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Can we hear a supersonic plane?

  1. Aug 5, 2016 #1

    andrewkirk

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    A very clever first-year physics student I know, who had just been learning about the Doppler effect, asked me a question. If a plane were flying a straight trajectory at Mach 2, playing a song on its speakers very loudly, and an observer with incredibly sensitive recording equipment were to position themselves in the wake of the plane just after it had passed, would they hear the song playing backwards?

    There are a number of interesting aspects to this question. One that I found myself puzzling over was whether one could hear any sound at all from the plane itself, other than the sonic boom as it passed. I am not interested in questions of relative loudness, such as that the roar of the engines would drown out the song. One imagines that, with the right equipment, one might be able to separate the two waveforms and detect that of the song - at least in theory, if not in practice.

    What I was wondering was whether the sonic boom phenomena around the plane actually prevents one from detecting any sound wave emitted by the plane itself. What made me think that was that behind the plane there will be a near vacuum, preventing the transmission of sound backwards. In front of the plane is the very high pressure front of the sonic boom, and any wave that would otherwise be transmitted forwards is immediately destroyed by the plane crashing into it. Maybe sound can only escape in directions that are close to perpendicular to the plane's direction of travel - away from the rear vacuum and out of the way of getting smashed by the plane. But in that case one would only detect the sound very briefly as the plane passed directly overhead (again ignoring issues of volume).

    My initial reaction was that of course that is wrong because a ground observer hears the sonic boom and then hears a lower, continuous roar. But I thought that what we might be hearing there is not the engines themselves, which are shielded from us by the vacuum/boom-front, but the noise of the jet stream that is fired backwards by the engines, as it hits the air behind the vacuum. Because that collision occurs outside of the sonically shielded region immediately around the plane, it is free to propagate and so we can hear it.

    Anyway, my question is this:

    Could sound produced by a plane - such as if it had a very loud police siren blaring - be heard away from the plane with suitably sensitive equipment if the plane were travelling supersonically?
     
  2. jcsd
  3. Aug 5, 2016 #2

    Bystander

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    Does the air move? Yes.
    Or stand still? No.
     
  4. Aug 5, 2016 #3
    I don't think so.
    Every noise made directly by plane will be heard compressed as the 'boom', a shock wave, by observers on the ground shortly after the plane passes overhead.
    (actually I think it sounds more like snap-crack than boom. although I only personally heard a sonic boom on two occasions)
    That might be followed by a series of indirect echos of the boom from buildings or high terrain in the area.
     
    Last edited: Aug 5, 2016
  5. Aug 5, 2016 #4

    andrewkirk

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    Can you elaborate? It is unclear how these questions and answers relate to my question.
     
  6. Aug 5, 2016 #5

    Bystander

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    An ideal (still) vs. a non-ideal (moving) medium.
     
  7. Aug 6, 2016 #6

    boneh3ad

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    You can still hear supersonic planes, and if you are directly behind it you actually won't hear a boom anyway since a sonic boom comes from the oblique shocks coming off the various parts of the plane (notably the nose and tail).

    There is no vacuum that prevents sound from traveling behind a supersonic plane, either. As a sound wave is emitted, it will travel out roughly spherically from the point it was created in all directions at the local speed of sound, including backward.

    Randall Munro sums it up more amusingly than I can: https://what-if.xkcd.com/37/
     
    Last edited: Aug 6, 2016
  8. Aug 6, 2016 #7

    russ_watters

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    And what does it sound like? Playing back? No. Just very doppler shifted.
     
  9. Aug 6, 2016 #8

    andrewkirk

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    I don't think so. I suspect you are thinking of the Doppler shift for EM waves. The shift for sound is different in a number of important ways.
     
  10. Aug 6, 2016 #9

    Nidum

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    For anyone that has never heard a sonic boom :

     
  11. Aug 6, 2016 #10

    andrewkirk

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    @boneh3ad Awesome link! It's uncanny. I had originally drafted in the OP a quip about the song being Stairway to Heaven and that you would hear it telling you to worship Satan after the plane passed, but I took it out because I thought people might not take my question seriously.

    And then in your link, that delightful graphic, with the person hearing 'Worship Satan'.

