Acoustic waves dissipate their energy into non-acoustic (molecular) degrees of freedom. In the absence of matter, I think there are no other degrees of freedom

Acoustic waves dissipate their energy into non-acoustic (molecular) degrees of freedom. In the absence of matter, I think there are no other degrees of freedom for gravitational waves to dissipate into, unless they are strong enough to form a black hole (which they can do under extreme conditions, I think). (This is waaaay outside my expertise though, so take my answer with a grain of salt. Everything I've said in this thread is basically based off the rudimentary understanding from taking a GR course in grad school. I've never done research on this topic.)