The Wall of Sound acted as its own monitor system, and it was therefore assembled behind the band so the members could hear exactly what their audience was hearing. Because of this, Stanley and Alembic designed a special microphone system to prevent feedback. This placed matched pairs of condenser microphones spaced 60 mm apart and run out of phase. The vocalist sang into the top microphone, and the lower mic picked up whatever other sound was present in the stage environment. The signals were added together, the sound that was common to both mics (the sound from the Wall) was cancelled, and only the vocals were amplified.
