Following the outbreak of World War II, German U-boats began targeting merchant and passenger vessels in 1939, prompting the U.S. Navy to review its readiness to counter this threat.
[1] From 1942-1943, UCDWR was mainly focused on researching methods of detecting submerged submarines acoustically in order to rectify the Navy's inability to effectively employ its sonar equipment.
Together with NRSL, UCDWR conducted research and experiments exploring how currents, organisms, water temperature, salinity, depth, and the sea floor affected noise.
[1] The joint efforts of NRSL and UCDWR created an oceanographic scientific field that added to the Navy's ability to accurately detect sounds underwater, as well as measure listening ranges.
The NAC,NAG, NAH, and NAD (which were improvements from their respective predecessors) were designed to be released by submarines and remain stationary in the water while transmitting a signal at the same frequency of the enemy vessel, effectively jamming its sonar.