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Originally Posted by ref
That's correct, even with the early asdics which had a narrow cone (about 15º), the operator rotates it to cover 360, although the own ship sounds make impossible to hear anything in the ship baffles.
Ref
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RN asdic sets did not use a fixed baffle, they had the baffle attached to the rear of the transducer so it rotated along with the head. The baffle then would blank out noise which would otherwise have been picked up at the transducer.
The USN however placed another baffle at the rear of the dome fixed to the dome itself which blanked off a portion of the rearmost arc.
Remember the operator was not listening for the sound of the submarine, rather he was watching for the return echo on his display which would have been a range recorder during an attack. Often overlooked but the range recorder proved a major advance over the CRT display as it provided a history of events and allowed the operator time to examine the trace so enabling him the chance to spot returns that would have been missed on a CRT. The signal fed to the recorder was also processed by passing it through a filter set which removed unwanted frequency bands from the source and was then amplified. By mid war the processing had moved further on so that frequency shift effects could be exploited to a small degree.