Sonar is used to detect objects and organisms in marine environments, such as submarines, ships and schools of fish. There are two types of sonar: active and passive.
Active sonar involves transmitting an acoustic signal and observing how it interacts with objects that are in its way. For example, if there is a submarine in the way of the signal, the sonar wave bounces off it and feeds data back to its source. Some transducers can measure signal strength, which allows them to see how big an object is or how far away it is. When investigators analyze how long it takes the signal to return, they can identify an approximate location. In saltwater, it takes approximately 1 second for the signal to return from 1,500 meters away. As well as examining objects, sonars can interact with schools of fish and other sea life.
Passive sonars do not emit their own signal, which makes them harder to detect. This makes them particularly useful for military applications, as military vessels may need to stay hidden. In order to measure an object's range, passive sonars need to work alongside other listening devices. It is particularly useful when trying to identify targets, as it allows military vessels to identify a shooting area, without making themselves known to opponents' vessels.