Echo Sounder

An echo sounder is an instrument that uses sonar to measure the depth of water beneath a vessel, a technique also called echo sounding or depth sounding, and it works by transmitting an acoustic pulse downward and timing how long it takes the echo to return from the seabed. Because sound travels through water at roughly 1.5 kilometers per second, multiplying half that round-trip time by the speed of sound gives the depth, a method far faster than the older practice of lowering a weighted sounding line until it touched bottom. The German inventor Alexander Behm patented an echo sounding device on July 22, 1913, and the French physicist Paul Langevin conducted related sonar research during the First World War, with successful trials by 1920; one of the first commercial units, the Fessenden Fathometer, was installed aboard the liner SS Berkshire in 1924 by the Submarine Signal Company.