Which 1976 system used two out-of-phase loudspeakers around a central microphone to cancel sources at that location?
Answer
The Chelsea dipole.
The Chelsea dipole, developed in 1976, was one of several ingenious analog geometric solutions engineered to combat the inherent acoustic feedback problem (hallowing) in fixed systems. This specific configuration involved positioning two loudspeakers directly out of phase relative to each other, strategically placed around a single central error microphone. Because the secondary sources were equal in magnitude but opposite in phase, they naturally canceled each other out precisely at the location of the microphone. This arrangement effectively prevented the secondary noise from corrupting the microphone's signal used for control, allowing for more stable operation within the limitations of analog processing.
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