Dipole in orthogonal direction as a secondary source for active noise control in ducts

An active noise control (ANC) method is particularly suitable for noise control in ventilation ducts. Usually a feed-forward configuration is used. Performance of a feed-forward ANC system in an air conditioning duct depends on the quality of the reference signal, which should be perfectly correlated with the primary noise. Acoustic feedback corrupts the reference signal and is one of the main problems of broadband feed-forward ANC systems in ducts. Directivity of the secondary source reduces the acoustic feedback effect. Two highly directional designs of the secondary source were developed: the Transmission Line (TL) secondary source and the Quarter Wavelength (QW) secondary source. It has been shown that this kind of acoustic feedback reduction unburdens the adaptive FX-LMS algorithm and speeds up its convergence. An additional advantage of both proposed secondary sources is that a significant transmission loss is achieved, even when the loudspeaker is turned off.