A hydraulophone is a water-based musical instrument that is played by pressing fingers against water jets to force water through or against an array of underwater whistles or similar sound-producing devices.
The world's largest hydraulophone is a large fountain that is the main centerpiece out in front of the Ontario Science Centre in Toronto, Ontario, Canada. It runs 24 hours a day so that anyone can play on it anytime.
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.