Oceanographers become increasingly interested in underwater sound. Acoustics allows us to 'see' and measure ocean properties. Sound rays approaching the surface are turned back downwards by warm water, which has a higher sound speed - and upwards again at depth due to the high pressure which also gives a higher sound speed, allowing rays to travel great distances without touching the surface or bottom. Acoustic frequency is a variable of fundamental importance to our understanding and use of sound in the ocean. Some whales communicate over vast distances at frequencies of 20Hz.
Dolphins use frequencies ten thousand times as great to locate prey.
Low frequencies travel great distances - useful if you are looking for a mate or trying to find your way in the open ocean. The way sound travels in water depends on temperature, currents and other properties. This was what William Derham showed in the atmosphere 200 years ago. If you understand this dependence, you can work backwards from the measured sound signal and try to figure out what the properties are. For example, by measuring the travel time you can determine an average temperature along the acoustic path.