In my physics program, “acoustics” was referring to any mechanical waves that use local compression/rarefaction of medium particles to propagate, whatever the propagating medium (gas, liquid, solid).
- In air and most liquids, an acoustic wave propagates with a longitudinal component only (same direction as propagation); the transversal component does not exist because the air/liquid viscosity is not strong enough to move the air particles toward the perpendicular direction of propagation.
- In solid, the shear stress allows for transversal waves, in addition to the longitudinal waves. They are the same physical phenomena than longitudinal waves, i.e. compression/rarefaction of medium particles, but in a different direction than the propagation (see these very nice animations of pure-longitudinal waves, pure-transversal and longitudinal+transversal waves)
This seems the usual definition of acoustics and this encompasses many disciplines, including vibroacoustics, underwater acoustics, ultrasonic, noise etc.
Is there any other types of mechanical waves that do not propagate with alternating compression/rarefaction of medium particles (or does, see EDIT below), and should not be described as acoustical and why?
EDIT: First, I thought that water waves (not water sounds) would be easily not classified as *acoustical", but @WMXZ pointed that they are also compression/rarefaction of water particles (even if they are driven by gravity, contrary to acoustic waves). They are governed by different equations, their speed are slower than sound in water, etc. but I don't find the fundamental difference with acoustical waves. Water waves are not just low-frequency surface-borne acoustic waves that travel slower than underwater, are they??