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Air is a gas, then how is sound wave propagation possible? I mean, gas particles have a tendency to travel in a straight line, so how does a sound wave occur via compression and rarefaction? Most textbooks model this propagation by means of projecting gas molecules as seemingly bound, and oscillating harmonically about their original positions(barring damping). Shouldn't a gas particle leave it's position as soon as mechanical force acts in it,rather than oscillating? A gas ideally should not possess elasticity as portrayed per se?

My question being, how correct is this demonstration of sound wave propagation as a transverse wave one? Do textbooks(the few that I have read), skip the real mechanism, or my assumption is wrong. Could someone please clear this up for me.

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  • $\begingroup$ Exactly what I thought. In the animation, it seems like all air particles are bound together as well as to the source of the energy. $\endgroup$ – funct7 Nov 30 '16 at 1:12
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A sound wave is a longitudinal wave - that is, when the membrane of a loudspeaker moves towards the air, it causes compression by pushing molecules towards the stationary air in front of it. That briefly raises the pressure, while those stationary molecules are accelerated and in turn push against the molecules in front of them, etc. So it is a bit like a "chain reaction" or, if you like, a rear-end collision in a traffic jam. Except that the collisions are elastic, so things "bounce back" as well.

An animation may show the local pressure "going up and down", but really, it you consider a sound wave traveling from left to right, then the air molecules are also moving left to right and back again - not up and down.

This is nicely explained with pictures on this web page , in particular this animation. I created a downsampled version of the animation (to fit the 2 MB limit of the site):

enter image description here

but I highly recommend looking at the original on the site. Clearly, the molecules move in the direction of the wave propagation (longitudinally).

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    $\begingroup$ The elastic part is what I want to clear up. The gas particles are not fixed, so they ought to 'run off',as soon as a pressure wave reaches them,shouldn't it? $\endgroup$ – Abhinav Mar 8 '16 at 3:50
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    $\begingroup$ When a bunch of gas particles come towards them they feel that pressure. Imagine trying to walk against a crowd going the other way - if a lot of people run at you quickly you will get knocked from time to time. That is how the gas pressure is transmitted. $\endgroup$ – Floris Mar 8 '16 at 4:02
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Consider that the amount of energy in the wave front overwhelms the energy in the particles ambient movement. The wave pushes the particle forward, perpendicular to the wavefront with others to create an area of high pressure, but as soon as the wave passes, the particle is sucked back into an area of low pressure that follows the wave front.

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  • $\begingroup$ But since the low pressure is not reserved for the particles that left the space, couldn't other particles fill that place also? Clearly that's not the case, so I know my understanding is a little off. Just trying to get a clearer picture. :) $\endgroup$ – funct7 Nov 30 '16 at 1:13
  • $\begingroup$ There is an underlying assumption here that this wave is by far the most energetic system in the area and thus controls the all the particles locally. Interference from other energetic events would create more complex particle patterns. $\endgroup$ – Seiscat Dec 4 '16 at 15:33

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