The gliding longitudinal wave contains three parts, longitudinal wave beam, negative exponential gliding ivro-tational wave and positive exponential gliding irrotational wave.
A little apart from the surf ace, the longitudinal wave beam may exist and propagate parallel to the surface, yet has to connect with its corresponding gliding irrotational waves.
It is the gliding irrotational wave pair, not the longitudinal wave beam itself that excites longitudinal wave in mud to generate signal.
The longitudinal wave cannot exist near the surface of a solid, and those exist near it are gliding irrotational waves.
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