There was no set operational procedure to determine when escorts used active Sonar (pinging), or settled for passive listening devices (hydrophones). Basically, escorts carried out three distinct ASW roles:
1. Hunt - Seek to find as many enemy submarines as possible with little, or no knowledge as to their previous locations. ( e.g. an escort commander passing through a known U-Boat "hot zone" would constantly scan the underwater environment with hydropphones and/or pings, assuming that U-Boats were present. ) "Hunting" involved using the sonar on a broad band setting to cover as much area as possible. Obviously, the probability of a ping returning an echo from a target depended on the distance and how often the escort sent out a searching ping.
2. Location - establish the position of a U-boat whose position has been reasonably accurately determined. ( e.g. an escort would go into location mode once a periscope had been spotted, or following a torpedo attack, when the direction (bearing) from which the torpedoes had been launched could be determined. ) This used a narrower Sonar beam and continuous pinging.
3. Screen - 1 & 2 are Offensive tactics, Screening is defensive and tried to establish a zone through which any enemy submarine making an attack would have to pass. Again, a broad Sonar Band was utilised. Screening was most effective when several escort vessels combined to overlap their detection areas ( e.g. to cover the area ahead and abeam of a convoy, giving almost instantaneous triangulation )
For 1 & 3, often passive listening via hydrophones and well-trained operators was used, supplemented by periods of searching with active Sonar.
Often the actual ping was not heard by the operators etc, as they could shut down that part of the system which let them hear the transmitter and listen purely for echoes.
Operationally, depending on the skill of the operator(s) WW II Sonar could determine the bearing and range of a submerged target. Analysis of the Doppler effect could also give the change in course, or bearing of the target submarine. The problem was in establishing the target submarine's depth. This was largely down to guesswork, based upon the point at which the submarine passed under the Sonar beam. This "dead zone" could extend to as much as 600 meters. Hence, until depth-finding equipment was introduced ( late in the War by the U.S. ) the practice was to saturate an area with depth-charges, rather than try for an "aimed hit."
Ultrasonic listening devices and transducers were used. They were, size, for size, considered more accurate than normal Sonar - especially in determining a target's bearing - , but they still relied on heterodyning to transform an inaudible ultrasonic signal into something an operator could hear and analyse.
Given all the variables due to refraction, salinity, temperature, the different probability of detection patterns given by different transducers etc etc. the detection of underwater targets by sound, either active, or passive, was one of the most skilled and difficult tasks undertaken by surface vessels.
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