The isentropic state equation for sea water has been derived on the basis of thermodynamic relations as follows;
p=P (ρ/ρ₀) ^μ-Q
It is a unified expression of isentropic state for sea water and air since this equation holds in an adiabatic process of an ideal gas. The parameters μ and P (or Q) have been obtained respectively as a function of salinity, temperature and pressure from a secant bulk modulus equation of state (Chen-Tung Chen and Frank J. Millero, Deep-Sea Research, 23, 595-612, 1976).μ equals B/A (parameter in nolinear acoustics) + 1. The calculation results of μ and sound velocity in the sea using above equations are in good agreement with values obtained by using Willson's sound speed equation over the oceanographic range of salinity (35 %o), temperature (0°-30°), and pressure (0.1-100 MPa). The expression for sound velocity of finite amplitude propagation in the sea has been also derived using this state equation, and the computer simulation results of the propagation of finite amplitude sound in the ocean may be shown.

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