"Publishing [verse] is like dropping a rose petal down the Grand Canyon and waiting for the echo."
Don Marquis, U.S. humorist, journalist, and author
Sonar (which started as an acronym for sound navigation and ranging) is a technique widely used in shipping to detect nearby vessels and underwater obstructions. The word was coined by the Americans during World War II. The British also call Sonar, ASDIC, which has been daimed to stand for Anti-Submarine Detection Investigation Committee.
There are two major kinds of sonar, active and passive. Active sonar produces a sound "ping"' and then measures how long it takes a reflected pulse to return to the ship. The sound source and receiver are continually rotated, so that the direction of the echoing body can be found. Lower frequencies are used if the reflector is a long way off. Passive sonars listen without transmitting. The first Sonar devices were passive listening devices—no signals were sent out. They are usually used by the military (although some are scientific).
Lewis Nixon (1861-1940), a U.S. shipbuilding executive, introduced a passive sonar system. There were two driving forces: One was the hope of hearing the grinding sounds of nearby icebergs; the other was hearing submarines. Active sonar development accelerated due to the sinking of the Titanic in April 1912, and naval activities during World War I. By the time of World War II all ships were fitted with sonar.
The system is complicated by the fact that the speed of sound depends on the temperature and salinity of the water, and thermal gradients can bend the sound waves in peculiar ways. Small sonar systems are used on fishing boats to detect shoals of fish.
