In spatial/360 audio, what changes when the listener's head moves: which parts rotate with the head, which stay fixed, and how many channels do the orders need?

Problem: planning audio for 360/VR/head-tracked playback (e.g. a need about head movement). Conditions: ambisonic (scene-based) delivery decoded to binaural headphones. Method: ambisonic scenes are decoded and rendered binaurally, and the whole sound field is rotated by the head/camera orientation; Omnitone, Google's Web Audio decoder, applies a 3x3 or 4x4 rotation matrix each animation frame (it can be wired to a three.js camera or device orientation) to rotate the entire sound field. Channel counts by order are 4 (first order), 9 (second) and 16 (third) channels in that library. YouTube's spatial audio spec uses AmbiX (ACN ordering, SN3D normalisation), first order only, W,Y,Z,X at 48 kHz in one audio track, and it also supports a 6-channel FOA plus head-locked stereo (L,R) layout, where the stereo pair does not change when the viewer moves their head. Production consequences: put narration, dialogue-over-picture, music beds and UI sounds in the head-locked pair; place only sources that belong to the world (voices on screen, ambience) in the ambisonic field; a single audio track must carry both (multiple audio tracks are unsupported). Room acoustics (inference): rotation-only rendering is 3DoF; it re-orients the baked reverb and early reflections with the scene but has no listener translation or room model, so distance cues, occlusion and room-dependent reverb must be mixed into the ambisonic file or handled by an object/room renderer. Failure modes: FuMa-ordered material labelled as AmbiX; head-locked material accidentally placed in the field (it will swing as the viewer turns). Limitations: Omnitone documents implementation, not perceptual studies; the 3DoF inference is not stated by either source. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: The 3DoF and room-acoustics consequences are inference. Neither source covers six-degree-of-freedom rendering. YouTube page layout may change; its full spec link was not fetched. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://raw.githubusercontent.com/GoogleChrome/omnitone/master/README.md",
      "note": "Rotation and Rendering Mode; Feature Highlights; Supported spatial audio formats; Minimum requirements"
    },
    {
      "url": "https://support.google.com/youtube/answer/6395969?hl=en",
      "note": "Rotation and Rendering Mode; Feature Highlights; Supported spatial audio formats; Minimum requirements"
    }
  ]
}