Recently, researchers from the Institute of acoustics, Chinese Academy of Sciences (anzhiwu, Wang Xiaojie) proposed a loudspeaker diaphragm structure using a new fan ring drive unit, which has excellent sound pressure level and flat high-frequency response. This research work provides a new idea for the design and optimization of piezoelectric MEMS speakers. The relevant research results have been published in the Journal of piezoelectric and acousto optic.

MEMS speakers have the characteristics of high precision, small size, low power consumption, on-chip circuit integration, mass manufacturing, low cost, high reliability and repeatability. Its driving mode is generally divided into electromagnetic, electrostatic and piezoelectric. At present, piezoelectric thin film materials (ZnO, ain, PZT, etc.) have been widely used in sensing and driving. MEMS speakers based on piezoelectric effect have the advantages of low power consumption, single diaphragm structure, simple manufacturing and processing, wide frequency response and so on. However, the insufficient output sound pressure level and uneven frequency response are still the key factors restricting its development.

Schematic diagram of existing MEMS speaker structure: fixed circular diaphragm piezoelectric MEMS speaker

In order to solve the above problems, researchers proposed a new speaker structure. The loudspeaker diaphragm consists of four identical fan ring driving units (including support layer, bottom electrode, piezoelectric layer and top electrode), which are distributed in a circle with a radius of 1.5mm, the middle is a circular mass block, the substrate is silicon on insulator (SOI), and the upper surface is covered with a polyimide flexible film to form a rigid flexible coupling closed diaphragm.

The diaphragm structure of the fan ring drive unit is consistent with the fixed circular multilayer diaphragm reference structure in terms of key parameters such as size. Researchers use finite element simulation software to analyze the radius R2 of circular mass block in the fan ring drive structure and the angle between adjacent edges of adjacent fan rings θ And other key parameters are optimized, and it is found that when θ = 50°，r2 = 700 μ m. When F0 = 9689.4hz, the corresponding sound pressure level is the maximum.

Sound pressure level at 1kHz and θ、 R2 relation chromaticity diagram

Through calculation, the researchers further obtained the frequency response curve of the piezoelectric MEMS loudspeaker of the new fan ring driving unit structure and the reference structure. Under the condition of maintaining the same diaphragm area and basically the same diaphragm resonant frequency, compared with the reference structure piezoelectric MEMS speaker, the sound pressure level of the new fan ring drive unit structure is 5dB higher than that of the optimized reference structure piezoelectric MEMS speaker in the range of 100hz~9.3khz, and the high-frequency response should be flatter.

Comparison of frequency response curves of piezoelectric MEMS loudspeaker between fan ring driving unit structure and reference structure

With the continuous improvement of piezoelectric film technology and material properties, the performance of piezoelectric MEMS speakers is also continuously enhanced. In order to further realize the miniaturization, high sound pressure level and flat frequency response of piezoelectric MEMS speakers, continuous innovation in diaphragm structure, electrode design, array design and other aspects is also needed in the future to better meet the needs of mobile phones, laptops, wearable devices, medical imaging devices and various emerging applications.