DIGITAL UNIDIRECTIONAL MICROPHONE FOR ACT IVE NOISE-CONTROL IN DUCTS

For a few years active control systems have been based on numerical ad aptive filtering. LMS algorithms are commonly used to compute the anti -noise signal. One of the major problems with those algorithms is thei r sensitivity to feedback from the secondary sources. Solutions such a s mechanical transducers or secondary source combinations have already been proposed to attempt to reduce this instability. However, these m ethods are not always technically applicable or economically attractiv e. Although common microphone are omnidirectional at low frequencies, we have developed one implementing digitally a unidirectional micropho ne at these frequencies and dedicated to perform active noise control in ducts. This system is controlled by a DSP implementing a digital fi lter, the characteristics of which are automatically computed. It proc esses the output signal of two correctly arranged microphones to calcu late the noise component due only to the primary source. The influence of the sound coming from the secondary source is thus strongly reduce d.