Source arrays for directional and non-directional sound generation

In this paper, methods are presented to design an acoustic source array for both directional sound generation and non-directional sound generation. The methods are based on measured transfer functions to be able to take into account different source sensitivities, to use extended sources that cannot be regarded as point sources, and complex geometries modifying the radiation characteristics. The aim of this paper is to develop a sound source that can be integrated in passenger vehicles with the objective to warn vulnerable road users while minimising noise pollution. Nowadays, sensor systems exist which are able to reveal the position of the vulnerable road users, which can be used to optimise the performance of the warning signal generator. Based on this information, the signal generator is designed to generate the specified warning signal at the location of the vulnerable road user while minimising the noise pollution at other locations. Such a directional sound beam was realised with an array of controlled acoustic sources. Changes of the relative positions of the vehicle and the vulnerable road user require continuous adjustments of the sound beam. In an alternative operation mode the sound field should not be directional but uniform in a wide horizontal sector. It will be shown how to combine the different requirements of directional sound radiation and uniform sound radiation with a single array configuration. Practical sound sources mounted in cars should be as compact as possible. Therefore the efficiency of the array is an important property. It will be shown that different beam forming approaches can lead to significant differences in the efficiency of the array. The influence of beamforming parameters on the efficiency will be discussed as well as the physical interpretation of the resulting beamformer solutions. The acoustic results shown in this paper to illustrate these effects are obtained using numerical simulations based on FEM and BEM transfer function models of acoustic sources mounted in the front of a car, combined with models of the reflecting ground and other reflecting objects.