A Speed of Sound in Sea Water Calculator

The specialized calculator below will allow you to compute the speed of sound in seawater at a fixed depth (or pressure) given the temperature and salinity. It can also be used to compute salinity for a given temperature and sound speed, or temperature for a given salinity and sound speed, again at a fixed depth. To use the calculator:

1. You must enter either the depth or the pressure.
2. Check any 2 of the bottom 3 checkboxes to indicate what parameters are being entered.
3. Enter changes to the checked parameters and the other parameters will be automatically recalculated.

The depth and pressure are related according to a simplified approximation for a standard ocean (by convention an ocean at 0 deg C and 35 psu). The depth label includes an approximation symbol as a reminder. The quantities below the horizontal rule (conductivity, density,...) are output only.

Return to main version of this calculator.

DISCUSSION:

This calculator is based on an algorithm for the speed of sound published by Chen and Millero, "Speed of sound in seawater at high pressures," J. Acoust. Soc. Am., Vol. 62, No. 5, 1129-1135, Nov 1977. The underlying equations are valid for temperatures from -2 to 35 deg C, pressures from 0 to 10,000 dbar, and practical salinity from 2 to 42.

The actual Javascript code that performs the calculations is easily accessible. With this page open in your broswer, select "Source" under the "View" menu. The functions within the code can be used in other Javascript programs or are easily ported to C or Java. C-language versions of the routines are also available by downloading the source code to the Palm OS version of this calculator. (You don't need a Palm to look at the source code.)

Relationship of Depth and Pressure:

This calculator relates depth and pressure according to the quadratic relationship proposed by Saunders (JPO, Vol. 11, pp. 573-574, April 1981) evaluated at a latitude of 30 deg. This is a simplified approximation to a more complicated form describing the depth/pressure relationship for a standard ocean (by convention this is an ocean at 0 deg C and 35 psu). The latitude enters in through the variation in gravity, effecting the depth of the 5000 dBar surface at a rate of 40 cm per degree of latitude at 30 deg. The net result is that the depth is at best an approximation. At 30 deg latitude it should be good to a few meters, but the errors will grow to as much as 15 m near the poles.

Units:

The pressures in this calculator are not absolute pressures, but gauge pressures. The difference is that gauge pressure is measured relative to 1 standard atmosphere, so the gauge pressure at the surface is 0, not 1000 mbar. I've also used decibar's throughout. This reflects common oceanographic usage, but is not accepted as an SI unit. The internationally preferred units are Pascals, where 1 dbar = 1e4 Pascal.

Auxiliary Parameters:

The calculator computes the values for several additional properties of seawater based on temperature, salinity and pressure. These include the following properties:

• Specific heat: the energy in Joules required to to raise the temperature of one kg of seawater one °C at constant pressure (units Joules/(kg °C))
• Freezing point: valid for practical salinity ranging from 4 to 40 (units °C)
• Adiabatic lapse rate: the change of temperature per unit pressure for an adiabatic change of pressure of an element of seawater (units millideg/decibar)
• Potential temperature: temperature an element of seawater would have if raised adiabatically with no change in salinity to atmospheric pressure (units °C)

The values for the other four parameters are based on algorithms described in Fofonoff and Millard, "Algorithms for computation of fundamental properties of seawater," UNESCO Technical papers in marine science No. 44, 1983.

Revision History:

Initial release on November 26, 2006.