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Homework Help: Audio cancellation

  1. Jun 23, 2016 #1
    1. The problem statement, all variables and given/known data
    I created a 180 degree phase shifted version of a song using matlab. They cancel each other out when i play them together using any mixer software,but do not do so when i play the actual and the shifted versions from different sources.

    2. Relevant equations
    Is this possible.Can they cancel each other out in space?

    3. The attempt at a solution
  2. jcsd
  3. Jun 23, 2016 #2
    When you say, "I play the actual and the shifted versions from different sources", I take that to mean that you are playing each through a speaker and listening to the result.

    Assuming that is the case, the problem you will have is that you need to have both ears positioned an equal distance from each speaker (so that the phase and time delay are the same for both) and both must be of equal amplitude. Moreover, echoes created by the walls of the room and other items will still be heard - so an anechoic environment would be needed.

    This is not an easy set up to arrange.
  4. Jun 25, 2016 #3
    Thanks for the suggestion.
    I have been thinking of using a pipe and placing the two speakers on the ends and somehow keeping a microphone in the center.
    Will an arrangement like this be beneficial?
  5. Jun 25, 2016 #4
    If two waves are 180 degrees out of phase they won't cancel unless they are pure sine waves. If you have two opposite polarity waves (e.g. balanced audio signal) they would be in phase but still cancel each other out.
  6. Jun 27, 2016 #5
    Yes. You may want a T shaped pipe with sound pushed through the top ends and the mike at the bottom. By adjusting the volume, you should be able to get that to work. Perhaps not creating silence, but at least a partial but very noticeable cancellation.
  7. Jul 3, 2016 #6


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    Staff: Mentor

    Make sure you pass the two versions through identical amplifiers, otherwise there may be phase shifts you hadn't anticipated.

    If you were to perform your tests outside in the open air over an expanse of lawn I think reflections should not be a problem.
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