Hearing by Bone Conduction. Physical and Physiological Aspects
Bone conduction as a phenomenon, physically and physiologically, is of vital importance in both the diagnosis of a hearing impairment and the development of bone conduction hearing aids. Understanding hearing by bone conduction is difficult, involving sound transmission by wave motion in a complex geometrical structure of layered bone covered with soft tissue and cartilage, finally received by the highly delicate cochlea. Current measurement techniques, with computers, FFT analysers, and miniature accelerometers, enable high quality measurements and analyses of complex structures. Objective investigations of a dry skull and of patients equipped with either binaurally or monaurally osseointegrated titanium fixtures, as well as subjective investigations of these patients, have yielded the results of the physical and physiological behaviour of bone conducted sound presented here.
A bone-anchored hearing system with the transducer implanted offers some potential advantages, not only for current users of the bone-anchored hearing aid but also for sensorineural hearing impaired people who use air conduction aids. A bone-anchored hearing aid of present standard does not provide sufficient gain to aid a pure sensorineural loss; however, to implant the system would make it more beneficial. An attachment close to the cochlea improves the transmission by 5 to 10 dB and yields a better directionality. Moreover, in comparison with air conduction, bone conduction has greater loudness sensitivity, with up to 10 dB more efficient transmission in the range 30 to 80 dB HL. This behaviour may be explained by the efferent nerves to the middle and inner-ear.
Although there is linear transmission for normal sound levels in the skull, bone conducted sound can be severely affected by structural characteristics such as the antiresonances of the skull. These antiresonances cause transcranial attenuation of up to 40 dB, resulting in a perceived sound lateralization. Also, a forced low-frequency antiresonance appears in the transmission to the ipsilateral cochlea when a transducer is applied in the mastoid area; this results in perception of the applied tone solely at the contralateral cochlea. For frequencies above the first free skull resonance, bone conducted sound is transmitted primarily by wave motion through the bones of the cranial vault and only secondarily at the base of the skull. The teeth are well suited for transmission of bone conducted sound and have a sensitivity close to that of the skin covered mastoid; both of these, however, are less sensitive than a percutaneous approach at the mastoid for frequencies above 1 kHz.