At 1:20:00 of the film the German destroyer starts pinging with its sonar after U-571 has submerged and passed beneath her. We hear the pings, then we hear the return signals at a lower frequency. For the purpose of interest, this shift in frequently is called the Doppler Effect. However; the sailors on U-571 would not have heard the return echoes only the initial pings. The return echoes were most likely added for dramatic effect.

Film

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