MINIMUM ATTENUATION GUARANTEED BY AN ACTIVE NOISE CONTROL SYSTEM IN PRESENCE OF ERRORS IN THE SPATIAL DISTRIBUTION OF THE PRIMARY FIELD

Abstract

The optimal attenuation of sound fields inside moving vehicles in cruise conditions by active acoustic control depends on the number and locations of acoustic secondary sources. When taking measurements is difficult or expensive, as in airplanes, launchers etc., the best number and locations can be determined, on the one hand, from harmonic numerical modelling to obtain the interior vibro-acoustic responses inside the vehicles, and on the other hand by the help of a calculated map providing the spatial distribution of the primary excitation. It turns out that the predicted attenuations are always optimistic when compared with the measured attenuation while the numerical response functions and the primary field correspond quite well to those measured. From among the reasons for the discrepancy, one is analyzed in the present paper: a numerical frequency model works with a perfectly stationary primary field in space, which is not the case in the real world. Indeed, the spatial distribution of the primary field always varies around a central one called here the primary field of reference. In these conditions, the present study shows the gap between the minimum attenuation guaranteed by a control system and the optimal attenuation associated with the primary field of reference. This gap increases with the latter reference attenuation. The study also gives the attenuation which a numerical model should provide in the reference situation in order to guarantee a minimum attenuation when knowing the errors in the spatial distribution of the primary field. This new information is of great importance for calculations carried out to predict the efficiency of active control of harmonic sound fields.