Wall of Sound

Imagine a wall built across a hall, with microphones covering one side, and loudspeakers covering the other side, connected to them. It might seem that the pattern of sound arriving at the microphones would be reproduced by the loudspeakers, so that the wall would appear not to be there. The loudspeaker side could then be taken somewhere else to reproduce the sound from the front of the hall.

This was the idea the Harvey Fletcher at Bell Labs had in the 1930s. But because of the impracticability of the number of microphones and loudspeakers that would be required, the system was simplified for practical experiments to a horizontal row of 80 microphones, and finally to just three.