The outer ear pathway during hearing by bone conduction

There have been conflicting reports in the literature about the importance of the induced ear canal sound pressure for the perception of bone-conducted (BC) sound. Here we investigated this by comparing the ear canal sound pressure at threshold for air-conducted (AC) and BC stimulation. Twenty-one adults with subjectively normal hearing function participated. They were tested for their hearing thresholds in the frequency range 250 Hz to 12.5 kHz with AC and BC stimulation and the ear canal sound pressure within 5 mm of the eardrum was obtained with probe tube microphones. Contralateral masking used with BC stimulation shifted the hearing threshold by 5 to 10 dB due to central masking effects. When the ear canal sound pressures at threshold were investigated, the results indicate that the ear canal component for hearing BC sound is around 10 dB below other contributors at frequencies below 2 kHz and similar to other important contributors at frequencies between 2 and 4 kHz. At frequencies above 4 kHz, the contribution from the ear canal sound pressure on BC hearing declines and was around 40 dB below other contributors at 12.5 kHz. The contribution of the ear canal sound pressure in the mid-frequency region is facilitated by the ear canal resonance occurring in this frequency area. The results were similar irrespective of stimulation position. The study also revealed problems estimating the force out of BC transducers caused by a shift in resonance frequency when the artificial mastoid impedance deviates from the impedance of human mastoids. The current study indicates that model predictions have underestimated the contribution from the ear canal sound pressure on BC hearing by around 10 dB.