## 6 Conclusions and Future Outlook  
There is an increasing trend of utilizing piezoelectric MEMS resonators for the design of AE sensors. While MEMS have the geometric flexibility in tuning the sensor frequency to a particular value with narrow bandwidth, mass manufacturing and low-cost design, the demonstrations of MEMS AE sensors for detecting real AE sources, rather than stimulations and field deployment, are limited. While the sensitivity of MEMS AE sensors needs to be improved, they are applicable to detecting AE sources with high energy release rates such as wire breaks. A robust packaging design encapsulated against a harsh environment for long-term field deployment is needed.  
A monolithic integration of multiple frequencies by combining individual resonators could overcome the sensitivity limitation. As an example, two highly resonant MEMS piezoelectric sensors are connected in an array such that the total response is increased by the constructive interference of carefully selected frequencies. Figure 8 shows individual responses of 200 kHz and 300 kHz sensors designed by PiezoMUMPs, and their combined response using a single channel output. The sensors are excited by a chirp signal in the range of 100-400 kHz. The frequency spectrum of the combined signal clearly shows two distinct frequencies with the constructive interference, such that the total response is a simple summation of two individual responses.  
In summary, while there are numerous examples of MEMS AE sensors in the literature, they have been evaluated in a laboratory environment, mostly using simulated AE sources. The transition of MEMS AE sensors into practice requires (a) comparable sensitivity to conventional AE sensors, (b) robust packaging to function well in harsh environments with long-term durability, (c) the ability to detect actual AE sources in a laboratory environment and (d) field deployment for side-to-side comparison with conventional AE sensors.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Design Geometry** & **Frequency Range** & **AE Demonstration** & **Specific Design** & **Reference** \\ \hline Thick poled PVDF film & 10 kHz to 1.4 MHz & Drilling experiment & \begin{tabular}{c} Sensitivity measured \\ with laser interferometry \\ \end{tabular} & [4] \\ \hline PZT nanofibers on flexible substrate & Wideband & Steel ball impact & Flexible AE sensor & [50] \\ \hline ZnO on rectangular silicon substrate & 78.9 kHz & None & \begin{tabular}{c} Mostly acoustic pressure \\ sensing in air \\ \end{tabular} & [51] \\ \hline \begin{tabular}{c} Cantilever beam with \\ PZT film \\ \end{tabular} & 200–390 Hz & \begin{tabular}{c} Stimulations and wear in \\ knee joint \\ \end{tabular} & \begin{tabular}{c} Thermoresponsive \\ polymer actuators added \\ to increase sensitivity \\ \end{tabular} & [48,49] \\ \hline AlN layer on octagon diaphragm & 40 kHz, 200 kHz & \begin{tabular}{c} Actuation using another \\ piezoelectric sensor on a \\ composite panel \\ \end{tabular} & \begin{tabular}{c} Super narrowband, good \\ amplitude/size ratio \\ compared to \\ conventional sensors \\ \end{tabular} & [39] \\ \hline \begin{tabular}{c} Square diaphragm \\ with a ZnO layer \\ \end{tabular} & 115 kHz & \
\begin{tabular}{c} Laser Doppler \\ Vibrometer \\ \end{tabular} &
\begin{tabular}{c} Acoustic vibration \\ sensing \\ \end{tabular} & [52] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Piezoelectric MEMS AE sensors and their specifications.  
**Funding:** This research was funded by National Science Foundation under Award No. IIP 2016444 entitled \"PIFI-TT: Multi-Frequency Acoustic Device for Rapid Infrastructure Damage Diagnostics\".  
**Conflicts of Interest:** The author declares no conflict of interests.