The plate reverb consists of a thin metal plate suspended in a soundproofed enclosure.
In the early days of recording, it was common to use a real physical room as a natural echo chamber. This is still a worthwhile technique, but obviously requires dedicating a whole room to this purpose. Also, the room needs to be soundproofed so that it is quiet.
In 1957, the EMT company developed the EMT 140 Reverberation Unit. Compared to modern digital reverb units, this one is a monster - 2.4 meters (8 feet) long by 1.2 meters (4 feet) high and weighing in at 270 kilograms (600 pounds)! However, it was a lot smaller than the size of room you would need to make a decent natural echo chamber.
Inside the casing of the EMT 140 there is a large metal sheet, which we call a 'plate' in this context. A transducer similar to the motor of a moving coil loudspeaker drive unit is mounted on the plate causing it to vibrate. Multiple reflections from the edges of the plates are detected by two (for stereo) microphone-like transducers.
The reverb time of the EMT 140 plate reverb is varied by a damping pad that is pressed against the plate thus absorbing the energy of the reflections more quickly. This is servo-controlled allowing the reverberation time to be controlled remotely.
Although the sound of the EMT 140, and any plate reverb, is not altogether natural, the reflections are very dense and smooth with a complexity that the ear appreciates. Unlike even modern digital reverberation units that are nowhere near as complex.
A top recording studio would still keep an EMT 140 plate reverb, or similar, for its unique sound that has so far not been successfully emulated in digital units or plug-ins. For vocals, the sound of the EMT 140 plate reverb is superb.