SONAR equipment is used on most ships for measuring the depth of the water. This is accomplished by sending an acoustic pulse and measuring the time for the echo, or return from the bottom. By knowing the speed of sound in the water, the depth is computed by multiplying the speed by one half of the time traveled (for a oneway trip). SONAR is also used to detect large underwater objects and to search for large fish concentrations. More sophisticated SONAR systems for detection and tracking are found aboard naval vessels and submarines. In nature, bats are well known for making use of echolocation, as are porpoises and some species of whales. SONAR should not be confused with ultrasound, which is simply sound at frequencies higher than the threshold of human hearing - greater than 15,000-20,000 cycles per second, or hertz (Hz). Ultrasound is used on a very small scale, at high power, to break up material and for cleaning purposes. Lower power ultrasound is used therapeutically, for treatment of muscle and tissue injuries.
SONAR is very directional, so the signals are sent in narrow beams in various directions to search the water. SONAR usually operates at frequencies in the 10,000-50,000 Hz range. Though higher frequencies provide more accurate location data, propagation losses also increase with frequency. Lower frequencies are therefore used for longer range detection (up to 10 mi [17,600 yd]) at the cost of location accuracy.
Acoustic waves are detected using hydrophones that are essentially underwater microphones. Hydrophones are often deployed in large groups, called arrays, forming a SONAR net. SONAR arrays also give valuable directional information on moving sources. Electronics and signal processing play a large role in hydrophone and general SONAR system performance.
The propagation of sound in water is quite complex and depends very much on the temperature, pressure, and depth of the water. Salinity, the quantity of salt in water, also changes sound propagation speed. In general, the temperature of the ocean is warmest at the surface and decreases with depth. Water pressure, however, increases with depth, due to the water mass. Temperature and pressure, therefore, change what is called the refractive index of the water. Just as light is refracted, or bent by a prism, acoustic waves are continuously refracted up or down and reflected off the surface or the bottom. A SONAR beam propagating along the water in this way resembles a car traveling along regularly spaced hills and valleys. As it is possible for an object to be hidden between these hills, the water conditions must be known in order to properly assess SONAR performance.
In location and tracking operations, two types of SONAR modes exist, active and passive. Echolocation is an active technique in which a pulse is sent and then detected after it bounces off an object. Passive SONAR is a more sensitive, listening-only SONAR that sends no pulses. Most moving objects underwater make some kind of noise. This means that they can be detected just by listening for the noise. Examples of underwater noise are marine life, cavitation (small collapsing air pockets caused by propellers), hull popping of submarines changing depth, and engine vibration. Some military passive SONARs are so sensitive they can detect people talking inside another submarine. Another advantage of passive SONAR is that it can also be used to detect an acoustic signature. Each type of submarine emits certain acoustic frequencies and every vessel's composite acoustic pattern is different, just like a fingerprint or signature. Passive SONAR is predominantly a military tool used for submarine hunting. An important element of hunting is not to divulge one's own position. However, if the passive SONAR hears nothing, one is obliged to turn to active mode but in doing so, risks alerting the other of his presence. The use of SONAR in this case has become a sophisticated tactical exercise.
Other, non-military, applications of SONAR, apart from fish finding, include searching for shipwrecks, probing harbors where visibility is poor, oceanography studies, searching for underwater geological faults and mapping the ocean floor.
Waite, A.D. Sonar for Practicing Engineers. John Wiley & Sons, 2001.
Canadian Center for Remote Sensing, "History of Remote Sensing." 2001 [cited February 1, 2003]. <http://www.ccrs.nrcan.gc.ca/ccrs/org/history/morleye.htm>.
K. Lee Lerner