Speed of Sound in Water at 20 °C

Sound speed in a liquid is √(K/ρ), the ratio of bulk modulus to density: 2.18 GPa over 998 kg/m³ gives 1478 m/s, close to the measured 1482. Unlike gases, liquids get faster as they warm, up to a maximum near 74 °C, and seawater is faster still (about 1500 m/s) thanks to salinity. This is the number behind sonar, ultrasonic flow meters, hull thickness gauging and medical ultrasound (soft tissue is near 1540 m/s).

In piping it sets water hammer. The Joukowsky surge Δp = ρ c Δv predicts about 1.5 MPa (215 psi) for every 1 m/s of velocity stopped instantly in a rigid pipe. Real pipes are elastic, and the wave speed in a thin-walled plastic or lined pipe drops to 300–600 m/s, which softens the surge substantially: one of the few genuine advantages of plastic pipe. A few percent of entrained air lowers c dramatically, since the mixture inherits the gas's compressibility and the liquid's density, which is exactly why air chambers and surge vessels work.