The term may be applied based on the interface used to play the instrument, in which a player blocks the flow of water through a particular hole in order to sound a particular note, or based on a hydraulic sound production mechanism. Hydraulophones use water-flow sound-producing mechanisms. They have a user interface, which is blocking water jets to produce sound. Those described in Mann's paper "Hydraulophone design considerations" use water jets striking perforated spinning disks, shafts, or valves, to create a pulsating water flow, similar to a siren disk. A single disk, shaft, or valve assembly can have rings or passages with different numbers of holes for different notes. Some hydraulophones have reeds (one or more reed for each finger hole) and some are reedless, having one or more fipple mechanism associated with each finger hole, thus having no moving parts to wear out.