what i mean is they could hear it most of the time... but not always. for example, late war sonar is ultrasonic... meaning it is beyond the range of human hearing. also, if you are beyond the range of the ASDIC transmitter you wont hear anything.
unless you were being pinged from close range with late war ASDIC you wouldnt hear it... in fact all you would really hear is the pressure wave of the sonar ping hitting your boat, you wouldnt actually hear the ping.
Furthermore... im pretty sure that when ASDIC pings reflect off of a thermal layer you cant hear them if you are under the layer.
certain conditions DO exist in which you will never hear pinging.
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