Dynamic Spatial Sidechain for First-Order Ambisonics

Much of the clarity and perceived separation in contemporary stereo production can be attributed to dynamic sidechain compression and frequency-selective sidechain attenuation, which enable precise temporal shaping of the auditory scene. New techniques for expressing dynamic relationships across the expanded sound field of 3D audio remain underdeveloped, despite their proven importance in stereo mixing. We present a novel, rotation-based dynamic spatial sidechain system for First-Order Ambisonics (FOA), enabling energy-driven shaping of directional relationships in immersive audio. Motivated by the time-varying interaction between control and target signals in sidechain compression, our system extends this mechanism into the spatial domain, treating directionality-rather than amplitude-as the primary control dimension. Both main (target) and sidechain (control) FOA signals are decoded into virtual loudspeaker feeds. From these, we compute instantaneous spatial vectors using energyweighted summation of loudspeaker directions, analogous to the summation stage in VBAP. By analyzing the angular difference between these vectors, we generate smoothed azimuth and elevation controls to drive a spatial rotator that dynamically steers the main signal relative to the sidechain. Implemented in Max/MSP, this technique enables realtime, energy-responsive rotation of audio signals in 3D space, with potential applications in VR/AR, spatial music, and immersive media production.