Natural echo chamber
The natural echo chamber is a room with hard irregular surfaces, in which are installed a loudspeaker and two microphones (for stereo).
The microphones should not point at the loudspeaker.
Signal is sent via an auxiliary send and power amplifier to the loudspeaker, and returned from the microphones to the console where it can be mixed in with the dry signal.
The natural echo chamber never sounds realistic, firstly because loudspeakers never sound realistic anyway, and it is generally too small to produce really good-sounding reverberation.
Even so, the result is quite unlike any simulation of reverb as it is a much denser sound, and it worth experimenting with even today.
It has also been known for classical music recordings to be 'improved' by using a large auditorium as a natural echo chamber.
If electrostatic loudspeakers are used then the result can be of excellent quality.