Understanding bone-conduction hearing: Measurements and model development

Bone conduction (BC), the transmission of sound to the inner ear by way of skull vibration, is thought to stimulate the inner ear by several mechanisms: external ear (vibration of the ear-canal walls), middle ear (ossicular inertia), and inner ear (inertia of the cochlear fluids, compression of the cochlear walls, and sound flow through “third windows”). In chinchilla, we explore the inner-ear mechanisms with measurements of compound action potentials and intra-cochlear sound pressures after manipulations of the middle and inner ear. These measurements suggest that while contribution of cochlear fluid inertia to BC is reduced by occlusion of both cochlear windows, effects of cochlear compression and/or sound pressure transmission via “third window” pathways increase. We also explore external-ear mechanisms by applying an analytically-derived reverse input impedance of the external-ear canal, pre-existing measurements of middle-ear input impedance, and new measurements of ear canal sound pressure. We define an equivalent volume velocity source representative of the combined BC external-ear mechanisms. Estimates of the source magnitude are in agreement with experimental data (Chhan et al., 2016), showing that during ear-canal occlusion, the ear-canal source is a significant contributor to BC hearing at mid-frequencies (1-3 kHz).