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Is a sonic boom a one-time bang, caused when an object initially goes supersonic, or is it a continuous noise emitted by the object as it's travelling?

To be more specific, will an aircraft which is continuously accelerating cause a boom only over the point where it first transitions to supersonic, or does it cause a 'boom' to be heard over the whole length of it's supersonic journey by all of the observers along it's route (the same way that a fast boat will create a bow wave which follows it wherever it goes)?

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up vote 8 down vote accepted

I've received an answer/explanation from Jim Wild at Lancaster University. I'll add it here in case anyone's interested. Full credit to him :)

Jim Wild: But basically, no it's not just a single one-time bang. The pressure wave (which we perceive as a boom) is generated continuously as long as the aircraft is moving supersonically. This is why supersonic flights are usually prohibited over land - there isn't just one bang, it would "follow" the aircraft and be heard by lots of people!

Back when Concorde was flying, had you placed a chain of listening posts across the Atlantic, you would have been able to detect the aircraft moving across the ocean as it overflew each station, even if it only "broke" (i.e. accelerated through) the barrier once just after leaving the coast.

See: http://www.bbc.co.uk/news/science-environment-17701155

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So what happens when the aircraft surpasses Mach 2? (and Mach 3...) –  Eugene Seidel Jun 17 '13 at 16:28
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I believe there's no additional sonic boom - there's nothing actually that special about Mach 2+ :) –  Nick G Jun 17 '13 at 16:33
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@EugeneSeidel, the angle of the shock wave with respect to the line of motion changes with the Mach number. –  Alfred Centauri Jun 18 '13 at 1:05
    
Good answer, but... I could be wrong, but I think you need to paraphrase Jim Wild's answer or get him to answer it here personally. Anything posted on this site is released according to a creative commons copyright licence, and it's non-trivial to get the legally-sufficient permission from Jim Wild necessary to post it here. –  Jess Riedel Jun 25 '13 at 22:18
    
@JessRiedel Thanks but quoting someone is not a copyright issue (newspapers do it every day). I did not take it from a copyrighted publication - just his personal reply to me. –  Nick G Jul 3 '13 at 8:13
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A sonic boom is a continuous event just so long as the aircraft initially producing the sonic boom stays above Mach 1. Pressure and temperature affect the actual speed of Mach 1 which at sea level is at least 750mph. A sonic boom only appears to be a momentary event as the individual who hears it is stationary position. The boom itself travels along the path that the aircraft while travelling supersonic. Now if the aircraft was at high altitude and then suddenly turned downward pointing directly at an individual on the ground while travelling supersonic, the sonic boom would be more continuous and not just be a passing event. There is an inventor in the US who has created a device that produces a continuous sonic boom from a stationary position. A very real sonic boom. Said device can be focused to point the sustained sonic boom (shock waves) in a particular direction. I hope this was helpful.

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If you have more to add just put it in your answer rather than giving your email (edited it out). –  Kyle Jun 25 '13 at 21:53
    
Hi Tony. I'm not sure why the edit button is grayed out, but you should know leaving your personal email address on a question is not appropriate for this website. Please remove it. Cheers. –  Jess Riedel Jun 25 '13 at 22:15
    
@JessRiedel in this case, there was a pending suggested edit (and you can't stack another suggested edit on it and you don't have the rep to bypass the suggested edits feature) –  Manishearth Jun 25 '13 at 22:18
    
@Manishearth: Ahh, learn something new every day. Thanks. –  Jess Riedel Jun 25 '13 at 22:20
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