Speed of sound super sonic jet

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Discussion Overview

The discussion revolves around the auditory experience of a supersonic jet passing directly in front of an observer, particularly focusing on the timing of sound perception relative to the jet's position. Participants explore the implications of being infinitely close to the jet and the nature of sound and shock waves produced by supersonic flight.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that if one is infinitely close to a supersonic jet, they would hear the sound as it passes due to being at the source of the sound waves.
  • Others argue that the sound of the jet approaching is not heard until after it has passed, as the sound is incorporated into the shock wave created by the jet.
  • A later reply questions the nature of sound waves and shock waves, suggesting that the sound produced before the jet passes is not left behind but rather compressed into the shock wave.
  • Some participants mention that the sound of a shock wave is perceived as a "crack," while at greater distances, it transforms into a "boom." This distinction is noted in relation to the sound of supersonic bullets as well.
  • There is a discussion about the conditions under which moisture clouds form around a supersonic jet, attributed to pressure drops associated with shock waves.
  • One participant raises a question about whether a pilot in a supersonic jet hears their own engines, leading to clarification that sound is transmitted through the aircraft itself.

Areas of Agreement / Disagreement

Participants express differing views on the timing of sound perception relative to the jet's position, with no consensus reached on whether sound is heard as the jet passes or only afterward. The nature of shock waves versus sound waves also remains a point of contention.

Contextual Notes

Participants acknowledge that the rapid sequence of events during a jet's passage may complicate the ability to perceive sound timing accurately. The discussion also highlights the complexity of sound propagation in relation to supersonic flight.

chrisakel
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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?
 
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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.
 
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?
 
Not toward you. Only away.
 
cool, never knew this :D
 
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
 
TVP45 said:
Not toward you. Only away.

Why?

He will hear the sound of the jet coming toward him after the jet has passed.
 
Shooting star said:
Why?

He will hear the sound of the jet coming toward him after the jet has passed.
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.
 
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Jeff Reid said:
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

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
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
ok
now why was there a big cloud of moisture around the plane when it went super sonic
 
  • #12
PatPwnt said:
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?
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
noagname said:
ok
now why was there a big cloud of moisture around the plane when it went super sonic
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
russ_watters said:
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.

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?
 
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  • #15
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
Jeff Reid said:
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".

Interesting!
 
  • #17
Ahmed Abdullah said:
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?
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
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?
 
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  • #19
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
russ_watters said:
The pilot, however, hears his own plane because of the sound transmitted through the plane.
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.
 
  • #21
thanks for all of the information...very interesting
 
  • #22
Theoratically u can hear the sound when the jet nears you, and after it has crossed you. but not at the particular moment when the jet is infinitely close to u. But practically since this all happens in just a blink of a moment, u won't be able to realize or differentiate it... Hope u understand what i mean..
 

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