# Sound waves: why do air molecules oscillate?

#### vela

Staff Emeritus
Homework Helper
You are confusing intra-atomic (within an atom) with inter-atomic (between atoms) effects.
No, I'm not.

#### jtbell

Mentor
So, I mean: these (elastic) collissions, like here,

are really "electromagnetic interactions", the molecules "pushing each other away" are really the electrons of the molecules repulsing each other by rheir electromagnetic fields, right? But do you explain that to (16 year old) students, or do you stick with the "direct contact marble model"?
I don't think the exact nature of the interaction matters, if the range of the interaction is short enough (much shorter than the average distance between molecules), and the collisions are elastic.

#### hutchphd

Have you taken into account the fact that the air molecules are already bouncing around MUCH faster than the speed of sound?
This is surprisingly not true. For an ideal gas$$\frac{v_s}{v_m}=\sqrt{\frac{\gamma}{3}} \qquad ,$$where ${v_m}$ is the rms speed and ${\gamma}$ is the ratio of specific heats

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#### Drakkith

Staff Emeritus
2018 Award
This is surprisingly not true. For an ideal gas$$\frac{v_s}{v_m}=\sqrt{\frac{\gamma}{3}} \qquad ,$$where ${v_m}$ is the rms speed and ${\gamma}$ is the ratio of specific heats
That is surprising. I thought they'd be going faster than roughly the speed of sound at room temp. That's what I get for not doing the math myself.

#### Delta2

Homework Helper
Gold Member
Have you taken into account the fact that the air molecules are already bouncing around MUCH faster than the speed of sound? It's basically an oscillation of a bulk behavior, like how electric current is the slight net motion of a chaotic mix of electrons moving in random directions in a conductor.
I wasn't talking about the microscopic thermal motion of the molecules, i was talking about their macroscopic net speed which i believe it is also oscillating when the pressure is oscillating.

#### sophiecentaur

Gold Member
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

Mentor
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

Mentor
Gold Member
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

Gold Member
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).

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