Sound waves: why do air molecules oscillate?

[sophiecentaur]

The air molecules aren’t oscillating, the air pressure is oscillating.

The molecules have a huge range of velocities; it's just the average of all the molecules' motions in a local region that's the density (pressure) and motion in the macroscopic sense.

 

[Nugatory]

The molecules have a huge range of velocities; it's just the average of all the molecules' motions in a local region that's the density (pressure) and motion in the macroscopic sense.

Yes. We plot the pressure at a point and we get a nice sinusoidal function of time, but that is a macroscopic phenomenon that emerges from averaging the much less orderly movement of the molecules. That orderly sinusoidal pressure oscillation does not imply a similar oscillation of the air molecules; they aren’t swaying back and forth in unison like a line of dancers with locked arms.

 

[anorlunda]

Yes. We plot the pressure at a point and we get a nice sinusoidal function of time, but that is a macroscopic phenomenon that emerges from averaging the much less orderly movement of the molecules.

Electric current is an even stronger analogy. The electrons move in every direction and bounce off the atoms, but the average of all their motions gives the direction and magnitude of current. Even in AC current, the electrons do not wiggle back and forth in unison.

 

[DrDu]

Actually, I think the marble analogy is quite a good one. There is this famous model of balls in a container whose bottom vibrates rapidly so that the balls jump around wildly. Would be interesting to see whether you can even simulate a wave when also one of the side walls is made to oscillate albeit more slowly.

 

[sophiecentaur]

Actually, I think the marble analogy is quite a good one. There is this famous model of balls in a container whose bottom vibrates rapidly so that the balls jump around wildly. Would be interesting to see whether you can even simulate a wave when also one of the side walls is made to oscillate albeit more slowly.

There are some good school models of kinetic behavior in gases but there are also some really poor ones with motors and rubber diaphragms which clatter a lot but their ‘wave’ patterns are mainly in one’s imagination. I suspect that small ball bearings in a container, excited and kept up by a high power piston / actuator using pink noise, would work well. But it would cost more than a budget level school demo.

 

[cjl]

Actually, I think the marble analogy is quite a good one.

Yes - you can treat gas molecule interactions as perfectly elastic hard-sphere collisions and you arrive at pretty much exactly the correct answer (so long as you appropriately scale the sphere radii in your model).