Sonar. In underwater communication, audible baseband
signals use water as the transmission medium.
Submarines and ships use sonar to detect the
presence of enemy submarines. Passive sonar uses
audio receivers to pick up water, propeller, and other
noises. Active sonar is like an underwater radar with
which refl ections from a transmitted ultrasonic pulse
are used to determine the direction, range, and
speed of an underwater target.