Why is there a sonic boom at Mach 1.5?

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  • Thread starter Dead Rabit
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  • #1
Dead Rabit
It's my understanding that a sonic boom is the doppler effect taken to the extreme, with a sound emitting object travelling at the speed of sound, your building the ultimate constructive wave.

I want to know, if you were travelling at Mach 1.5, why doesn't the sound you're generating cancel itself out? (in layman's terms)

More precisely, if you understood how the frequency of the sound generated by your aircraft varied at different speeds as it's travelling, why can't you:
a) choose to travel at a speed that would destroy the sound wave produced?
b) deliberately place the elements of your design that cause drag (and hence, noise) offset by half the wavelength?

Or at least I assume you can't because it still takes 8 hours to fly to America from the UK, 50 years after Concorde was invented =P.

Cheers,
J
 

Answers and Replies

  • #2
Drakkith
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Once you hit Mach 1, you're no longer generating sound waves in the traditional sense. Further increases in velocity do not create sound waves with different frequencies. What's happening is that you're pushing the air out of the way faster than it can form a wave in the first place. This creates a cone-shaped region of high-pressure air which expands outwards after the plane passes, which is where the sound comes from. Increasing the plane's velocity just elongates this cone.
 
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  • #3
Dead Rabit
Interesting, are there any topics you can recommend I read up on to improve my understanding of this phenomenon?
"sonic boom" only seems to bring up high level synopsis.
 
  • #4
Drakkith
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Other than wikipedia, I'm afraid I can't. :cry:
 
  • #5
FactChecker
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The sonic boom is simply a moving shock wave. Shock waves form in many places on the wing and in the jet exhaust. In fact, in the jet engine exhaust, there is a nested set of shock waves, one inside the other. There is a great deal of information in books on aerodynamics. You can go to Amazon and find many books by searching "aerodynamics shock wave". I have never studied it, so I can not recommend any particular book.
 
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Nidum
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Search on ' formation of shock waves on aircraft ' and ' NASA supersonic aircraft '
 
  • #7
sophiecentaur
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At a distance from the high speed object, the air will have slowed to below sonic speed. (i.e. When the object has passed) the impulse will have spread out and the particle speeds will no longer be sonic. What you hear is not a 'shock wave' but the remnants of it, spread out in time on the journey to the ground into a single pulse of long duration. Boom not crack.
You get more than one boom when there are multiple initial shock waves.
 
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