First-order Ambisonics consists of four signals corresponding to one omnidirectional and three figure-of-eight polar patterns aligned with the Cartesian axes. We obtain these signals from a matched pair of dual-diaphragm microphones, where each diaphragm output is accessible individually. While the signals for figure-of-eight patterns in x- and y-direction are calculated directly from the microphone outputs, the z-direction figure-of-eight signal is calculated as differential signal from omnidirectional patterns of both microphones, resulting in low-frequency attenuation and a phase shift. We equalize the frequency response using a combination of a model-based IIR and a measurement-based FIR filter and provide an open-source plugin performing the processing.
Authors: Thomas Deppisch (University of Technology and University of Music and Performing Arts, Graz, Austria) and Christoph Frank (Austrian Audio GmbH)