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.

Objective: This study explores the influence of stimulation position on bone conduction (BC) hearing sensitivity with a BC transducer attached using a headband. Design: (1) The cochlear promontory motion was measured in cadaver heads using laser Doppler vibrometry while seven different positions around the pinna were stimulated using a bone anchored hearing aid transducer attached using a headband. (2) The BC hearing thresholds were measured in human subjects, with the bone vibrator Radioear B71 attached to the same seven stimulation positions. Study sample: Three cadaver heads and twenty participants. Results: Stimulation on a position superior-anterior to the pinna generated the largest promontory motion and the lowest BC thresholds. Stimulations on the positions superior to the pinna, the mastoid, and posterior-inferior to the pinna showed similar magnitudes of promontory motion and similar levels of BC thresholds. Conclusion: Stimulations on the regions superior to the pinna, the mastoid, and posterior-inferior to the pinna provide stable BC transmission, and are insensitive to small changes of the stimulation position. Therefore it is reliable to use the mastoid to determine BC thresholds in clinical audiometry. However, stimulation on a position superior-anterior to the pinna provides more efficient BC transmission than stimulation on the mastoid.

Objectives: As prostheses and techniques related to stapes surgery develop and improve, there is a need to assess the functional outcomes of the surgery objectively. This study provides a bench test method to assess the functional results of stapes surgery by measuring volume displacement at the round window (RW), which is closely related to pressure propagation of the travelling wave inside the cochlea and thus to hearing. Design: Motion of the RW membrane in fresh temporal bones was measured using a scanning laser Doppler interferometry system for normal and reconstructed conditions, and the performance of the reconstruction with stapes surgery was quantitatively assessed by comparison of the volume displacements at the RW between the two conditions. To obtain optimal measurements, reflectivity of the laser beam of the scanning laser Doppler interferometry system was improved by retroreflective beads coated onto the surface of the RW, and orientation of the RW membrane relative to the laser beam was obtained using micro-computed tomography imaging. Results: From measurements in 12 temporal bones, difference in the RW volume displacement between normal ears and ears reconstructed with stapes surgery was approximately 15 dB below 2 kHz and approximately 10 dB above 4 kHz, which was comparable with air–bone gaps in patients after stapes surgery. Two different sizes of the stapes prostheses were also tested (n = 3), and a tendency toward a better outcome with a larger diameter was found. Conclusion: The method developed in this study can be used to assess various prostheses and surgical conditions objectively in controlled laboratory environments. It may also have potential for providing ways to assess other middle- and inner-ear surgeries, and to study other aspects of hearing science.

Recent measurements of vibrational motion to assess sound transmission through ear structures and skull contents have included three-dimensional (3D) behavior. The 3D motion of a point has been described with the three orthogonal components in the 3D space. In this article, a method to represent the 3D velocity with the magnitude and phase of the resultant velocity is introduced. This method was applied to the measurement of cochlear promontory motion as an indication of bone conduction (BC) sound transmission. The promontory motions were measured on the ipsilateral and contralateral sides, and the transcranial attenuation and phase delay of the contralateral side relative to the ipsilateral side were calculated. The transcranial attenuation and phase delay calculated with the maximum magnitudes and corresponding phases of the resultant were a better fit to the interaural threshold difference and transcranial time interval between the ipsilateral and contralateral sides as reported in the literature, than the attenuation and phase delay calculated with any individual Cartesian motion component.