The velocity of a density wave may change, for instance it may slow down in a way that it would be impossible for an ordinary wave.

Lusenzo, 2013-08-26 u1606; spatiole B u1631; moon declination N 15°13; Moon distance km 393800; Eq t +1'40; moon angular velocity 126.78.4 deltins/hr; variation zero. Water level: cm 45.