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.