surface ship hydrophones only detect objects below the surface. ie your sub.
in real life this tactic might work to some degree, but for the most part, in game AND in real life... surface ship hydrophones lose their efficiency nearer to the surface as the intent of the design is to search for underwater objects and not surface objects.
so while you had the right idea... the DD never was detecting the surface ships at all... except visually... so your tactic of using their sound to help you evade the DDs was of no use im affraid.
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