Sonar

Sonar is a technique that uses sound to navigate, communicate with or detect other vessels, and to observe the distance and velocity of underwater objects. The acoustic frequencies used vary from extremely low (infrasonic) to very high (ultrasonic). The word “sonar” also describes the equipment used to gather and analyze acoustic information. Sonar is used in depth sounding, fish finding, seafloor mapping, Doppler navigation, and acoustic location systems for divers. The term for the scientific study of underwater sound is hydroacoustics.

Three types of technology are categorized as "sonar":

1. Passive sonar uses receiving sensors to listen for and analyze the sound made by vessels
2. Active sonar uses an acoustic projector to emit pulses of sounds and a receiver which detects the echoes to find the range, bearing and relative motion
3. Acoustic communication systems use a projector and receiver at both ends of the acoustic path

Sonar was first proposed as a means of detecting icebergs, but government interest in and funding of sonar systems did not take off until World War I highlighted the threat posed by submarine warfare. Modern technological innovations have included rapid-scanning and side-scan sonar, the echo sounder (or depth detector,) and within-pulse electronic sector scanning sonar (or WPESS). Military uses of sonar include systems used in acoustic homing torpedoes, in acoustic mines and mine detection.