JohnCockburn Posted May 31, 2020 Report Share Posted May 31, 2020 Change of plan. I'll answer the points here. David Beard: 4 hours ago, David Beard said: ??excuse me?? There is no simple relation between the pressure wave frequency and motion of individual molecules. Only the collective presure directly has the frequency. Individual molecules of the medium will move move in response, but not necessarily simply and directly tracking the frequency as pulses of the wave pass through. I didn’t mention individual molecules. Each individual molecule in the air is undergoing translational motion in random directions, with a spread of velocities given by the Maxwell-Boltzmann distribution. The root mean square speed of a nitrogen molecule at room temperature is about 500 m/s. What’s relevant for a sound wave isn’t this individual motion. It’s the collective displacement of all the molecules in an arbitrary “slice” of gas in the direction of propagation that’s important. The molecules are, of course all still undergoing random motion, but they all have collective oscillatory motion in the direction of the pressure wave propagation. It's a bit like the drift velocity in the direction of the current experienced by the randomly-moving conduction electrons in a metal. I don’t get what you mean by “only the collective pressure has the frequency” The acoustic pressure and the displacement oscillations are fundamentally linked. Displacement oscillations lead to local particle density fluctuations, which lead to local acoustic pressure fluctuations, as they must from the ideal gas equation of state. And all vice-versa of course, if you prefer. Quote Link to comment Share on other sites More sharing options...
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