Sonar is an acronym for "Sound Navigation and Ranging." It is a tool that uses underwater acoustics to determine water depth, the location of mines, and the presence of submarines. It serves as the Navy's front line of defense from underwater attack, and is the only reliable way to identify, track, and target submarines. There are two major classes of sonar: passive and active.

Passive Sonar

Passive sonar is a listening device that uses hydrophones (underwater microphones) that receive, amplify, and process underwater sounds. It is primarily used to detect the presence of submarines. Determination of the type of submarine can be made by matching the frequency of sound detected with the known frequencies of various submarine engine plant sounds and the unique sounds emitted by the different types of propellers in water (cavitation). The advantage of passive sonar is that it places no sound in the water, and thus does not reveal the location of the listening vessel. Passive sonar can indicate the presence, character, and movement of submarines, but it is ineffective at detecting modern, quiet submarines running submerged on batteries, as well as submarines that are motionless (lying in wait) or running at very low speeds. Active sonar is needed for precise location, classification, and rapid targeting.

Active Sonar

Active sonar emits pulses of sound that travel through the water, reflect off objects, and return as an echo to an underwater acoustic receiver. It uses an underwater speaker to generate sound waves. The sound waves travel until they strike an object and are reflected in various directions. Some of the reflected waves return to the receiver, where they are converted back into electric signals, amplified and displayed. A careful interpretation of the sound can provide the direction and distance of the object, as well as its size and speed. This is accomplished through "echo ranging," which measures the time it takes for a sound wave to travel from the transducer, reflect off the object, and return to the receiver. Active sonar is critical for locating and tracking submarines. It is used sparingly, however, because it also allows an enemy submarine to pinpoint the position of the ship emitting the sound. To put Navy active sonar use in perspective, it is important to note that, of the U.S. Navy's approximately 280 surface ships, only about 58% are equipped with mid-frequency active sonar. About half of these ships are underway at any given time, and for each ship, active sonar is turned on only a small percentage of the time (during certain types of training and maintenance activities).

Types of Active Sonar

Different types of active sonar operate at different frequencies, according to their purpose.

High Frequency: High frequency sonar (>10 kHz) is primarily used for determining water depth (fathometers), hunting mines, and guiding torpedoes. At higher frequencies, the sound energy is greatly attenuated (weakened due to scattering and absorption) as it travels through the water. This results in shorter ranges, typically less than five nautical miles.

Mid Frequency: Mid frequency sonar, which includes the AN/SQS-53 system, has been in use since World War II, and is the primary tool for identifying and prosecuting submarines. Mid frequency sonar (1 kHz - 10 kHz) suffers moderate attenuation and has typical ranges of 1-10 nautical miles.

Low Frequency: Low frequency sonar (< 1 kHz) produces sound that suffers less attenuation as it travels through the water, providing greater range than other sonar. Achieving ranges up to 100 nautical miles, low frequency sonars are primarily used for long-range search and surveillance of submarines. Surveillance Towed Array Sensor System Low Frequency Active (SURTASS LFA) is the U.S. Navy's low-frequency sonar system.

Sonar Configurations

Sonar systems can be mounted to the hulls of various combat ships, towed behind ships in an array, dipped into the water from helicopters, or attached to free-floating buoys (sonobuoys).

Using Sonar

Sonar contacts are charted visually as scrolling lines on a digital display (see image, right), and the sounds also pass through headphones to sonar technicians. Each line on the display corresponds to a sound source, and each must be investigated and classified as a potential target, a commercial ship, or other source.

The classification process is very manpower intensive, and is dependent on highly skilled operators. This particular sonar display shows several commercial ships.