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Sound as Sine Wave

  1. Nov 23, 2011 #1
    Hi, easy one but can't really get my head around it. The physics teacher today showed sound as Sine wave on an oscilloscope. If I was actually listening to this sound wave, where would it be silent, on the x axis or at the trough? Please explain.
  2. jcsd
  3. Nov 23, 2011 #2


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    Sound is dependent on the amplitude, so it is zero on the axis.
  4. Nov 23, 2011 #3


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    If you are listening to it, you are "hearing the sound" with your brain. Not with your ears, and certainly not with a scientific measuring instrument! For almost all the frequency range where you hear anything at all, the loudness you hear will be constant.

    On the other hand it we are talking about measuring what happens to the air wth scientific instruments rather than "what you hear", loudspeakers are designed so the position of the moving cone follows the voltage graph that you see on the oscilloscpe. (But a real loudspeaker isn't perfect, and won't follow it exactly). The change in air pressure, close to the speaker, is zero when the cone is at its mid point. However the velocity of the air close to the speaker is zero when the speaker is at the max and min displacements.

    But further away from the speaker, there is a "time delay" because the speed of sound is finite - about 340 m/s so the peaks and troughs of the pressure and velocity further away from the speaker "lag behind" the movement of the speaker by an amount that is proportional to the distance away.
  5. Nov 24, 2011 #4
    It wouldn't be silent anywhere. It would only be silent if there was no wave at all; just a flat line. At the x-axis the wave actually has the highest rate of change. The rate of change is zero at the peak and the trough but even then there would still be sound.
  6. Nov 24, 2011 #5
    Thanks everybody
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