# Does gravity affect sound on different mass planets?

Do gravitational forces affect how sound travels?

For example would two different planets with similar atmospheres, but greatly differing in gravity make any differences to how sound travels?

• I'm having trouble figuring out why anyone would communicate with sound rather than radio on any but heavily-terraformed planets, but other than that I guess this is a valid question. – Nathan Tuggy Feb 23 '16 at 0:11
• My thinking was that even if you communicate by radio you would still need sound to travel to the mic. – Terry Feb 23 '16 at 0:12
• Right, but unless the atmosphere is suitable for breathing, you're going to be using a suit, which will have enough air for the mic and headset to work, and the distance between mouth and mic is not going to be enough for much effect to show up. – Nathan Tuggy Feb 23 '16 at 0:16
• Large artificial habitats could have enough room inside for this to be relevant, though. – Nathan Tuggy Feb 23 '16 at 0:19
• Assuming the air was breathable and of similar composition to Earth's, greater gravity would result in a denser atmosphere, which would result in a greater speed of sound. – BillDOe Feb 23 '16 at 1:13

Yes and no. It's a yes in that gravity is often responsible for the density of the atmosphere, at least on a planet or gas cloud. Higher air density often means a higher speed of sound for the same composition of atmosphere, although there are other factors involved. It's a no in that gravity does not change the path of sound.

Say you had 2 identical habitats pressurized to 1 earth atmosphere at the same temperature and humidity levels. One habitat is on a 1G planet, the other is on a 5G planet. Sound would travel the same speed and reach the same point on the other side of the hab at the same time.

• Higher density does not mean a higher speed of sound. The speed of sound in an ideal gas is a function of temperature and the makeup of the gas only: $c_{\text{ideal}} = \sqrt{\gamma R T/M}$ where $\gamma$ is the specific heat ratio, $M$ is the molar mass of the gas, $R$ is the ideal gas constant, and $T$ is the temperature of the gas. – David Hammen Feb 23 '16 at 18:02
• Things are more complicated in a real (non-ideal) gas. If the air in those habitats is at 100% humidity, the speed of sound in the 0.5 atmosphere habitat will be considerably faster than the speed of sound in the 1.0 atmosphere habitat. If the air is completely dry, the speed of sound in the 0.5 atmosphere will be very slightly slower than the speed of sound in the 1.0 atmosphere habitat. Here's a nice graph that shows these effects: phy.mtu.edu/~suits/SoundSpeedFreq.gif . – David Hammen Feb 23 '16 at 18:06
• I take your point @DavidHammen, I was extrapolating from the pv = nrt. – GdD Feb 24 '16 at 9:14

For example would two different planets with similar atmospheres, but greatly differing in gravity make any differences to how sound travels?

That depends on what you mean by "similar atmospheres." If you mean similar chemical constituents, similar surface temperature, and similar surface pressure, then the answer is no. In an ideal gas, the speed of sound depends only on temperature and chemical constituents of the atmosphere. Pressure does come into play in a non-ideal gas.

How the speed of sound varies with pressure depends very much on the components of the non-ideal gas. With dry air, the speed of sound increases very slightly as pressure increases (and temperature remains constant). Water vapor makes this slight increase in the speed of sound with increasing pressure change into a decrease, a rather marked decrease with high humidity. That said, if the surface pressure is "similar", then there is no effect.

There is one significant effect in these similar atmospheres, and that's how the speed of sound varies with altitude. The dry adiabatic lapse rate (the rate at which temperature nominally drops with altitude outside of clouds) is proportional to g. Assuming similar atmospheres, the planet with a higher surface gravity will have a much higher lapse rate than the planet with a lower surface gravity. The speed of sound in air strongly depends on temperature, proportional to the square root of absolute temperate.

This means that the planet with the greater value of g will see a slower speed of sound at altitude, which in turn means sounds will attenuate faster (at least during daytime) on the planet with the greater value of g. A decreasing speed of sound with altitude refracts sound up, away from the surface. (Aside: On Earth, a temperature inversion often sets up at night. This has the opposite effect, refracting sound down. This is why you can hear a far-away train whistle at night but not during the day.)