Distance coding and performance of the mark 5 and st350 SoundField microphones and their suitability for Ambisonic reproduction.

Abstract

Capturing and replaying distance cues for multi-channel audio is currently an under-explored and under-exploited area. Panners that successfully give control of distance do not, currently exist. However, recordings made with 1st order ambisonic, Soundfield microphones replayed over an ambisonic rig can give realistic results with respect to distance perception (particularly when bringing sound sources inside the speaker array). Near-field effect, resulting from the wave front curvature of near-field sources, is one cue recorded by the microphone, but not reproduced by software or hardware panners. Papers by Daniel (2003, 2004) discuss the encoding and decoding of ambisonic material with particular reference to higher-order ambisonics, and describe ‘near-field coding’ filters which encode near-field effect while pre-compensating for finite loudspeaker reproduction distance. While existing research concentrates on its simulation, this report documents an investigation into near-field effect in Soundfield ST350 and MK V tetrahedral microphones. It is found that, as a result of calibration for a flat frequency response at a practical source distance, the Soundfield microphone responses bear strong similarity to various near-field coding filters, suggesting the existence of an optimum loudspeaker array radius for positional localisation. On determination of this distance, recordings may be adapted for proper reproduction at any chosen reference distance using the WigWare ambisonic plug-ins created at the University of Derby.