Transcranial attenuation of bone-conducted sound when stimulation is at the mastoid and at the bone conduction hearing aid position.

HYPOTHESIS: The transcranial attenuation of bone-conducted (BC) sound depends on the stimulation position and the stimulation frequency.
BACKGROUND: Subjective transcranial attenuation of BC sound has previously only been measured at a few audiometric frequencies and with the stimulation at the mastoid. The results reported are on average of 5 to 10 dB with large intersubject variability and inconsistent with results obtained using vibration measurements of the cochlea.
METHODS: Pure tone hearing thresholds were measured in 28 unilateral deaf subjects at 31 frequencies between 0.25 and 8 kHz. The stimulation was provided at 4 positions: ipsilateral and contralateral mastoid, and ipsilateral and contralateral position for a bone conduction hearing aid.
RESULTS: With stimulation at the mastoid, the median transcranial attenuation is 3 to 5 dB at frequencies up to 0.5 kHz; between 0.5 and 1.8 kHz, it is close to 0 dB. The attenuation increases at higher frequencies; it is close to 10 dB at 3 to 5 kHz and becomes slightly less at the highest frequencies measured (4 dB at 8 kHz). When measured at the bone conduction hearing aid position, the median transcranial attenuation is 2 to 3 dB lower than at the mastoid. The intersubject variability is large at each frequency (around 40 dB), but there are small differences in general trends of the transcranial attenuation between individuals.
CONCLUSION: The median transcranial attenuation depends on stimulation position and frequency. However, the variability is great, both between individuals and within subjects for adjacent frequencies.