Near-field spherical microphone array processing with radial filtering

This paper presents an analysis of spherical microphone array capabilities in the near-field, with an emphasis on radial filtering of sources in a given direction. The near-field of the array is defined in terms of frequency and distance from the array. Directional beamforming is demonstrated given the near-field radial compensation filter, which yields a desired directional beam-pattern at a chosen distance from the array. This pattern deteriorates as the source draws away from the array. Next, a framework is presented for radial filter design, enabling distance discrimination between sources positioned in the same direction relative to the array. Design examples include Dolph-Chebyshev radial filtering, radial notch filtering, and numerical design. Performance is analyzed in terms of spatial response and robustness to noise. Results show radial filtering is practical for improving attenuation of far-field and near-field interfering sources relative to a desired source positioned in the same direction.