Evaluation of a photoacoustic bone-conduction vibration system

This article proposes a bone conduction vibrator that is based on a phenomenon by which audible sound can be perceived when vibrations are produced using a laser beam that is synchronized to the sound and these vibrations are then transmitted to an auricular cartilage. To study this phenomenon, we measured the vibrations using a rubber sheet with similar properties to those of soft tissue in combination with an acceleration sensor. We also calculated the force level of the sound based on the mechanical impedance and the acceleration in the proposed system. We estimated the formant frequencies of specific vibrations that were synchronized to five Japanese vowels using this phenomenon. We found that the vibrations produced in the rubber sheet caused audible sound generation when the photoacoustic bone conduction vibration system was used. It is expected that a force level that is equal to the reference equivalent threshold force level can be achieved at light intensities that lie below the safety limit for human skin exposure by selecting an irradiation wavelength at which a high degree of optical absorption occurs. It is demonstrated that clear sounds can be transmitted to the cochlea using the proposed system, while the effects of acoustic and electric noise in the environment are barred. Improvements in the vibratory force levels realized using this system will enable the development of a novel hearing aid that will provide an alternative to conventional bone conduction hearing aids.