Generation of a Near-field Sound Zone Using Broadside Differential Array

This work proposes the use of a broadside differential array to generate a near-field sound zone. Unlike many personal audio techniques utilizing optimization techniques to focus energy into a bright zone while suppressing in a dark zone, this work focuses on deriving an explicit formula for boosting near-field energy using a small number of loudspeakers. The broadside differential principle is advantageous when the listener’s ear is in the broadside direction from the array axis. From the preliminary study on the loudspeaker array installed on a neckband headset, it is shown that the optimal driving signal calculated from an optimization technique follows the broadside differential principle. The driving signals for a three-element linear array are derived for a simple model of three monopoles, and the near-field gain is calculated as a function of listener distance and wavelength. Throughout the experiment conducted in an anechoic chamber and car cabin, it is demonstrated that the broadside differential array consisting of three loudspeakers and two independent signal channels can boost the near-field sound pressure level up to 6 dB over a wide frequency range with only two second-order filters.