# Sound and gravity

How would heavier gravity (or lighter gravity) affect sound in terms of what I hear?

For example, say there is a planet that is capable of supporting human life, but it has heavier gravity than Earth. Would things sound deeper? Higher? Or would sound mostly just get from point A to point B faster/slower?

Thanks for satisfying my curiosity.

• Sound velocity does not depend on pressure, if that is what you are asking about. – Pieter Dec 20 '18 at 7:21
• No, I'm more interested in what changes a person might notice. Helium being lighter than oxygen makes our voices higher, but what if it's something less direct than a different gas? – ratgirl34 Dec 20 '18 at 7:58
• Actually there is not a real direct influence of gravity. As you can see from the answers what can come of importance are P and gas composition. But it is correct that the gravity of a massive earth will nfluence both. – Alchimista Dec 20 '18 at 11:43

Surprisingly for an ideal gas the speed of sound depends only on that temperature and the molecular weight. The equation is:

$$v = \sqrt{\frac{\gamma RT}{M}} \tag{1}$$

You may have seen the speed of sound written in terms of the pressure and density:

$$v = \sqrt{\gamma\frac{P}{\rho}} \tag{2}$$

but you can show that equations (1) and (2) are equivalent. I go into this in my answer to Estimating the current speed of sound but briefly at constant temperature the pressure and the density are proportional so the ratio $$P/\rho$$ is a constant.

So assuming the heavier planet has the same atmosphere as Earth, and that we can treat the atmosphere as an ideal gas, the speed of sound on the heavier planet would be the same as the speed of sound on Earth if we measure at the same temperature. That means sounds will be the same on the heavy planet and on Earth.

Giorgio points out in his answer that non-ideal behaviour of the gases in the atmosphere will change this, but only at pressures too high for us to live there.

• You said "molecular weight" not "molecular mass" which in the context of this question is perhaps a little unfortunate ... – Andrew Steane Dec 20 '18 at 14:01
• There are explicit density dependent effects in acoustics. These are common in underwater acoustics. – ggcg Dec 21 '18 at 0:13
• Okay, so for the sound to have any significant change there would need to be a different gas or massive pressures. Thank you. – ratgirl34 Dec 21 '18 at 7:11