A hydraulophone is a tonal acoustic musical instrument played by direct physical contact with water (sometimes other fluids) where sound is generated or affected hydraulically.

A single disk, shaft, or valve assembly can have rings or passages with different numbers of holes for different notes.

[3] Blocking water from coming out of a given hole produces a given note, which, in some embodiments, is electrically amplified by a hydrophone.

[9] Some hydraulophones include an underwater hydrophone pickup to allow the sounds produced by the water to be electrically amplified.

Moreover, the instrument provides the unique capability of polyphonic embouchure, where a player can dynamically "sculpt" each note by the shape and position of each finger inserted into each of the mouths.

Whereas the piano and organ both have a similar kind of keyboard layout, the response ("key action") is different.

(12 sets of dots typically made from brass pins above each finger hole) Whereas park and pool hydraulophones are usually 12-jet diatonic, concert-hydraulophones are usually 45-jet chromatic.

An example of a theme-specific hydraulophone is the Aquatune located at the main entrance to the Legoland waterpark in Carlsbad California.

In this kind of hydraulophone (e.g. balnaphone, from the Greek "balnea" meaning "bath") the hydraulist is immersed in the hydraulic fluid used by the instrument.

The WaterHammer Hydraulophone uses the water hammer principle for hard-hitting percussive musical notes.

[13][14] Presently the world's largest hydraulophone is located in the outdoor plaza of the Ontario Science Centre, one of Canada's landmark architecture sites.

It is also Toronto's only freely accessible aquatic play facility that runs 24 hours a day.