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Sound from a hydraulophone (underwater or water-filled musical instrument) where the sound originates from vibrations in water.
Dolphins and porpoises do not produce hydraulophonic sound, i.e. they breathe in air from the surface, and use the air to "originate" the sound which subsequently travels through the water. Divers signal each other by banging rocks together which produces idiophonic sound that subsequently travels through water. But hydraulophones make sound hydraulophonically, such that when played underwater, the sound originates in the same medium in which it subsequently propagates.
by Hydraulist February 14, 2012