

4. Oct 31, 2007

### TVP45

Not toward you. Only away.

5. Oct 31, 2007

### AFG34

cool, never knew this :D

6. Nov 1, 2007

### rcgldr

Also, the sound of a shock wave up close is a "crack", only as it dissipates over a longer distance does it turn into a sound wave where you get a boom. In the second half of this video, the F14 flyby is close enough to produce the cracking sound (very loud).

f14flyby.wmv

7. Nov 2, 2007

### Shooting Star

Why?

He will hear the sound of the jet coming toward him after the jet has passed.

8. Nov 2, 2007

### Staff: Mentor

No he won't. That sound is entirely contained in the shock wave. It is the shock wave. It sounds like you are thinking the sound waves just get 'left behind' while trying to travel forward from the plane. They don't get "left behind", they get crushed together by the plane's passage.

Immediatly after a supersonic jet passes, you hear the doppler-shifted sound of the plane heading away from you.

Last edited: Nov 2, 2007
9. Nov 2, 2007

### TVP45

A good example of that is what you hear when a supersonic bullet e.g. a 30-06) goes over your head. You hear a long CRACKKK, shifting down in frequency, then nothing until you hear the BOOM from the rifle.

10. Nov 2, 2007

### PatPwnt

If you are infinitely close to the jet, that means the explosion from the jet engine is in your ear. So yea you would hear it, then you would hear the the sound of it coming from both the left and right because you would hear the sound it made while back before it passed you and the sound it made a little while after it passed you. Am I wrong?

11. Nov 4, 2007

### noagname

ok
now why was there a big cloud of moisture around the plane when it went super sonic

12. Nov 4, 2007

### Staff: Mentor

Yes, you are wrong. As I said above, the sound a supersonic jet makes before it passes you isn't following behind the jet, it is incorporated into the shock wave.

13. Nov 4, 2007

### Staff: Mentor

There is a big drop in pressure from one side of the shock wave to the other - enough to condense water vapor on a very humid day.

14. Nov 5, 2007

### Ahmed Abdullah

Isn't the shockwave a kind of soundwave ?
So the sound a supersonic jet makes before it passes someone is really following behind the jet with higher frequency and much much higher density of energy. Am I right?

Last edited: Nov 5, 2007
15. Nov 5, 2007

### rcgldr

Most of the interaction between air and a supersonic aircraft are below the speed of sound. Shockwaves are basically produced at the points in the aircraft where cross-sectional area increases. The shockwaves accelerate the air forwards, enough so that the differential in airspeed behind the shockwaves and aircraft (and aircraft engines) are sub-sonic. This sub-sonic differential generates normal sound waves (although they are traveling faster than the speed of sound compared to the surrounding air), and this explains the normal sounds you hear after a super-sonic aircraft passes by.

Shock waves and sound waves are different. The speed of sound can be considered to be the fastest speed that "information" can travel through the air. This means the air can't respond to a super-sonic aircraft until the aircraft passes through (to be nit-picky, there are very small stagnant pockets of supersonic air moving along with some of the leading edge surfaces of the aircraft). Over time and distance, the shockwave dissapates into a conventional sound wave, and with enough distance, you'll hear a "boom" instead of a "crack".

16. Nov 5, 2007

### Ahmed Abdullah

Interesting!

17. Nov 5, 2007

### Staff: Mentor

Sort of - I mean directly behind. For the scenario in the OP, the plane passes so close, the plane and shock wave pass at the same time.

But yes, for someone below the plane, the shock wave passes after the plane passes. I don't think that's what the others were referring to, though.

18. Nov 9, 2007

### chrisakel

So would a pilot traveling in a supersonic jet hear his own engines, or would he only hear them after a certain time period once he had decelerated to a sub-sonic speed?

Last edited: Nov 10, 2007
19. Nov 10, 2007

### Staff: Mentor

Again, the sounds of the plane are not following behind in a way that would allow them to "catch up".

The pilot, however, hears his own plane because of the sound transmitted through the plane.

20. Nov 10, 2007

### rcgldr

Also because the shockwave accelerates the air in the same direction as the plane this accelerated air is moving at sub-sonic speed relative to the plane.