To ready the chasers for antisubmarine warfare required designing, testing, and producing listening devices. This photo, from a folio of Submarine Signal Company photographs, shows the K Tube hydrophone and compensators, built at the Nahant facility, complete and ready to ship.
The three main components of the K Tube were a triangular frame with microphones mounted on it and a float to keep it at a fixed distance under water; a reel of cord, also with floats; and the compensator device (the stacks of wooden boxes on the right contain the compensators) for determining the direction of the sound.
During a listening period, the frame was put in the water and the line reeled out. A listener turned a hand wheel on the compensator, which adjusted the length of the air passage from each of a pair of recievers to each ear. Once the sound level had been tuned to be the same in each ear, the listener could view on a scale how much the hand wheel had been turned, and use this measurement to determine the angle of bearing of the sound source.
For a detailed description and photos of the compensator and crewmen using it on deck, see my book, starting on page 39 (Chapter 2 covers the ASW equipment on chasers).