In salt water that is free of air bubbles or suspended sediment, sound travels at about  ( at , 10°C and 3% salinity by one method). The speed of sound in seawater depends on pressure (hence depth), temperature (a change of  ~ ), and salinity (a change of 1‰ ~ ), and empirical equations have been derived to accurately calculate the speed of sound from these variables. Other factors affecting the speed of sound are minor. Since in most ocean regions temperature decreases with depth, the profile of the speed of sound with depth decreases to a minimum at a depth of several hundred metres. Below the minimum, sound speed increases again, as the effect of increasing pressure overcomes the effect of decreasing temperature (right). For more information see Dushaw et al.
An empirical equation for the speed of sound in sea water is provided by Mackenzie:
where
* T is the temperature in degrees Celsius;
* S is the salinity in parts per thousand;
* z is the depth in metres.
The constants a, a, ..., a are
with check value  for , , . This equation has a standard error of  for salinity between 25 and 40 ppt. See [http://resource.npl.co.uk/acoustics/techguides/soundseawater/] for an online calculator.
(Note: The Sound Speed vs. Depth graph does not correlate directly to the MacKenzie formula.
This is due to the fact that the temperature and salinity varies at different depths.
When T and S are held constant, the formula itself is always increasing with depth.)
Other equations for the speed of sound in sea water are accurate over a wide range of conditions, but are far more complicated, e.g., that by V. A. Del Grosso and the Chen-Millero-Li Equation.