In the dark ages prior to the Rise of the Digital FX Boxes, there were three common ways to achieve a reverb effect: Reverberation Room (large and seldom portable), Spring Reverb (Which didn't sound very realistic) and the Plate Reverb.

The Plate Reverb uses the same principle as the other types - sound vibrations are induced into a medium, allowed to bounce around for a bit and then picked up with one or several - that's right - pickups. The medium in this case is a large metal plate suspended in mid-air so it can oscillate freely. The whole contraption - frame, plate, transducer and pickups - is then placed in a sound- and vibrationproof box to avoid interference from other sound sources. The signal one wants reverbed is then transferred to the plate by a transducer, basically a loudspeaker driver connected to the plate rather than the loudspeaker cone. Supplying an amplified signal to the transducer puts the plate in motion and vibrations travel outward from the transducer and are reflected back again as they hit the edge of the plate, eventually reaching a pickup. By placing the transducer and pickups at different distances from the edges, a larger variation in the travel time of the vibrations is achieved, giving a more realistic, irregular reverb as well as a wider stereo field if two pickups were used. Some models also had a motorized felt pad that could dampen the vibrations in the plate to avoid very long reverb times, since the metal plate could continue ringing for several seconds in some configurations.

As with several other vintage analog effects like the Leslie cabinets, tape echoes and tube amplifiers, plate reverb enjoy somewhat of a cult following (most digital reverb boxes has a 'Plate' preset included with the other, more 'acoustically correct' ones) despite being quite unwieldy - since sound travels much faster in metal than in air, the plate has to be quite big to give delay like a real room, and the sound is very bright due to the resonant characteristics of the metal plate.

Sources: "Understanding Vintage Effects", http://www.sospubs.co.uk/sos/jan01/articles/vintage.asp
"Sweetwater Word of the Day", http://www.sweetwater.com/insync/word.tpl?find=PlateReverb

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