    The link also covered something else I had figured out, which is that you would hear both forwards music and backwards music simultaneously.

    I haven't worked out yet why it says the backwards music would be rising in pitch. I thought it would be at a constant interval above the original pitch. I'll need to do some more calcs.
     
  12. Aug 6, 2016 #11

    russ_watters

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    Please explain what you mean in more detail: what do you think you will hear and why? What is wrong with my explanation?

    There is no vacuum behind a supersonic plane. The plane makes noise, so the air carries away the sound.
     
  13. Aug 6, 2016 #12

    andrewkirk

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    See link in post 6. Although I wasn't aware of that linked web page when I started the thread, it presents a good summary of why - to the extent that one could discern anything of the music - one would hear it backwards as well as forwards. Anything I wrote would just overlap it.
     
  14. Aug 6, 2016 #13

    boneh3ad

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    For one, it is possible to travel faster than the speed of sound but not the speed of light. Given that this is a question about a supersonic sound source, it doesn't have an EM equivilant since superluminal travel is not thought to be possible.
     
  15. Aug 6, 2016 #14

    nsaspook

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    If you listen closely you can hear the jet thrust pitch change as it moves overhead after the shock wave. It would take one really loud boom box to be heard over that.:biggrin:

     
  16. Aug 6, 2016 #15

    russ_watters

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    That's fine, but I don't see what it has to do with my post.
     
  17. Aug 6, 2016 #16

    russ_watters

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    Fair enough. I did miss that it can go backwards for a short time. But you mentioned Doppler shift as being an error in my post. The link mentions Doppler shift, though not by name. Just to make sure it is clear, there are several things happening and also some parts of the link that are misleading:

    1. Sound waves don't quite propagate in all directions from an object moving faster than the speed of sound, as implied by the diagram with the circles. The front quadrant (roughly) is cut off by the shock wave (they are part of the shock wave) and the airplane (to propagate forward with respect to the ground and backwards with respect to the plane, they would have to propagate through the plane). At mach 2, by a 30 degree wedge, that's 90 degrees cutoff.

    2. Because of #1, the "forward" propagation of the sound waves is limited by the mach cone. That means that the amount of backwards playback is similarly limited. By my calculation with some simple geometry and arbitrary altitude of 11,000 feet, you get 4 seconds of backwards and forwards sounds mixed together. After that, for as long as you can still detect it, you only get forwards sounds, doppler shifted.

    3. The link says after that, the sound would be heard progressing forward at half speed and an octave lower. That's not quite true: just like with a moving train or ambulance siren, the sound frequency changes as the angle between the object and observer changes. But it will eventually settle on that maximum doppler shift.
     
  18. Aug 6, 2016 #17

    JBA

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    The Concord video is interesting but due to its altitude doesn't have the same sensory impact as that demonstrated in one of the US Air Force air shows that I attended many years ago during which, while everyone is focused on the activities above the airfield, you see a fighter flying over Mach 1 pass directly overhead from behind totally silent until shortly after it passes its sonic cone (boom) hits you.
     
  19. Aug 8, 2016 #18

    mfb

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    You cannot beat the speed of light in vacuum, but you can be faster than the speed of light in a medium, where you get the same effects as we get with the supersonic jet. Plus Cherenkov radiation.

    Sound backwards and forwards.
    There is no limit on the time you get backwards sounds. The sounds played 1 minute before you hear the airplane have been played at a distance of about 2 "sound-minutes", so you'll hear them two minutes after the airplane passes you. Well, in theory, because hearing sound over ~40 km distance is impractical.
     
  20. Aug 8, 2016 #19

    CalcNerd

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    My opinion only: As the plane passes overhead, you will hear two sonic booms, one from the nose and the other from the tail. Usually, the plane is to short to actually distinguish these two booms. On a larger Concord or B-1 bomber, you could probably be able to hear these two booms. The second boom emanating from the tail of the plane destroys all sound information that could possibly be heard from the plane.
     
  21. Aug 8, 2016 #20

    mfb

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    Sound waves are linear to a good approximation, there is no destruction going on.

    You could indeed hear two separate booms from the Concorde. From Falcon 9 you can even hear three (engines, landing legs as they start to deploy, grid fins).
     
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