Sound travels about 1500 meters per second in seawater. Sound travels much more slowly in air, at about 340 meters per second.
The speed of sound in seawater is not a constant value. It varies by a small amount (a few percent) from place to place, season to season, morning to evening, and with water depth. Although the variations in the speed of sound are not large, they have important effects on how sound travels in the ocean.
Sound speed is affected by the oceanographic variables of temperature, salinity, and pressure. Here we are referring to the ocean pressure due to the weight of the overlying water (equilibrium pressure), not to the pressure associated with a sound wave, which is much, much smaller. We can look at the effect of each of these variables on the sound speed by focusing on one spot in the ocean. When oceanographers look at the change of an oceanographic variable with water depth, they call it a profile.
Here are the basic profiles of temperature, salinity and pressure for a mid-latitude location in the deep ocean.
From these profiles, you can see that temperature changes a large amount, decreasing from 20 degrees Celsius (°C) near the surface in mid-latitudes to 2 degrees Celsius (°C) near the bottom of the ocean. On the other hand, salinity changes by only a small amount, approximately 34 to 35 Practical Salinity Units (PSU)[Glossary-add]. Finally, pressure increases by a large amount, from 0 at the surface to 500 atmospheres (atm) at the bottom.
The speed of sound in water increases with increasing water temperature, increasing salinity and increasing pressure (depth). The approximate change in the speed of sound with a change in each property is:
- Temperature 1°C = 4.0 m/s
- Salinity 1PSU = 1.4 m/s
- Depth (pressure) 1km = 17 m/s