Air particles have random zigzag motion. However,in a longitudinal sound wave in air the particles vibrate to and fro about their mean position in the direction of propagation of sound wave(ie, the air particles don't move along with the wave)

The question arises that if air particles show random zigzag motion throughout the space they can occupy(as per the Kinetic theory of matter), then how and why air particles don't leave their mean position in a longitudinal sound wave? OR Have I misunderstood some concept?


1 Answer 1


Have I misunderstood some concept?

Visualising what a sound wave is on a molecular level can be difficult.

You can think of air molecules as being under the influence of local fluctuations in pressure (differing concentrations of air molecules) superimposed on a random (thermal) motion and via the fluctuations in pressure energy (and momentum) is transferred.

So there is no net transfer of air molecules due to a sound wave rather an increase and a decrease in their concentration in a region which averages out to a zero change.

There is no one molecule oscillating about a mean position rather think of it as the position of the centre of mass of a group of molecules contained in a very small volume and that centre of mass oscillating about a mean position but noting that the molecules which are contained within that small volume are continually changing due to the random motion of the molecules.


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