Most of the applications for underwater acoustics rely on an accurate knowledge of the sound velocity in water. The speed of a compressional sound wave (c) is given by:
Both the axial modulus (a measure of elasticity or compressibility) and the density of sea water are complex functions of temperature, salinity and pressure. However, due to the complex interdependency of these parameters it is very difficult to apply this theoretical equation for sound speed with any confidence. For this reason it is often necessary therefore to resort to empirical relationships.
Sound velocity increases with temperature (T), pressure (p) and salinity
(S) in this order of importance. As a rule of thumb the rate of change of
velocity with these parameters is:
T - 3m/s/degree C
p - 0.017m/s/m
S
- 1.2m/s/ppt
(Mazel, 1985)
An more accurate empirical relationship relating the sound velocity to T, S and p (or equivalently depth) is:
(Range of use: 0 to 30 degrees C, 0 to 45ppt, 0 to 100m, Clay and Medwin, 1977)