the thermodynamic sound velocity 'c' or 'a' is the slope of the isoentrope passing through -rho^2*c^2 and that is equivalent to [dp/dv]s
so c =[1/rho^2* [dp/dv]s}^1/2
but for flight this is more useful
http://www.faa.gov/library/manuals/a...apter%2015.pdf
