Dobrev Ivo, Sim Jae Hoon, Stenfelt Stefan, Ihrle Sebastian, Gerig Rahel, Pfiffner Flurin, Eiber Albrecht, Huber Alexander M, Röösli Christof
Department of Otorhinolaryngology, Head and Neck Surgery, University Hospital Zürich, Switzerland; University of Zürich, Zürich, Switzerland.
Department of Otorhinolaryngology, Head and Neck Surgery, University Hospital Zürich, Switzerland; University of Zürich, Zürich, Switzerland.
Hear Res. 2017 Nov;355:1-13. doi: 10.1016/j.heares.2017.07.005. Epub 2017 Sep 23.
Bone conduction (BC) is an alternative to air conduction to stimulate the inner ear. In general, the stimulation for BC occurs on a specific location directly on the skull bone or through the skin covering the skull bone. The stimulation propagates to the ipsilateral and contralateral cochlea, mainly via the skull bone and possibly via other skull contents. This study aims to investigate the wave propagation on the surface of the skull bone during BC stimulation at the forehead and at ipsilateral mastoid.
Measurements were performed in five human cadaveric whole heads. The electro-magnetic transducer from a BCHA (bone conducting hearing aid), a Baha Cordelle II transducer in particular, was attached to a percutaneously implanted screw or positioned with a 5-Newton steel headband at the mastoid and forehead. The Baha transducer was driven directly with single tone signals in the frequency range of 0.25-8 kHz, while skull bone vibrations were measured at multiple points on the skull using a scanning laser Doppler vibrometer (SLDV) system and a 3D LDV system. The 3D velocity components, defined by the 3D LDV measurement coordinate system, have been transformed into tangent (in-plane) and normal (out-of-plane) components in a local intrinsic coordinate system at each measurement point, which is based on the cadaver head's shape, estimated by the spatial locations of all measurement points.
Rigid-body-like motion was dominant at low frequencies below 1 kHz, and clear transverse traveling waves were observed at high frequencies above 2 kHz for both measurement systems. The surface waves propagation speeds were approximately 450 m/s at 8 kHz, corresponding trans-cranial time interval of 0.4 ms. The 3D velocity measurements confirmed the complex space and frequency dependent response of the cadaver heads indicated by the 1D data from the SLDV system. Comparison between the tangent and normal motion components, extracted by transforming the 3D velocity components into a local coordinate system, indicates that the normal component, with spatially varying phase, is dominant above 2 kHz, consistent with local bending vibration modes and traveling surface waves.
Both SLDV and 3D LDV data indicate that sound transmission in the skull bone causes rigid-body-like motion at low frequencies whereas transverse deformations and travelling waves were observed above 2 kHz, with propagation speeds of approximately of 450 m/s at 8 kHz.
骨传导（BC）是刺激内耳的一种替代气传导的方式。一般来说，骨传导刺激发生在颅骨上的特定位置，或通过覆盖颅骨的皮肤进行。刺激主要通过颅骨并可能通过颅骨的其他内容物传播到同侧和对侧耳蜗。本研究旨在探究在前额和同侧乳突进行骨传导刺激时颅骨表面的波传播情况。
在五个完整的人类尸体头部进行测量。将来自骨传导助听器（BCHA）的电磁换能器，特别是Baha Cordelle II换能器，通过经皮植入的螺钉固定，或用5牛顿的钢头带固定在乳突和前额处。Baha换能器直接由频率范围为0.25 - 8kHz的单音信号驱动，同时使用扫描激光多普勒振动计（SLDV）系统和三维激光多普勒振动计（3D LDV）系统在颅骨的多个点测量颅骨振动。由3D LDV测量坐标系定义的三维速度分量，已在每个测量点的局部固有坐标系中转换为切向（面内）和法向（面外）分量，该局部固有坐标系基于尸体头部的形状，由所有测量点的空间位置估计得出。
在低于1kHz的低频下，类似刚体的运动占主导，而对于两个测量系统，在高于2kHz的高频下观察到明显的横向行波。在8kHz时，表面波传播速度约为450m/s，对应的经颅时间间隔为0.4ms。三维速度测量证实了SLDV系统的一维数据所表明的尸体头部复杂的空间和频率相关响应。通过将三维速度分量转换为局部坐标系提取的切向和法向运动分量之间的比较表明，法向分量在2kHz以上占主导，其相位在空间上变化，这与局部弯曲振动模式和表面行波一致。
SLDV和3D LDV数据均表明，颅骨中的声音传播在低频时会引起类似刚体的运动，而在2kHz以上观察到横向变形和行波，在8kHz时传播速度约为450m/s。
