Active vs. Passive sonar
Active sonar emits pulses of sound waves that travel through the water, reflect off objects, and return to the receiver on the ship. By knowing the speed of sound in water and the time for the sound wave to travel to the target and back, we can quickly calculate distance between the ship and the underwater object. As examples, active sonar systems can be used to track targets and realign internal navigation systems by identifying known ocean floor features. Whales, dolphins and bats use the same technique, echolocation, for locating prey.
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. The advantage of passive sonar is that is 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 direction of submarines.
Frequencies of sonar: The U.S. Navy in Hawaii uses mid-frequency active sonar
Mid-frequency active sonar, which operates between 1 and 10 kHz, has been in use since World War II (1939) and has evolved since then into the version in use today. It is the primary tool for identifying and targeting submarines. Mid-frequency sonar has typical ranges of up to 10 nautical miles. Mid-frequency sonar is a key element of anti-submarine warfare as employed by the United States, NATO, Russia and most other navies. It is used to detect submarines that may be operating within strike-range of U.S. Navy ships. Without it, men and women serving aboard ship are blind and vulnerable to attack from submarines.
To better understand the science and uses of sound in the sea, please visit the Discovery of Sound in the Sea website from the University of Rhode Island: www.dosits.org.