The Oliver Hazard Perry FFG has a "Prairie Masker" noise reduction system that, it has been reported, is very effective in masking noises at low speeds.
I'm not sure, but I'd imagine that it is an active sound dampening system, similiar to what is used in the latest Russian submarines, meaning that it is only effective at fairly low speeds because it generates vibrations that cancel out the normal vibrations generated by the machinery of the ship.
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