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Question about sound.

  1. Feb 18, 2014 #1
    Okay, so I was listening to someone via tinychat explaining how our current understanding of sound was wrong in some sense. He claimed that the source of the propagation of sound waves HAD to be moving faster than the speed of sound (in air).

    One particular analogy he used in order to explain his argument is that the frequency of a rope was directly dependent on the motion of your fist moving the rope in a wave-like manner (which is true). Yet, he extended this analogy with the snap of your finger, in that the interactions between your thumb and ring finger moved faster than the speed of sound in order to produce the sound in the air. Well in air, sound waves 'travel' ~340.2 m/s or something like that...
    I had a problem with his claim, because I could slam my fist very slowly against a table much slower than the actual speed of sound, and still produce waves through the air.

    He kept claiming that every source producing these waves had to make a sonic boom, because it dictates the frequencies of energy propagated.

    I'm not a physicists, but should this claim be taken seriously? Or simply ignored?
    Thanks for your time.
  2. jcsd
  3. Feb 18, 2014 #2


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    not sure about the snapping of the finger and thumb ... possibly producing a supersonic velocity ?
    am sure some one else will confirm that

    But I do know of one example of a supersonic snapping ... did watch a video on marine life in the last 12 months
    where a shrimp type animal could snap its claw fast enough to produce a supersonic pulse of sound and water to stun its prey
    ( will have to do a google search and see if I can find the reference)

  4. Feb 18, 2014 #3
    Conducted some brief research, so I think you're talking about the Pistol Shrimp. In this case, I'd think the claws would absolutely be moving faster than the speed of sound (supersonic like you pointed out) in order to produce a temperature hotter than the sun in the water. In fact, the force of the claw is probably moving faster than the sound produced under the water..however I'm not too sure about that.

    Yet, for the user I was talking to, who tried to expand this case across ALL effects of the production of sound is extremely extraordinary in my opinion. Based on my conversation with him, I think that HE thought that sound was simply limited to what we hear, as opposed to what sound actually is.
  5. Feb 18, 2014 #4


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  6. Feb 18, 2014 #5
    That's interesting! This is why I love physics...so much to learn. Alright, I'll have a look at this article while simultaneously learn more about sound and give a response..thanks for the link!
  7. Feb 18, 2014 #6
    Yes, you can produce sound by having something move with supersonic speed. The most common example is the noise made by a whip.

    But the supersonic speed is not necessary.
    Most sound sources have parts moving with subsonic speeds and they do not produce sonic "booms". They just produce acoustic waves.

    It's actually pretty easy to estimate the speed of various vibrating membranes and other sound source.
    If you have a tuning fork producing a sound of 1000 Hz, the amplitude of the tip is of the order of 1 mm.
    The speed will be of the order 2πfA where f is the frequency and A the amplitude.
    So for the above values we'll have a speed of about 6 m/s. Not supersonic at all.:smile:
    Last edited: Feb 18, 2014
  8. Feb 18, 2014 #7


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    Although sonic booms do produce audible sound waves, sounds can emanate from other sources. I don't think anyone can reasonably claim that a human voice is caused by parts of the anatomy travelling faster than Mach 1.
  9. Feb 18, 2014 #8


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    Just consider your vocal cords . . . . .
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