Investigation of an active noise control system to reduce the noise emitted by an industrial stack

A precise analysis of the noise emitted by an aluminum plant has shown that one of the main sources of noise is the noise emitted by an exhaust slack 40 m high and 1.8 m diametre, emitting a pure lone at 320 Hz. Because a classical cost effective solution is not available for this source, active noise control (ANC) solution was investigated. Commonly available ANC systems, however, are not applicable to the chimney considered because the diameter of the slack (phi) is too important compared to the wave length (lambda) of the frequency to control. In fact, until now, active noise control (ANC) has been successfully used to reduce the acoustical energy emitted at the end of ducts in the case of plane wave propagation, i.e. lambda/phi < 1.7 [1,4]. When phi > lambda/1.7, high order modes propagate in the duct and a single canal active noise control system is not sufficient. This paper presents an ANC system developed to control higher modes in a circular duct. The controller used is based on a multiple-input multiple-output adaptive filtered X-LMS and allows up to 11 error microphones and controls sources. The system has been used in a laboratory on a finite duct of 3.5 m long and 0.30 m diameter which corresponds to the same ratio lambda/phi for the real stack. The efficiency of the system with different numbers and location of control sources and errors sensors has been analyzed. Noise reduction of 15 dB(A) could be obtained with some configurations. Actually the system is being considered for installation on the real chimney stack.