Efficiency testing of active noise control in the duct outlet and effect of the error microphone location

In a duct, the performance of the active control system depends among other things on the position of the loudspeaker. In ventilation systems, they are usually inserted in the ductwork along the duct wall or in the duct if its diameter is large enough not to generate too much pressure drop. In this paper, we describe an active noise control system placed not in the duct but at its outlet, instead of the ventilation grille. The control loudspeaker being no longer in the duct, it must remain close enough to the outlet to "colocalize" the two sources of sound waves (ventilation noise and control) and keep a global noise control while leaving a sufficiently large opening to allow a similar ventilation flow obtained with a standard ventilation mouth. The error microphone is placed outside the duct and the influence of its position is also analyzed on a ventilation system prototype. The obtained attenuation is good and extends over a wider range of frequencies than when the loudspeaker is placed at the periphery of the duct. Moreover, its position at the outlet of the duct is better for using the speaker as a source of sound diffusion.