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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??

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If we take the Wikipedia entry on mechanical waves then

a mechanical wave is a wave that is an oscillation of matter, and therefore transfers energy through a medium

The same reference states

There are three types of mechanical waves: transverse waves, longitudinal waves, and surface waves. Some of the most common examples of mechanical waves are water waves, sound waves, and seismic waves.

If we further take etymology of acoustics then term acoustic waves was used for waves that could be heard (from Greek 'akoustikos', meaning "of or for hearing, ready to hear".

If we follow these descriptions then:

  • a sudden rupture in the earth crust generates a seismic waves that is sensed by a geophone as seismic wave, but can very often be heard by a human as acoustic waves. The human does, however, not hear the rupture of the earth crust directly but the vibration of the earth.

  • picking a guitar string, generates a mechanical waves that propagates the guitar string, and the human can hear the sound generated by the vibration of the guitar string.

So, in sensu stricto acoustic waves should be only mechanical waves that propagate as compressional waves in air. In a wider sense, it seems acceptable to extend the term acoustic waves to all longitudinal waves that can be sensed by pressure sensors.

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  • $\begingroup$ Summarized to: acoustic waves are one type of mechanical wave, but there are other types of mechanical waves too $\endgroup$
    – LouR
    Aug 2, 2022 at 20:39

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