Comparison of two algorithms for modal active control of fan noise in a duct

Abstract

The reduction of fan noise in circular ducts has been realized using active control. Preliminary modal decomposition has allowed the determination of preponderant acoustical modes. The comparison of two methods has been executed both in simulation and experimentation. First, the x-filtered LMS algorithm has been tested. The second algorithm, based on a harmonic controller specially adapted for tonal noise, was composed of multiple in-phase and quadrature weights; it only needs two complex coefficients per frequency and per mode. Simulations have been first performed to evaluate filter lengths and step sizes. In an initial experiment, the fundamental has been reduced by 15 dB thanks to a SISO environment acting on the (1,0) mode. Then, with a MIMO controller, the (0,0) and the (1,0) modes could have been cancelled simultaneously at the fundamental frequency and the first harmonic. The test bed is composed of an axial fan whose fundamental frequency is 2150 Hz and a 0.1-m-diam duct; the actuators and sensors are, respectively, crowns of four loudspeakers and four error microphones mounted on the wall. With regard to these experimental results, both control methods have been compared, looking at efficiency, stability, and speed, in accordance with computational complexity.