A design of array‐microphone system using directional microphones and two‐dimensional FIR filters

Abstract

AbstractDelay‐and‐sum array microphones consisting of a linear array of microphones can achieve a sharp directivity at high frequencies. A disadvantage is that the directivity depends on the frequency. In order to resolve this problem, an array microphone using a fan filter has been proposed. However, in order to form a sharp beam at a low frequency, it is necessary to increase the filter order and the array length. In this paper, we report improvement of the directivity of an array microphone realizing a narrow beam with a relatively low order of the filter by means of a two‐dimensional frequency domain design. Employing two microphones, a first‐order pressure‐gradient microphone is formed. The spacing of the microphones is set for high sensitivity and broad bandwidth in the operating frequency range. The amplitude characteristics are obtained in the two‐dimensional frequency domain. These microphone units are placed along a line and are cascaded to a two‐dimensional fan filter so that the load is distributed. In the proposed configuration, a cardioid microphone is cascaded with an axially symmetric fan filter in a configuration that forms a single broadside beam. Another configuration forms a single beam in the endfire direction by cascading the bidirectional microphone to a diagonally symmetric fan filter. Deterioration of the pressure‐gradient microphone is compensated for by the two‐dimensional FIR filter. As a result, the directional characteristics of the pressure‐gradient microphone are reflected in the array microphone and a sharp directivity not dependent on the frequency is obtained over a wide frequency range. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 90(1): 46–58, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20233