Answer: Fessender oscillator
Created by Reginald A. Fessenden, motivated by The Titanic tragedy. It was the first successful acoustical echo ranging system. Given its properties (540‐Hz air‐backed electrodynamically driven clamped‐edge circular plate), morse code could be used to communicate between submarines. During the First World War, the Royal Navy also used it for detecting submarines.
The only disadvantage that this system had, it's that it was reasonably easy to be heard by other ships, even on the surface. On the other hand, it was only useful for detecting big targets, as for little ones, it lacked power
