Design of a multipolar weighting for acoustic antennae

This work describes the development of an acoustic antenna for which several kinds of weightings have been studied to obtain a narrow directivity with attenuated rear lobes. Later, this antenna will be used in a new device to measure the sound absorption coefficient of flat panels in industrial sites. It is often necessary, although quite difficult, to measure acoustic absorption coefficients in partially diffuse conditions, because predicting the sound pressure level at, for instance, a workplace requires in situ characterization of the facings. This antenna has been optimized so that the major part of the received acoustic energy would come from one portion only of an investigated facing present in an industrial room. The multipolar weighting is quite efficient because the associated directivity can be carried out with a limited number of microphones. The calculation technique is improved to achieve a constant sensitivity antenna within the desired frequency range with most of the secondary lobes rejected. The designed receiving system is composed of four antennae, each equipped with five sensors. The directivity, at 150-5000 Hz range, has been verified in an anechoic room.