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I Sound source moving at the speed of sound

  1. Mar 21, 2016 #1

    Suppose that a plane that is travelling at the speed of sound has a loudspeaker attached to it. Along the he trip the loudspeaker releases specific sounds. What does the pilot ear when the first sound is released? And after the second sound is released, does he ear both sounds at the same time ?
    If someone is at rest in the ground, when the plane passes does he ear all the sounds at the same time ?

    Thank You!!!
  2. jcsd
  3. Mar 21, 2016 #2
    For simplicity, let's say that the plane is actually travelling a bit faster than the speed of sound.

    The pilot will hear the sound through the frame of the aircraft - without interference from the air stream that he is flying through. So he will hear the sound as it was played. Outside the aircraft, a shock wave forms at surface feature of the aircraft - and a modulating shock wave forms at the speaker. In theory, you could design and position the speaker so that it's output was strong enough to avoid colliding with those other shock waves. But even then, someone on the ground will only hear the sound made by the speaker at a particular instant. If he wanted to hear more, he would have to chase the plane and match it's speed.

    Realistically, some of the shock waves from the aircraft body will catch up to the waves produced by the speaker and merge with those waves. So what is heard on the ground is just the sonic boom.
  4. Mar 21, 2016 #3
    If i travel at the speed of sound and i say a word, the sound of that word will travel alongside me. If a minute later i say another word, now i have the sound of word 1 and 2 traveling with me. I presume that if i pass someone in the way, he will hear both words as i pass by. What am i seeing wrong ?

    Thank you already for your answer :)
    Last edited: Mar 21, 2016
  5. Mar 21, 2016 #4
    The expression 'speed of sound' is usually referring to the speed at which sound travels through air.
    It travels at a different speed through air depending on the temperature and density of the air.
    A ball park figure for average pressure and temperature at Earth's surface is around 1,236 km/h.
    Presumably when you utter your words you are doing so in enclosed space (let's assume a plane), the air inside the plane is travelling at the same speed as yourself.
    The sound of your words will propagate through air within your plane at it's normal speed, regardless of your speed in relation to the ground.
    Anyone else on board the plane would hear you speaking normally.

    If you are not in an enclosed space you will of course have great difficulty in saying any words anyway.
    However any sounds you do manage to make will be compressed to be a single shock wave, as experienced by somebody on the ground.
    They will hear some kind of sonic boom exactly as they would hear a boom when a supersonic aircraft flies overhead.
    Your word one and word two become indistinguishable contributions to the shock wave (boom) which is heard on the ground.
    Last edited: Mar 21, 2016
  6. Mar 21, 2016 #5
    Got it ! Thank you very much!
  7. Mar 21, 2016 #6
    No. The sound from your voice will travel in all directions. Some will travel directly forward, so directly in front of you, where the forward tip of the plane divides the air stream, you could imagine all of your words piling up as part of that shock wave. But the other parts of you voice are going to spread out from the plane - and fall behind. None of your voice will follow along side.
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