Spaced A-B placements of higher-order Ambisonics microphone arrays: Techniques for recording and balancing direct and ambient sound

In acoustic music recording, spaced microphone positions typically give an impression of spaciousness and depth in the rendered sound scene. Inter-channel decorrelation, particularly at low frequencies, is thought to contribute to this effect, and cannot be achieved through coincident techniques. Higher order Ambisonics microphones have seen a rapid increase in popularity, despite their limitation as a coincident recording technique. In this article, recording and mixing strategies for the A-B spaced placement of HOA arrays are described combining the advantages of both: sound stage depth and stability, and the compatibility of b-format based mixing with respect to rendering for binaural and immersive 3D reproduction loudspeaker systems. Two tests were conducted to evaluate a) the suitability of various A-B spacings with regards to direct and/or ambient sound, and b) a novel two-dimensional mixing control tool, to study direct and ambient sound optimization. Results show that increased A/B spacing of HOA receivers correlates to more optimal presentation of ambient sound. Furthermore, users interacted with the mixing control tool differently when asked to optimize the sound scene for either direct or ambient sound, or both, providing support for our microphone placement and mixing techniques. This work points to new directions for the integration of HOA microphones with traditional recording techniques towards improved spatial representation of mediated music.