Plate reverb
Reverberation made by setting a suspended steel plate vibrating and recording how it rings out.
Before the plate, a studio that wanted reverb needed a real echo chamber: a tiled room with a speaker and a microphone in it. Germany's EMT launched the EMT 140 in 1957 — a thin steel sheet of roughly two square metres tensioned in a frame, with a driver in the middle and contact pickups near the edges. A damping pad cranked closer or further away set the decay time.
The sound is dense and bright, without the distinct early reflections of a room, and it sat on vocals and snares throughout the 1960s and 70s. The unit weighed several hundred kilos and was sensitive to footsteps in the corridor. Today it's imitated by algorithmic reverbs or by impulse responses sampled from surviving units.