OVERVIEW AND RECENT ADVANCES IN BONE CONDUCTION PHYSIOLOGY

Abstract

During the mid 18th century it was found that sound could be transmitted through solids and in the 19th century it was generally accepted that a person can perceive sound by bone conduction (BC). Since then, the research community has tried to understand its fundamental mechanisms. This report provides an overview of the present state in BC physiology. Five factors contributing to BC hearing are identified: 1) sound radiated into the ear canal, 2) middle ear ossicle inertia, 3) inertia of the cochlear fluids, 4) alteration of the cochlear space, and 5) pressure transmission from the cerebrospinal fluid. Of these, inertia of the cochlear fluid seems most important. The vibration modes in the skull can be divided into three frequency ranges. At the lowest frequencies, approximately below 400 Hz, the skull moves as a whole with rigid body motion. At higher frequencies, up to 1 kHz, the skull motion can be modelled as a mass-spring system whereas at frequencies above 1 kHz, wave propagation dominates the skull vibration response. The wave propagation differs between the thin-boned cranial vault and the thick and dense bone of the skull base. Both vibration measurement of the cochlea and ear canal sound pressure can estimate BC perception changes caused by stimulation alterations for frequencies above 0.8 kHz; below 0.8 kHz measured hearing thresholds differ from skull vibration and ear canal sound pressure data.