We investigated the multipole loudspeaker array based on spherical harmonic expansion in the Cartesian coordinate system. The arrangement of the multipoles with the least number of loudspeaker units has been studied for 2nd order of spherical harmonic expansion. The directivity of the multipole array does not achieve ideal directivity of the spherical functions due to the asymmetrical arrangement. The multipole array was also applied to an end-fire array because a multipole array uses less loudspeakers than the spherical array if the look-direction is fixed. We compared the performance of the conventional least-squares design method and multipole array method for an end-fire array. The least squares method had a higher directivity due to the large number of variable coefficients. Nevertheless, the multipole array based on spherical harmonic expansion has advantages of easy setup in the Cartesian coordinate system, analytical directivity, and small number of filter coefficients.

An acoustic evanescent wave reproduction method is presented. Evanescent waves have faster distance attenuation. In most sound field reproduction approaches, the evanescent wave is a spatial aliasing component and is an undesirable wave field for reproducing a pure plane wave. However, because of its faster distance attenuation property, it does not affect these approaches. In this work, we attempt to reproduce only the evanescent wave. If only the evanescent wave is reproduced, it can be applied for a local area sound reproduction system that can localize a sound field around one specific location. The evanescent wave reproduction system is constructed by using a linear loudspeaker array and digital filters. Computer simulation results showed that our method achieves faster distance attenuation than a simple sound source.Index Terms-evanescent wave, fast distance attenuation, local area sound reproduction, linear loudspeaker array, digital filter

We present a localized sound reproduction method in which sound energy is high in specified target areas and low everywhere else. Such a system needs to reproduce a sound which attenuates faster than a point source. Our study focuses on an acoustic evanescent wave that has faster distance attenuation. The evanescent wave can be generated when a wave transmitted at subsonic speed between two media. Previously, we used a linear loudspeaker array to reproduce the evanescent wave. Nevertheless, since this method requires a line source of infinite length, a large number of loudspeakers are needed to realize sufficiently fast distance attenuation. To reduce the number of loudspeakers, we propose an evanescent wave reproduction method by using a circular loudspeaker array. Computer simulation results show that our method achieves faster distance attenuation than a point source and reduces the number of loudspeakers to less than that of the linear loudspeaker array.