No speech articulator robustly favours retroauricular electrodes over canonical jaw sites: a volume-conductor model with orientation and electrode-count controls

Abstract

Objective. Ear-worn biopotential devices are being designed around a coupling that has not been computed: how strongly each speech articulator reaches electrodes on the jaw and around the ear. We compute it, and test the answer against the three things that could produce it spuriously - source orientation, electrode count, and the level of anatomical detail in the volume conductor. Approach. Articulator-to-electrode coupling is computed by reciprocity on MIDA, a head model with 116 labelled compartments at 500 microns, treating muscle as both the generator and its own conducting compartment. Twenty-two electrode positions - spanning the canonical jaw montage, a retroauricular cluster, and a cEEGrid C-path - are compared against ten individually segmented articulators. Source orientation is swept over the hemisphere, not assumed, and every comparison is repeated at matched electrode counts. Results. Five articulators - orbicularis oris, buccinator, mentalis, depressor anguli oris, and platysma - favour the jaw montage at every sampled orientation and at every electrode subsample. No muscle robustly favours the retroauricular montage. The strongest ear-leaning candidate, temporalis, reaches -2.57 dB under a uniform orientation sweep; but derived from the label volume, its fibre field gives -1.15 dB with an interval of [-1.45, +5.46], and only half of four-site retroauricular subsets favour the ear. Significance. For device design the result is one-sided, not a trade: a retroauricular montage loses lip and chin activity entirely and buys no muscle back reliably in exchange. What remains a design lever is placement - a four-site cluster chosen by anatomical target outperforms arbitrary placement around the ear by up to 1.07 dB.