Sonobuoys are expendable underwater listening devices. The sonobuoy has 4 main components: a float, a radio transmitter, a saltwater battery, and a hydrophone. The hydrophone is an underwater sensor that converts the pressure waves from underwater sounds into electrical voltages that get amplified and sent up a wire (length of released wire can be set to 90, 400, or 1000 feet) to the radio transmitter that is housed in the surface float. The radio signal is picked up by an antenna and a radio receiver on the ship, then reviewed and simultaneously recorded onto a digital audiotape. Sonobuoy can transmit for a maximum of 8 hrs. before scuttling and sinking.
       There are 2 types of sonobuoys. Omnidirectional sonobuoys have hydrophones that can register signals up to 20 kHz, but they cannot determine the location of the sound source. DiFAR (DIrectional Fixing And Ranging) sonobuoys also have an omnidirectional hydrophone for recording sound, but it is limited to frequencies lower than 2.5 kHz. However, DiFARs also have 2 pairs of direction sensors, which along with an internal compass can determine the exact bearing of the sound relative to the sonobuoy. With 3 or more sonobuoys in the water it is possible to pinpoint the exact location of the sound source. This can then be correlated to visual observations of the species of marine mammal in that location, along with behavior and grouping information.