Sonar

Sonar was a detection system based on sound waves.

A sea-fort in the English Channel was converted into a sonar testing centre. (TV: The Sea Devils)

A signal transmitted by the Game Station masked the Dalek Fleet from sonar, radar and scanners. After he was taken to the Game Station in 200100, the Ninth Doctor cancelled this signal, revealing who was manipulating mankind. (TV: Bad Wolf)

The First Doctor used a sonar device called an echo-sounder to determine the depths of the marsh he, John and Gillian were walking through, enabling them to get to the Pied Piper's castle. He later used it to find the Piper after he turned himself invisible. (COMIC: Challenge of the Piper)