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Sound waves question

  1. Apr 1, 2012 #1
    Hey guys, I have an average-man knowledge in physics so please simplify your question so anyone could understand :) I had no place for this so I'm writing it here in the general forum.

    I asked a friend of mine if you can supress sound waves entirely, so you won't hear a thing. to keep things short, he answered but I thought of something else:

    I thought of when you hear music with headphones for example and can't hear your surroundings. well, what if you did the same thing, only with sound at frequency that humans can't hear? you won't hear anything.

    I asked my friend, he told me that the sound from your surroundings aren't getting "canceled" - so it won't work, so I told him that if it's a wave, a stronger wave should completely runover a weaker wave, just like at the beach/sea. so we both don't know why it doesn't work. I'm hoping to find an answer here.

    What I tried:
    I'm a programmer so I built a quick program to generate those waves.
    I turned on the TV and played music in my headphones to make sure the volume is high enough not to hear anything from the TV.
    So I (turned the music off of course and) played a sound with a frequency that's out of the ear's range, and I really couldn't hear it but I did hear the TV (and so it failed).

    Could you guys explain to me please why it didn't work? (to be honest, I wasn't sure at all that it will work, but had to give it a shot. Thanks!

    By the way, just to make things clear:
    my goal was not to hear the TV + nothing at all! the wave (which I can't hear) would "block" anything else, just like the music did.
    Last edited: Apr 1, 2012
  2. jcsd
  3. Apr 1, 2012 #2


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    Don't have a direct answer to your question but you should check out the Bose noise-cancelling headphones. They do NOT cancel out anything like ALL of the ambient sound, but they come close (it's quite remarkable really) and I was told by the salesman (no idea whether this is true or not) that they could have made them even better at the cancellation but that it was considered a bad idea for marketing and legal reasons (for example, if someone wore them while driving in traffic and couldn't hear other car horns it could be dangerous).
    Last edited: Apr 2, 2012
  4. Apr 1, 2012 #3
    I can't imagine they produce a sound like I was takling about, plus I googled and by the looks of it, they're just really tight (don't know how this sells anyways, must be so goddamn hot inside them).

    But thanks for your reponse.

    By the way, just to make things clear:
    my goal was not to hear the TV + nothing at all! the wave (which I can't hear) would "block" anything else, just like the music did.
  5. Apr 1, 2012 #4


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    They are not tight at all. They produce sound with the opposite phase from the ambient sound so that it cancels. If you hook it up to an iPod, for example, you hear the music and nothing else. If someone speaks loudly right next to you, you will be able to hear it but it will not be loud. These are by far the best noise cancellation headset on the market and Bose is a top quality stero company that commands premium prices.
  6. Apr 1, 2012 #5


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    Comparing acoustic pressure waves with water waves on a beach has a limited value. They have only a few characteristics in common and drawing conclusions about sound based on water waves may be misleading.

    Active noise cancellation headphones like the Bose models operate by

    a. Receiving ambient sound
    b. Inverting that signal (phase shift by 180 degrees) and
    c. Transmitting that result to the listener.

    Since the generated sound waves are exactly out of phase with the ambient waves, when they mix near the listener’s ears they totally cancel each other due to destructive interference. The result is to completely remove that ambient sound.

    In your experiment, ultrasonic sound is, by definition, sound above the human hearing threshold. So even if you generate it and then transmit it from your headphones you could not hear it. Nor will it block (cancel) any other sound like the TV because there is no phase cancellation.
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