Echo chamber (studio)
A hard, empty room containing one loudspeaker and one microphone, built for no purpose other than adding reverb to recordings.
The signal is played into the room, the microphone picks it up along with the room's reflections and tail, and the result is blended back into the mix. Bill Putnam is generally credited with turning this into a production technique when he used the studio bathroom as a chamber on The Harmonicats' "Peg o' My Heart" in 1947. The record sold over a million copies, and suddenly everyone wanted a tiled bathroom.
Bigger studios built purpose-made rooms of concrete, angled walls and tile. The Capitol Records Tower in Hollywood got its underground chambers in 1956, and they are still in use. The costs were floor space and a reverb time you could only change by moving furniture, which is why the plate took over. Today the chambers live on as impulse responses.