Yes, it's actually two sine waves with a different base frequency and a random offset, one for the large waves, the other for the smaller waves.
As their offset changes, the whole wavefront stays still, then it grows up to a random level which can get higher and higher as the levels progress!
The most interesting part of the wave is the way it's drawn by picking random columns of pixels from a very tiny palette. I'm quite proud of how it came out!