Asdic, the 'underwater ear'
An Asdic operator. Image copyright Imperial War Museum
Asdic, later known as sonar, was a secret device for locating submerged submarines by using sound waves. It was named after the Anti-Submarine Detection Investigation Committee. It consisted of an electronic sound transmitter and receiver. This was housed in a metal dome beneath the ship's hull. High-frequency beams - audible 'pings' - were sent out and bounced back when they hit a submarine. The time that passed before an echo was received showed the range of the submarine. The pitch of the echo revealed if it was approaching or moving away.
Despite these advantages, a highly trained operator was required to use the equipment effectively. However, as the war progressed technology improved. This made using Asdic easier and improved its success rate.
Listen to the Asdic 'pings' here (MP3 format, 274kb)