Water instruments generate pressure waves by oscillation, turbulence, or similar distrubance in water. Water instruments are like woodwind instruments in some ways, except that the operative fluid (liquid) is for the most part incompressible fluid, as compared with wind (air) which is highly compressible). Water instruments are called hydraulophones and work best underwater, but are often designed so that they can be heard in a surrounding air medium even though they are typically water-filled or operate in-part underwater.