High performance piezoelectric acoustic actuators based on optimization of the triple-layered multimorphs and vibrating diaphragms

Abstract

Recently, as mobile IT products have required smaller, slimmer, lighter, and more power-efficient components with the advent of smartphones and tablet PCs, piezoelectric acoustic actuators have attracted a great deal of attention as the most promising devices to overcome the thickness limitation of the traditional dynamic speakers. The piezoelectric acoustic actuators have significant benefits of being thinner, more power-effient and lighter than the traditional voice-coil speakers. However, general piezoelectric acoustic actuators have still suffered from limitations of acoustic performance such as high driving voltage, lower acoustic output, and bad sound playback quality by the fluctuating frequency response. In order to improve these disadvantages, the piezoelectric ceramic and acoustic diaphragm should be respectively designed to provide a relatively larger mechanical deflection at low voltage and be more flexible. In other words, higher acoustic performance of piezoelectric ceramic speakers can be achieved by design optimization of a piezoelectric ceramic and an acoustic diaphragm. In this study, we have investigated the influence of the optimized triple-layered multimorph ceramics and acoustic diaphragms on the vibrational and frequency response characteristics of the piezoelectric acoustic actuators.

Firstly, in order to select the best suitable material composition of PZT ceramics for high performance piezoelectric acoustic actuators, we investigated the influence of Zr/Ti ratio of PZT materials on their structur-al and piezoelectric properties. For Nb-doped PZT samples with Zr/Ti ratios of 53/47, 55/45, and 56/44, it was found that higher crystallization of perovskite phases can be observed with the increase in Zr content. In addition, the atomic concentrations and grain sizes measured by AES and EDX, and FE-SEM showed that all fabricated Nb-doped ‘soft’ PZT ceramics had lower lead concentration which can result in large piezoelectric coefficient, large electromechanical coupling factors and low coercive fields, and had larger grain size with the increase of Zr content. The larger grain size can lead to higher piezoelectric polarization properties due to their easy polarization switching characteristics. The measured hysteresis loops also confirmed that the in-crease of the Zr content in PZT results in higher $P_r$. Therefore, we selected Nb-doped PZT (56/44) with highest Pr as best suitable material composition for high performance piezoelectric ceramic speakers.

Secondly, the triple-layered multimorph piezoelectric ceramic speakers were successfully fabricated and studied to investigate the effect of the electrical configurations of the piezoelectric ceramics on the volt-age-displacement and frequency response characteristics. The serial and parallel multimorph ceramic speakers were electrically connected in two different configurations of series and parallel, respectively: the serial multimorph in series connection (SMS), the serial multimorph in parallel connection (SMP), the parallel multimorph in series connection (PMS), and the parallel multimorph in parallel connection (PMP). The resonant displacements of the fabricated piezoelectric speakers increased with the increase of the voltage but decreased abruptly above the coercive voltage due to the domain reorientation induced from the applied field. Below the coercive voltage, the maximum resonant displacements for SMS and PMP types were at least 4.8 times higher than SMP and PMS types. It is because the out-of-plane vibrational displacements for SMS and PMP types are higher than SMP and PMS types as well as their resonant displacements are generated by the coupling between the out-of-plane and in-plane motions. Due to these larger displacement characteristics, the triple-layered multimorph ceramic speakers for SMS and PMP types had around 20 dB higher sound pressures than those for SMP and PMS types. Moreover, the speakers with the triple-layered SMS and PMP types had at least 10 dB higher sound pressures at a low-frequency range less than 1 kHz, compared to a commercial piezoelectric speaker.

Thirdly, the vibrational characteristics of 3 types of the acoustic diaphragms were investigated to en-hance the output acoustic performance of the piezoelectric ceramic speaker at a low-frequency range. In other to achieve both a higher output sound pressure level and wider frequency range of the piezoelectric speaker, we have proposed the rubber/resin bi-layer acoustic diaphragm. The theoretical square root dependence of the fundamental resonant frequency on the thickness and Young`s modulus of the acoustic diaphragm was verified by the FEA simulation and LSV measurement. The simulated resonant frequencies for each diaphragm correspond well to the measured results. From the simulated and measurement resonant frequency results, it is found that the fundamental resonant frequency of the piezoelectric ceramic speaker can be designed by adjusting thickness ratio of the rubber/resin bi-layer acoustic diaphragm. Compared to a commercial piezoelectric speaker, the fabricated piezoelectric ceramic speaker with the rubber/resin bi-layer diaphragm has at least 10 dB higher sound pressures at a low-frequency range of less than 1 kHz.

Finally, we presented an optimal design of the acoustic diaphragm to improve the frequency response characteristics of the piezoelectric ceramic speaker. The FEA simulation and laser scanning vibrometer (LSV) measurement results showed that the dip and peak correspond respectively to (1,3) and (3,1) resonant modes of the acoustic diaphragm. By attaching the elastic mass to the acoustic diaphragm, the resonant frequencies corresponded to (1,3) and (3,1) modes shifted to higher frequencies and the vibrational displacements at each mode were quite reduced by about 40 %. In addition, the significant dip of (1,3) mode in SPL of the piezoe-lectric acoustic actuator was dramatically improved by the elastic mass attached to the acoustic diaphragm. This study presents that optimized elastic mass attached to the acoustic diaphragm allows flat and smooth frequency response characteristics for the improved sound quality of the piezoelectric acoustic actuator.