Sonobuoys use a transducer and a radio transmitter to record and transmit underwater sounds. There are three types of sonobuoys: passive, active, and special purpose buoys. Passive sonobuoys use a hydrophone to listen for sound energy from a target. Active sonobuoys use a transducer to send an acoustic signal and then listen for the return echo off an object. Special purpose buoys provide additional information about the environment such as water temperature, wave height, etc. Airplanes can drop these instruments into the water from altitudes as high as 30,000 feet. A pattern of sonobuoys can be deployed and used to determine the exact location of a target.
The Directional Frequency Analysis and Recording (DIFAR) sonobuoy is a passive acousticsonobuoy used by the Navy to detect underwater submarines. There are several types of DIFAR buoys, but the main component of each is a directional hydrophone that gives bearings to where the acoustic signal originated. Bearings are measurements of the angle, with respect to magnetic North, to where the target is located. The DIFAR sonobuoy detects acoustic energy from 5 to 2,400 Hz and can operate for up to eight hours at depths of up to 1000 feet. These sonobuoys have also been used for research to track whale populations and monitor underwater volcanic activity.