# Coupled D33 Mode-Based High Performing Bio-Inspired Piezoelectric MEMS Directional Microphone
## 4 Results and Discussion  
The experimental measurements were performed in an anechoic chamber using the setup shown in Figure 8. This included a reference microphone (GM1351, digital sound-level meter) to calibrate the incoming sound, a lock-in amplifier (SR830, Stanford Research Systems, Sunnyvale, CA, United States) for data recording, a function generator (DS345, Stanford Research Systems, Sunnyvale, CA, United States) to produce frequency-dependent sound and a speaker to emit the produced sound. The inset of Figure 8 shows the real view of the microphone in the anechoic chamber. The details of this setup can be found in our previous work [8]. In the experimental measurements, the measured sensitivity at 1 kHz frequency and 1 Pa/94 dB of sound pressure level (SPL) sound pressure was found to be 110.46 mV/Pa considering the entire microphone. The measured sensitivity deviated by 6.8% in comparison with the simulated sensitivity, as shown in Figure 6d. Moreover, the deviation could be the reason for the fabrication tolerance as well as the mismatch of the positioning of the sound source relative to the microphone. In comparison with other D33 mode-based works, the measured sensitivity was \(\sim\)876 times higher than the double-layered D33 mode from 126.21 mV/Pa to 110.46 mV/Pa [24] and \(\sim\)7.36 times higher than distributed D33 mode 15 mV/Pa to 110.46 mV/Pa [27]. Moreover, the measurement was extended by varying the acoustic pressure from 0.02 Pa/60 dB SPL to 3.56 Pa/105 dB SPL, and the responses were found to be linear from 1.61 mV at 60 dB SPL to 327.78 mV at 105 dB SPL, as shown in Figure 9a. Taking this one step further, the noise of the microphone was found to be 0.03 mV at 1 kHz frequency and 0 dB of sound pressure, which led to a 71.32 dB signal-to-noise ratio (SNR) together with a sensitivity of 110.46 mV/Pa. The measured results showed a \(\sim\)32 and \(\sim\)1.042 times higher sensitivity and SNR, respectively, compared to the regular D33 mode based on our previous work [8]. Furthermore, the self-noise at 1 kHz frequency was reduced by 11.13% from 25.52 dB SPL to 22.68 dB SPL due to the improved sensitivity and SNR [8]. Figure 8: Experimental setup in an anechoic chamber with a zoomed-in real view of the microphone setup. Figure 9b shows the directional sensitivity (i.e., directionality) of the fabricated bio-inspired piezoelectric MEMS directional microphone with the coupled D33 mode. In the measurements, the fabricated device was placed horizontally on a rotation stage (shown in Figure 8) which provided the flexibility to rotate the whole setup in the azimuth plane from 0\({}^{\circ}\) to 360\({}^{\circ}\). At each rotation, the responses of both diaphragms were measured and summed [8,23]. On the other hand, the formation of the directionality of this kind of directional microphone follows the in-plane directivity and/or cosine dependency [8,23]; as a result, the ideal pressure-gradient formula corresponding to P \(\times\) cos(\(\theta\)) (where P and \(\theta\) are the sound pressure and angular rotation of incoming sound, respectively) was used for comparison with the normalized measured response. Moreover, the normalization of the directionality was performed using the maximum response (110.46 mV/Pa) at 0\({}^{\circ}\)/180\({}^{\circ}\) and minimum response (102.91 mV/Pa) at 90\({}^{\circ}\)/270\({}^{\circ}\) followed by the results found for this kind of microphone [28]. Moreover, the measured directionality showed full compliance with the ideal-pressure gradient sensor, as shown in Figure 9b, and could be extended towards the localization of the incoming sound. In summary, the primary intent of this study was not to describe an optimized D33 mode but rather to present a way of coupling the D33 mode. Due to the coupling, the proposed D33 mode uses the maximum effective area compared with the regular D33 mode. As a result, the proposed D33 mode is capable of transducing a high amount to the mechanical deflection of the diaphragm caused by the applied mechanical stimuli.