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# Calculation of absorption of sound in seawater

The absorption of sound in seawater forms part of the total transmission loss of sound from a source to a receiver. It depends on the seawater properties, such as temperature, salinity and acidity as well as the frequency of the sound. The details of the underlying physics of absorption are quite complex. Note that the absorption causes only part of the transmission loss. Usually, the major contribution to transmission loss is the spreading of the acoustic wave as it propagates away from the source.

To use the calculator below, enter the frequency of interest and the values of water temperature and depth. Default values for salinity and acidity (pH) are provided, but these can also be modified if this data is available. The values for the absorption are calculated automatically. The three alternatives are derived using algorithms from the sources specified.

Frequency,  f  (kHz)
Temperature,  T  (Celsius)
Depth,  D  (km)
Salinity,  S  (ppt)
Acidity,  pH
 (dB/km) (Fisher and Simmons 1977) Absorption,  α (dB/km) (Francois and Garrison 1982) (dB/km) (Ainslie and McColm 1998)

When using water other than the Lyman and Fleming standard (S = 35, pH = 8), the Fisher & Simmons algorithm is invalid, and so this field becomes blank. Use the reset button to return to these values for typical seawater.

## Range of validity

Francois and Garrison estimate their model to be accurate to within about 5%. For frequencies of 10-500 kHz (where the MgSO4 contribution dominates), the limits of reliability are:

 -2 < T < 22 °C 30 < S < 35 ppt 0 < D < 3.5 km

At frequencies greater than 500 kHz, the pure water contribution exceeds that of MgSO4, and the limits are:

 0 < T < 30 °C 0 < S < 40 ppt 0 < D < 10 km

The Ainslie and McColm formula retains accuracy to within 10% of the Francois and Garrison model between 100 Hz and 1 MHz for the following range of oceanographic conditions:

 -6 < T < 35 °C (S = 35 ppt, pH=8, D = 0 km) 7.7 < pH < 8.3 (T = 10 °C, S = 35 ppt, D = 0 km) 5 < S < 50 ppt (T = 10 °C, pH = 8, D = 0 km) 0 < D < 7 km (T = 10 °C, S = 35 ppt, pH = 8)

## References

Ainslie M. A., McColm J. G., "A simplified formula for viscous and chemical absorption in sea water", Journal of the Acoustical Society of America, 103(3), 1671-1672, 1998.

Fisher F. H., Simmons V. P., "Sound absorption in seawater", Journal of the Acoustical Society of America, 62, 558-564, 1977.

Francois R. E., Garrison G. R., "Sound absorption based on ocean measurements: Part I:Pure water and magnesium sulfate contributions", Journal of the Acoustical Society of America, 72(3), 896-907, 1982.

Francois R. E., Garrison G. R., "Sound absorption based on ocean measurements: Part II:Boric acid contribution and equation for total absorption", Journal of the Acoustical Society of America, 72(6), 1879-1890, 1982.