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I'm not sure if this is a stupid question. I've been considering the deviation from the equilibrium reflects the air pressure, with larger deviations reflecting higher air pressure. But in Reetz and Jongman's Phonetics, it's said that one reason to use RMS amplitude is to reflect the fact that more energy is required to move the air molecule further away. This reason seems to assume that waveform deviation also reflects the air molecule movement. My question is, does larger deviation necessarily reflect the further movement of the air molecule, or, is the higher air pressure must be derived from the further movement of the air molecule?

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  • $\begingroup$ well... I want to know what the deviations of a waveform mean in terms of air molecules. $\endgroup$
    – mooninnap
    Mar 6, 2018 at 2:56
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    $\begingroup$ In other words, what is the relationship between the acoustic waveform and displacement of air molecules? When you blown on a feather, why isn't that incredibly loud? $\endgroup$
    – user6726
    Mar 6, 2018 at 5:25
  • $\begingroup$ As I continued reading, I found in a later chapter the book circumvents the issue of the relationship between the waveform deviation and the air molecule displacement by stating that "large deviations from the atmospheric air pressures are much more prominent than small deviations". Given @jknappen's answer, I suppose the relationship between the waveform deviation and the air molecule displacement is trivial. $\endgroup$
    – mooninnap
    Mar 8, 2018 at 18:13

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The Mean Free Path of a molecule in the air at normal pressure and temperature is about 70 nm which is very short compared to the wavelength of typical sound waves (34 cm at 1 kHz). Therefore the motion of single molecules is not affected by the fact that a sound wave is propagating through the air.

What really propagates is a small difference in air pressure.

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The answer by jknappen gives the reason why one can treat are as a continuous medium instead of trying to calculate movements of molecules. One usually regards sound pressure. But there is also a longitudinal displacement of volume elements of the gas. It is less convenient to use displacement because it is further at low frequencies for the same sound level..

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