People and marine animals use sound in the sea to accomplish many tasks. Since light travels relatively short distances in the ocean, sound is often used by animals for such basic activities as finding food or a mate, navigating, and communicating.
In a very general sense, there are two ways that sound is used: passively and actively. In passive acoustics, the organism or user makes no sound of their own, but listens to sounds that are being made by others. The ocean, particularly coastal environments, is a very noisy place. By listening to the sounds, much can be learned about the environment.
The second way to use sound is called active acoustics. In this case, sound is created by a source and then received by a receiver. Some active acoustic systems, such as underwater telephones, have a separate source and receiver that are located in different places. Other active acoustic systems have the source and receiver located at the same place. In this case, sound goes out from the source, bounces off an object, and returns to the receiver. The sound returning to the receiver is called backscatter. By analyzing the sound received (the return echo), much information can be learned about the object, such as its distance, size, and composition.
Read on to learn how people use sonar and animals use echolocation to explore their ocean environment: