Basic physics. Passive sensors (radar warning, hydrophone) go as the square of the distance. Twice as far, four times as weak. Active sensors (radar, ASDIC) go as the fourth power of the distance. Twice as far, sixteen times as weak! You can hear him pinging you four times as far away as he can locate you. (All things being equal, which they never are.) So when you hear him pinging, he's searching. When you hear him make an attack run, he's probably found you. When the cans go off nearby, he's got you.
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