# Speech Communication System Based on Piezoelectric Electret Mechanical Antenna
## 2 Materials and Methods  
### Mechanism of the PZT-42 Mechanical Antenna  
Lead zirconate titanate piezoelectric ceramic 42 (PZT-42) is a widely utilized piezoelectric ceramic material with a high dielectric constant [20; 21]. Mechanical antennas made of PZT-42 have been shown to result in substantial enhancements in the radiation intensity of EM waves [17]. The piezoelectric ceramics will be stimulated to generate an inverse piezoelectric effect and produce elastic displacement when the external electric field is applied [22; 23; 24], as shown in Figure 1a. At the same time, the dipole moment inside the electret follows the material displacement and oscillates, thus radiating EM waves externally. The mechanical antenna, with dimensions of 30 \(\times\) 15 \(\times\) 3 mm\({}^{3}\), is comprised of a PZT-42 piezoelectric ceramic substrate with silver electrodes on its upper and lower surfaces, serving as a mechanical antenna in speech communication systems. As illustrated in Figure 1b, the carrier signals amplified by the power amplifier are used as an AC source and fed to the mechanical antenna through the silver electrodes to stimulate the mechanical antenna for radiating EM waves in wireless communication [25]. As demonstrated in Figure 1c,d, the proposed mechanical antenna exhibits a resonant peak and a minimum impedance within the range of 165-170 kHz. At its resonant frequency of 167.34 kHz, the PZT antenna obtains maximum radiation intensity, proving that it is capable of radiating EM waves externally as an antenna. The signal strength of the permanent magnet mechanical antenna depends on the magnetic field strength of the permanent magnet, and the operating frequency and modulation frequency are limited by the performance of the external motor, which also limits the transmission distance and transmission speed of the permanent magnet mechanical antenna. While the signal strength of a piezoelectric mechanical antenna can be improved with an increase in feed voltage, the operating frequency can be modified by changing the material and size of the piezoelectric antenna, and the modulation frequency can be controlled by the carrier signal. Therefore, the operating frequency, transmission distance, and transmission speed of piezoelectric mechanical an tennas are generally higher than those of permanent magnet mechanical antennas, which leads to better communication performance.  
The resonant frequency of piezoelectric mechanical antennas is usually dependent on the materials and dimensions of the antenna. Changing antennas with different piezoelectric materials and dimensions to tune the operating frequency can make the system apply to different applications [17]. Piezoelectric electret mechanical antennas could achieve resonant frequencies in the ULF, VLF, and LF bands with different piezoelectric materials and antenna size designs, allowing the proposed system to work in a variety of applications. The LF antennas can be used for real-time communication and RF control, the VLF antennas for underwater submarine communication, and the ULF antennas for mineral detection and seismic research.