u-boats were equipped with such a device as well.
it was called "TAG".
It was installed in the control room of the XXI and was set to listen for pre-determined high pitched prop sounds, if one of these pre-programmed sounds was heard, a warning would sound in the u-boat would alert the crew.
I would imagine the allied system was very similar.
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Last edited by GoldenRivet; 09-08-08 at 01:10 PM.
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