Through WWII submarine sonar worked by sending out sound waves to find objects in the water. Because the submarines were sending out sound this made them vulnerable to being detected. It was not until after WWII that the new sonar system was starting to be used and perfected. This relied on just listening for the sound made by other vehicles in the water.

(Book – Blind man’s Bluff: The Untold Story of American Submarine Espionage – Sherry Sontag and Christopher Drew with Annette Lawrence Drew)