Javel Eric, Grant Iain L, Kroll Kai
Department of Otolaryngology, University of Minnesota Medical School, Minneapolis, MN 55455, USA.
Otol Neurotol. 2003 Sep;24(5):784-95. doi: 10.1097/00129492-200309000-00016.
Piezoelectric bimorph transducers may be used at the input stage of implantable hearing aids to convert ossicle vibrations into electrical waveforms, and at the output stage to convert electrical signals into mechanical motion that drives the ossicles. This study assessed transducer performance in anesthetized, acutely implanted cats using computer-averaged, laser-Doppler vibrometer measures and cochlear potentials.
Measures of output linearity and distortion for a transducer placed on the umbo were obtained from averaged laser-Doppler vibrometer outputs. Frequency response and equivalent sound pressure level for transducers placed against the stapes were estimated by comparing compound action potentials and cochlear microphonics elicited preoperatively by acoustic signals with responses elicited postoperatively by signals presented through transducers.
The transducer placed on the umbo exhibited an effective bandwidth that exceeded 8 kHz, linear response behavior for driving voltages up to 2 Vrms, and harmonic distortion of -40 dB or better at all frequencies greater than 250 Hz. Except for a shorter latency, transducer-elicited cochlear potentials were indistinguishable from acoustically elicited responses. Frequency response varied widely across transducers, ranging from reasonably flat to possessing a bandpass characteristic with a peak at 2 to 4 kHz; 1-Vrms signals applied to transducers with various geometries yielded equivalent intensities of 62 to 108 dB sound pressure level at 4 kHz, 51 to 98 dB sound pressure level at 2 kHz, and 55 to 80 dB sound pressure level at 1 kHz. Differences in frequency response and equivalent sound pressure level stemmed from different resonance frequencies in transducers with dissimilar lengths and, more importantly, from variation in transducer-stapes contact force.
Appropriately designed piezoelectric transducers can provide the cochlea with high-fidelity, wide-bandwidth signals. However, using them in implantable hearing aids requires that geometry and contact force be optimized to reduce variability in output level. Recording cochlear potentials is a cost-effective means of assessing transducer performance intraoperatively, but care must be exercised to take into account any temporary, drill-induced sensitivity loss.
压电双压电晶片换能器可用于植入式助听器的输入阶段，将听小骨振动转换为电波形，在输出阶段将电信号转换为驱动听小骨的机械运动。本研究使用计算机平均激光多普勒振动计测量和耳蜗电位评估了麻醉下急性植入猫体内的换能器性能。
从平均激光多普勒振动计输出中获得置于鼓膜脐部的换能器的输出线性度和失真度测量值。通过比较术前由声信号诱发的复合动作电位和耳蜗微音电位与术后通过换能器呈现的信号诱发的反应，估计置于镫骨上的换能器的频率响应和等效声压级。
置于鼓膜脐部的换能器有效带宽超过8kHz，对于高达2Vrms的驱动电压呈现线性响应行为，在所有大于250Hz的频率下谐波失真为-40dB或更低。除了潜伏期较短外，换能器诱发的耳蜗电位与声诱发的反应无法区分。不同换能器的频率响应差异很大，从相当平坦到具有在2至4kHz处有峰值的带通特性；施加到具有各种几何形状的换能器上的1Vrms信号在4kHz时产生62至108dB声压级的等效强度，在2kHz时产生51至98dB声压级，在1kHz时产生55至80dB声压级。频率响应和等效声压级的差异源于长度不同的换能器的不同共振频率，更重要的是源于换能器与镫骨接触力的变化。
设计得当的压电换能器可为耳蜗提供高保真、宽带宽信号。然而，在植入式助听器中使用它们需要优化几何形状和接触力，以减少输出水平的变异性。记录耳蜗电位是术中评估换能器性能的一种经济有效的方法，但必须注意考虑任何由钻孔引起的暂时敏感性损失。
