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Why no noise is added when we hear someone's voice from the other side of a wall?

  1. Feb 3, 2012 #1
    We know that when sound travels through solids like a wall, the air molecules interact with solid molecules in porous medium to transmit the information hidden in sound wave.The question is that why no noise is added to someone's voice when we hear it from other side of a concrete wall ?
  2. jcsd
  3. Feb 3, 2012 #2


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    Staff: Mentor

    What noise do you think should be added? I don't understand the question.
  4. Feb 3, 2012 #3
    Same reason you don't feel the minute vibrations of the particles in the air. Pressure is a very 'macro' thing, it's averaged over the many particles taking part in the interaction.

    If that's what you meant.
  5. Feb 3, 2012 #4
    And why don't we see the sound waves?
  6. Feb 3, 2012 #5


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    Depends what you mean by 'noise'. If you are referring to some characteristic like a 'loss of information:signal ratio', then noise is, indeed, added because some frequencies will be attenuated and you will get dispersion across the spectrum (some frequencies will get to your ear at different times wrt each other than per the original signal).

    If you were to do this as an experiment with objective measurement gear which outputted (by amplification) a signal of the same magnitude with or without the wall in the way, you would, indeed, get more noise.

    What you maybe asking, then, is why does your ear not hear extra noise? The answer is that your ear-brain system is a fantastically complex Fourier sound analyser that can accommodate all these variables and differences in signals, and presents you with a perception of what you are hearing.
  7. Feb 3, 2012 #6


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    Simply, the displacements (that is, the degree of longitudinal compressions) caused by sound waves of audible level are too small to be seen.

    If you watch a [video of a] detonation of explosives from a distance, you can often see the percussive wave quite clearly.
  8. Feb 3, 2012 #7


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    Distortion is added due to frequency-dependent attenuation of the person's voice. No noise is added, but a lot of the signal is attenuated.

    Actually, I shouldn't say that no noise is added, because the resonance of wall might contribute to the sound that you hear, but essentially, the wall attenuates the voice.
  9. Feb 3, 2012 #8
    Sometimes you do, although in this example of the video of an ATLAS V rocket as it breaks the sound barrier and passes through a cloud layer, they are shock waves:

    Last edited by a moderator: Sep 25, 2014
  10. Feb 4, 2012 #9
    Hello arashmh

    Try this (thought) experiment.

    Stand with you friend. one on each side of a wall.

    Now before your friend speaks or makes a sound, what do you hear?


    If then, as you say, your friend makes a sound or speaks you can hear this in addition to whatever sound (noise) was there before.

    But there was silence there before!

    This is a demonstation of the fact that noise averages out to zero, whereas definite sounds or speech do not.

    Does this help?
    Last edited: Feb 4, 2012
  11. Feb 4, 2012 #10
    I had the same thought when I first saw this question. But try shouting through a sheet of paper and you will hear plenty of nonlinear distortion, or "noise". This is why I think Cmb's answer above which starts out "Depends what you mean by 'noise'", is so excellent.
  12. Feb 4, 2012 #11
    Yes you (and cmb) are right there are many 'definitions' of noise and I was only answering the most obvious one about acoustic background noise.

    go well
  13. Feb 4, 2012 #12


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    The important issue here is the.signal to noise Ratio. If the wanted signal is reduced (attenuated) then any noise which adds to it after that will reduce that SNR. Adding gain will not help the situation.
    Noise due to random thermal motion in the concrete will degrade the SNR but only by a tiny amount. BUT no amplifier / sensor is noiseless so the overall SNR will be significantly degraded.
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