An empirical equation for the speed of sound in sea water is provided by Mackenzie: c ( T , S , z ) = a 1 + a 2 T + a 3 T 2 + a 4 T 3 + a 5 ( S − − 35 ) + a 6 z + a 7 z 2 + a 8 T ( S − − 35 ) + a 9 T z 3 , where