Oceanography

Velocity of sound waves

The most important acoustical parameter in the ocean is the velocity of sound. Sound is a form of wave motion.

The velocity of sound waves, V in a liquid is determined by the Laplace equation. 

However, in practice it is more convenient to use the following expression --1

Where g is the ratio of specific heat at constant pressure (C_p) to specific heat at constant volume (Cv), i.e., C_p/C_v,

K is the compressibility of the transmitting medium, i.e., liquid and r is the density of the liquid.

Since sound pulse is a wave of compression, and therefore, heats the liquid as it passes through, the ratio C_p/C_v i.e., g is introduced in the expression.

The three variables ( g , r and K) in  the above expression  depend on temperature, salinity and pressure. Hence, the velocity of sound waves in the ocean depends on temperature, salinity and pressure i.e., depth. A simplified form of the dependence of sound velocity on temperature, salinity, and pressure is given in the following equation (Medwin, 1975):

V = 1449.2 + 4.6 T – 0.055 T² + 0.00029T³ + (1.34 – 0.010T) (S – 35) + 0.016Z (3)

Where V is the velocity of sound in m/s, T is the temperature in ^oC, S is the salinity in parts per thousand, Z is the depth below the surface in metres. As the temperature of sea water increases, its density decreases and so from expression (1), the velocity of sound should increase with an increase in sea water temperature. Normally, an increase in salinity leads to an increase in density and therefore from expression (1), the velocity of sound should decrease with increase in salinity. But, an increase in salinity also decreases the compressibility of sea water, i.e., K in expression (1) and this overtakes the effect of increase in density due to increase in salinity. Generally, the velocity of sound waves increases with increase in depth of the ocean.