# Why does sound move at it’s current speed? For example in air

I know that the sound is vibrations (pressure?) between the atoms. But i would like to know why does it move at that speed let’s say in the air. Why not faster or slower. I think this has to do with the interactions between individual atoms. But i really wonder the deep, fundamental explanation.

If you could give me that picture, i really appreciate it. Thank you.

How do molecules in air communicate with one another?

They do it by collision and the time one molecules takes to collide with another molecule and hence convey information is dependent on the speed of the molecule.

So it might be no surprise to you that the speed of sound in air at room temperature is approximately $340 \, \rm m\,s^{-1}$ and the rms speeds of oxygen and nitrogen molecules are $480 \, \rm m\,s^{-1}$ and $520 \, \rm m\,s^{-1}$ respectively ie the same order of magnitude.

Also as the temperature increases so does the rms speed of the gas molecules and the speed of sound in the same proportion.

• So it is just because of the rms speeds of molecules in the air right?
– Tuna
Commented Oct 28, 2017 at 10:13
• "It might be no surprise..." I don't follow the logic behind that. The speed of sound in both solids and fluids depends on the elastic modulus and the density. The elastic modulus and density of a solid have nothing to do with the rms speed of the solid molecules, which is zero, of course. Sinilarly, the propagation speed of an EM wave in a conductor has nothing to do with the drift velocity of the charges in the conductor- it is many orders of magnitude faster than the drift velocity. Commented Oct 28, 2017 at 10:31
• @alephzero My answer was relating to sound waves in gases as that is what the OP’s question was about. Commented Oct 28, 2017 at 11:14
• I did not only mean to get answers on gases. It was for an example. I wonder about the sound of wave in every material.
– Tuna
Commented Oct 28, 2017 at 19:38

I think it might be easier to understand in a solid rather than a gas. You can think of a solid block of, say, aluminium, as atoms in a regular pattern connected by linear springs (this is usually valid for small amplitudes of oscillations). A very simplified version of this would be just a linear chain of balls connected by springs. If you now push the first ball towards the others, the first spring will compress and push the second one, which will push the next and so on. As you can imagine, the speed at which this happens, depends on the stiffness of the springs. Softer springs make this process slower, harder ones faster.

In gases and liquids it is a little more involved, because the molecules are not bound to an equilibrium position, but I think to get a grasp of what is going on, this picture can help.

• Thank you for answering. But isn’t there a deeper explanation, say, involving the electrons?
– Tuna
Commented Oct 30, 2017 at 8:31
• The way atoms push on each other is a quite complicated topic I can't go into here. But for small amplitudes they can always be assumed as linear springs (from their Taylor expansion). The strength of the springs, however, is subject to microscopic electron interaction, for which I cannot give you an intuitive answer.
– noah
Commented Oct 30, 2017 at 9:00

This is answered by thermodinamics. Basically it depends on the interactions between particles, and more interaction means more speed. Thats why it also depends on temperature and other factors. Consider the sound as a specific interaction. If there are more interactions the sound is transmitted faster.

But that not answer the fundamental question: why is this speed and why is not faster or slower. This is a question in all physics and for the moment I think is just philosophy (we know, for example, the speed of light, but we dont know why is $3\cdot 10^8 m/s$, for example).

• Well, the speed of light is a speed limit for massless particles and why it is 3 . 10^8 m/s is beyond our current understanding. But since the sound is just a consequence of electromagnetism, i believe there is a fundamental understanding of it.
– Tuna
Commented Oct 28, 2017 at 10:09
• Yep, that's why I answered you on the first paragraph that it is only based in the number of interactions between the particles. Commented Oct 28, 2017 at 21:25
• Thank you for your answer. But that’s what i want to learn in the first place. Why they interact the way and the speed they do. Fundamental reason.
– Tuna
Commented Oct 28, 2017 at 21:32