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The speed of sound in seawater depends on pressure (hence depth), temperature (a change of 1 °C ~ 4 m/s), and salinity (a change of 1‰ ~ 1 m/s), and empirical equations have been derived to accurately calculate the speed of sound from these variables.
The speed of sound in any chemical element in the fluid phase has one temperature-dependent value. In the solid phase, different types of sound wave may be propagated, each with its own speed: among these types of wave are longitudinal (as in fluids), transversal, and (along a surface or plate) extensional. [1]
c is the speed of sound in the medium, which in air varies with the square root of the thermodynamic temperature. By definition, at Mach 1, the local flow velocity u is equal to the speed of sound. At Mach 0.65, u is 65% of the speed of sound (subsonic), and, at Mach 1.35, u is 35% faster than the speed of sound (supersonic).
The speed of sound in the ocean at different depths can be measured directly, e.g., by using a velocimeter, or, using measurements of temperature and salinity at different depths, it can be calculated using a number of different sound speed formulae which have been developed.
The speed of sound in water increases with increasing pressure, temperature and salinity. [ 23 ] [ 24 ] The maximum speed in pure water under atmospheric pressure is attained at about 74 °C; sound travels slower in hotter water after that point; the maximum increases with pressure.
A sound wave propagates through a material as a localized pressure change. Increasing the pressure of a gas or fluid increases its local temperature. The local speed of sound in a compressible material increases with temperature; as a result, the wave travels faster during the high pressure phase of the oscillation than during the lower pressure phase.
ion speed of sound, the speed of the longitudinal waves resulting from the mass of the ions and the pressure of the electrons: = () , where is the adiabatic index Alfvén velocity , the speed of the waves resulting from the mass of the ions and the restoring force of the magnetic field:
The linear formula commonly used for the speed of sound as a function of temperature is the first-order approximation of the square root formula. In other words, it gives the tangent line approximation to the parabola using zero degrees Celsius as the point of tangency.