Active attenuation of noise--The monopole system

An approach to active noise attenuation is proposed, which attenuates noise propagating down a duct by adding an antiphase copy of the primary noise. The upstream radiation from the secondary source is used as a compensating signal, which adds to the primary source radiation and maintains attenuation over a full range with the proper signal processing. A compensating network based on the requirements of the system is introduced, and experimental results show the effectiveness of the technique which enables one source to give similar results to those with two or three sources in the active attenuator.