Can sound fall?

D H

Wrong. Take away the heating from below and cooling from above and you get an isothermal atmosphere. Imagine a tall, thermally isolated, gas-filled cylinder that is oriented vertically in a gravity field. The condition that maximizes entropy is isothermal conditions, even with that gravity field. An atmosphere with a non-zero lapse rate would allow violations the second law of thermodynamics. The reason we get a lapse rate in our real atmosphere is that it isn't an isolated system (it is heated from below, cooled from above) and because it is far from the equilibrium state. It doesn't have a chance to reach that equilibrium state.

K^2

You don't need heating from bellow cooling from above, still. Yes, you need mixing. But uneven surface heating is sufficient. In fact, day/night cycle is sufficient, because that already produces convective currents. Heating AND cooling can both take place at the surface.

Like I said, adiabatic model matches the actual atmospheric temperature. You don't need any net heat flow through atmosphere, which is, in fact, very minor compared to heat exchange at the surface.

And yes, this would be a violation of thermodynamic laws if there was no energy input. But there is. You have the Sun driving the convection currents, which basically means you have an AC cycle in atmosphere. Power for it comes from Sun, but the PV changes come from gravitational potential. You can't have an atmosphere with this temperature profile without gravitational potential.

Ken G

The reason the atmospheric temperature gradient equals the adiabatic lapse rate is not because of gravity, it is because of the greenhouse effect. The greenhouse effect attempts to produce a temperature gradient that exceeds what is convectively stable, so convection appears. The convection involves buoyancy, so gravity, but it is not there inescapably because of gravity-- as D H said, it is easy to imagine a situation where you have gravity but no convection. Just take away the greenhouse gases! It is as we have been saying-- there is just no direct or inescapable connection between the presence of strong gravity and the presence of a changing sound speed, nor is there a requirement that temperature must fall as you go up (it doesn't in some layers of the atmosphere, in fact), nor is there a requirement that the sound speed must fall as you go up against gravity. Our own atmosphere proves all these points.

ImaLooser

I always wondered about sound inside neutron stars, which are extremely conductive of sound. I think there would be gravitational lensing of sound. You could say that that was sound falling.