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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)?

NickG
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5 Answers5

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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

NickG
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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+ :) – NickG 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 01:05
  • 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
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    @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. – NickG Jul 03 '13 at 08:13
  • Nick: in the US, everything everyone writes is copyrighted from the moment they set pen to paper. http://en.wikipedia.org/wiki/Copyright_law_of_the_United_States#Registration_procedure Newspapers quoting falls under "fair use", and a key aspect is whether the article discusses the quote, or whether is uses the quote at the content itself. I have no idea whether there is a simple reason why you don't have to worry about copyright here, but "I'm just quoting someone" is not a sufficient argument. – Jess Riedel Jul 06 '13 at 15:05
  • See here for an accessible summary: http://www.nolo.com/legal-encyclopedia/fair-use-rule-copyright-material-30100.html Importantly, you may have built up an intuition that most of the things you do are covered under fair use, because most of the things you do are not for profit. But StackExchange is a for profit enterprise (notice those ads). It's not crazy to think that Jim Wild is protected from having his words from being used to make money on some website he's not familiar with. (In this case, I think the chance that any legal conflict develops is zero, but knowing the law is valuable!) – Jess Riedel Jul 06 '13 at 15:10
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    Well ask a mod to delete this question then. I'm beyond caring now to be honest. If that's how anal this site is (when he hasn't even lodged a complaint!) then I want no part of it. No other StackExchange sites are subject to this problem. Thousands of articles have copied snippets from articles or blog posts etc and as long as it's properly credited, nobody should care. Ever heard of Wikipedia? Nearly everything on that site has been copied from somewhere else and credited with a citation link (without the authors consent). Has wikipedia been taken down? No. – NickG Jul 11 '13 at 09:07
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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.

Kyle Oman
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Tony Cocucci
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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 Oman 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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A sonic boom is a continuous high pressure sound wave following the aircraft. If a plane is flying over a long path say 100 mile, an observer is standing at (say) 10th mile will hear this boom as the plane will pass by and the observer standing at 100 mile will hear this boom after some time as the plane will pass by him.

Hashim
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Reducing an accelerating SST aircraft to point size, the sound (aerodynamic and mechanical) it creates are confined within a perfect cone. At Mach I, the angle of the cone margins to the line of flight is 45 degrees (the sound radiating laterally from the line of flight the same distance as the plane moves forwards). As the aircraft approaches an observer, the accumulating sound energy in the anterior sound cone displays rising sound frequency (Doppler Effect). with the wavelength decreasing proportionately.

Theoretically, at Mach I, the frequency should reach infinity, but this is impossible because the lower limit of the inversely related wavelength is restricted by the dimensions of the space occupied by the sound-transmitting adjacent air nuclei. As the aircraft passes the observer at Mach I, the frequency of the pent up Doppler-related sound energy in the anterior sound cone explodes into the ultimate example of low frequency sound – one single massive vibration, like the clap of a supersonic thunderbolt striking near the observer.

Noise interferes with the development of laminar flow. The violent reverberation of molecules with the intensifying noise in the anterior sound cone as Mach I is approached renders the anterior sound cone air refractory to laminar flow, converting the air into the equivalent of a gaseous gel, with greatly increased resistance to penetration by the leading edges. This resistance to thus gaseous gel effect is responsible for the increased air pressure anterior to the leading edges (the sound barrier effect so noticeable to earlier, less aerodynamic aircraft). The release of this pressure band as an SST aircraft passes rapidly through the sound barrier, potentially might result in a second source of a sonic boom, unrelated to the Doppler effect.

Gavin Hamilton
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"As long as an airplane travels at Mach 1 or faster, it will generate a continuous sonic boom. All those in a narrow path below the airplane's flight path will be able to hear the sonic boom as it passes overhead. This path is known as the “boom carpet." https://wonderopolis.org/wonder/what-is-a-sonic-boom

Wikipedia has an excellent entry showing more technical detail: https://en.wikipedia.org/wiki/Sonic_boom

Nasa Sonic Boom Fact Sheet https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-016-DFRC.html

blackweta
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