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Sound frequency dissapate's, By how much?

  1. Apr 12, 2012 #1
    Hi there, im currently got a little project in my head, and though im good at science i dont know enough physics to work this solution out and i was hoping someone would be able to help.



    Now i know sound wave diminish over time, but by how much and how fast,

    here my problem if i was to emit a sound from a location at so many cycles a second, 0 hertz, 50 hertz, 1000 hertz whatever, how quickly in respect to distance and fequeny could i expect that sound to dissapate over say 50 meters. and would the frequency have changed in that fifty meters.

    so say i emitted a sound at (for the sake of easyness) 1 hertz to an reciever (ear/mic whatever) 50 meters away would the sound still be at 1 hertz when it reached the reciever?
     
  2. jcsd
  3. Apr 12, 2012 #2

    haruspex

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    The frequency will not change unless the emitter and receiver are in relative motion. Think about it: the emitter is generating so many cycles a second; if the receiver hears less than that number per second, where are the other cycles going? They must be building up in the gap in between.

    The power loss with distance, in a 3-d medium, is an inverse square law. As each wave front spreads out, the area it has to cover grows as the square of the distance.
    If confined in a tube, the loss will be only what's absorbed by the tube. That probably makes the power vary with distance as a negative exponential.

    To be clear, the decay is with distance, not time. On a day when the speed of sound in the air is a bit lower, it will still decay the same amount for the same distance, thought it take longer to get there.
     
  4. Apr 13, 2012 #3

    Bobbywhy

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    Kahlenda, Welcome to Physics Forums. Here is a place where some members are higly educated, some are highly experienced, and some, both. Everyone here is ready and willing to assist with your progress in learning science.

    Acoustic engineers study sound in High school physics, then in College physics, and then in their work they continually study and learn more during their whole working career. You seem to be at the beginning of the process. Welcome!

    haruspex has answered your questions: emitted frequencies will not change over distance, and the sound power level (intensity) does drop off with the square of the distance. In air or water sound waves are said to radiate outward by "spherical spreading"...imagine a sphere of pressure waves expanding in all directions.

    Just like any area of science you need to build a good "foundation" of the basics. I suggest you visit this Wiki page, study it completely, and then check out interesting references near the bottom of the page.

    http://en.wikipedia.org/wiki/Acoustics

    Enjoy, Bobbywhy
     
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