Sonar (an acronym for sound navigation and ranging), is a system that projects acoustic energy into the water and measures the speed of its return from a reflective surface as an echo to detect an underwater object and determine its range and bearing. A simplified block diagram of an active sonar system is shown in Fig. 23-7. It is useful for detecting submarines, sunken ships, obstructions such as rocks, and naval mines. A sonar system typically includes a video display that provides a visual indication of the target and an audio speaker that provides audible information about the identity of the target or contact. The change in pitch of the audible signal indicates relative motion of the target with respect to the sonar system in accordance with the Doppler principle.

Figure 23-7: Active sonar system block diagram.

An active sonar system is analogous to an active radar system except that the transmission medium is sound rather than radio energy. A sonar transducer or transducer array is analogous to an antenna. Some transducers can be trained through 360 , while others are trainable through a more limited sector such as 90 . From knowledge of a target's change in range and bearing over time, its course, speed, and closest point of approach can be determined either by manual plotting or by computer.

The transmitter formats the signals that drive the transducer or transducers which project the sound into the water. Sonar transducers are made from materials that demonstrate...