Examination of bone-conducted sound pathways

Evidence of bone-conducted sound pathways through the skull have been explored with growth of loudness tests, auditory brainstem response, and otoacoustic emissions; however a comprehensive understanding of these pathways has not been achieved. Less is known about the usefulness of behavioral thresholds, sound pressure levels in the ear canal, and the auditory steady-state response (ASSR) as a function of ear canal occlusion in understanding propagation pathways of bone-conducted signals. Results acquired from multiple, repeated test sessions with a single-subject design, revealed lower behavioral thresholds and higher sound pressure level in the ear canal (Real Ear) in the occluded condition below 2,000 Hz; however, this trend was reversed above 2,000 Hz. The ASSR revealed lower thresholds in the occluded condition at all frequencies. Real Ear measured the bone-conducted signal while the behavioral thresholds show the product of the signal after following the traditional bone conduction pathway through the ear structures and the brain. The ASSR results indicate the responses recorded were suprathreshold, and are the product of bone conduction sound excitation at the level of the auditory nerve. Understanding bone-conducted sound pathways could provide guidance for designing enhanced hearing protection, new diagnostic protocols, and novel approaches for signal processing. [Work supported by AFOSR FA9550-06-0128.]