This is the Wikipedia definition of acoustic waves:
Acoustic waves are a type of energy propagation through a medium by means of adiabatic compression and decompression
Rayleigh surface waves (solid-surface-borne sounds) are a mix of transverse and longitudinal mechanical waves which propagates through compression/decompression, so they are acoustical by definition.
However, what about the transverse component alone of a solid-surface-borne mechanical wave (e.g. seismic S-wave)? While there are compression and decompression along the transverse component, there isn't any compression/decompression of particle in the direction of propagation (longitudinal). So I have 3 related questions:
- Do pure transversal mechanical wave exist in practice or is there always a small component of longitudinal wave that allows the propagation with compression/decompression?
- If it is possible to have no longitudinal component at all in mechanical pure-transverse waves, how do they propagate?
- I guess the wave could propagate through the friction of particles that belong to a different transverse plane. In this case, since the propagation is not directly performed through compression/decompression but via friction, could we say that pure transversal mechanical waves are not part of acoustics?
Some animations of longitudinal, transversal or a mix of the two help to think about it.