人類目前使用的電子產品除了不斷朝著輕、薄、短小的目標進步，在研究擴音器的領域中亦不例外，但傳統動圈式之擴音器乃磁力所驅動，產生磁推力之線圈卻有縮小化反而不利其輸出表現的缺點，因此具有微小化可能性之壓電式擴音器逐漸受到重視。目前市面上的壓電式擴音器之設計均採用傳統壓電材料如壓電陶瓷 (PZT)，雖壓電效率佳但因材質較硬，故輸出的音質均較偏高，且於低頻的反應仍不如動圈式擴音器。故本研究所開發之壓電式微擴音器，目的即在改善壓電式擴音器於低頻反應不佳的缺點，並同時保有高聲音品質、低耗電量之軟性微型壓電式擴音器。研究中使用高分子壓電材料 (PVDF)，其材質較軟故可發出較低頻率的聲音，以改善傳統陶瓷壓電材料之缺點，但 PVDF 之壓電係數不如陶瓷壓電材料，故在結構上利用雙層環狀高分子壓電材料 (ring-type bimorph) 作為激振器，並於雙層環狀壓電材料中挾持一高分子薄膜聚二甲基硅烷 (PDMS) 作為振動薄膜，此三明治結構具有加強壓電激振效果，使低頻效果與音質保真度
(damping effect) 更佳。此外，可撓性之優點，適合於各種小型電子發聲產品以及穿戴式軟性電子的應用。

Micromachined acoustic devices are recently being focused on a lot of
applications such as hearing aid, cellphone, earphone as well as wearable electronics.
However, the traditional dynamic loudspeaker has the limitation on size reduction due
to the magnet, coils and chamber. On the other hand, piezoelectric loudspeaker not only
can generate good quality of sound without back chamber but also has the possibility of
integration with micromachining technologies. For piezoelectric microspeaker, the
performance in the range of low frequency is still inadequate compared with dynamic
loudspeaker. In this study, in order to enhance the sound pressure in the low frequency
range, we utilize polymer-based piezoelectric thin film (PVDF) instead of ceramic
piezoelectric material such as PZT. In addition, an innovative design of the ring-type
bimorph structure was introduced for compensation on the relative low
electromechanical coupling coefficients (EMCCs) of PVDF. A polymeric thin film,
PDMS, was laminated between the bimorph piezoelectric layers as a vibrating
membrane. The enhancement of vibrating amplitude on PDMS membrane was
confirmed according to the simulation and experimental results due to the better
compliance. Therefore, present study provides not only an efficient way to improve the
performance of piezoelectric type microspeaker at low frequency range but also the
feasible processes to fabricate a flexible microspeaker.