# Speech Communication System Based on Piezoelectric Electret Mechanical Antenna
## 1 Introduction  
In solid and liquid environments, high-frequency electromagnetic (EM) waves will rapidly attenuate due to the skin effect, making them more challenging to apply in wireless communication areas like underwater and underground communications [1; 2; 3; 4]. With the advantages of strong penetration and low attenuation, low-frequency (LF, 30-30 kHz), very-low-frequency (VLF, 3-30 kHz), and ultra-low-frequency (ULF, 300-3000 Hz) EM waves have excellent propagation performance in solids and liquids and are widely used in underwater communication, earthquake prediction, and other applications [5; 6; 7; 8]. However, the size of traditional resonant antennas is typically comparable to the wavelength of EM waves. When resonant antennas are used for long-wave communication, they inevitably grow to huge sizes beyond a hundred meters, facing problems such as high cost, high energy consumption, and a low power conversion rate [9; 10; 11].  
Currently, mechanical antennas have become a research hotspot for LF, VLF, and ULF antennas. Mechanical antennas are appealing due to their ease of miniaturization, low cost, and simple impedance matching network for low-frequency communication. Mechanical antennas are driven by mechanical rotation or vibration, and they radiate EM waves via the movement of internal electric charges or magnetic dipoles [12; 13]. Permanent magnet mechanical antennas and piezoelectric mechanical antennas are the two types of mechanical antennas. Permanent magnet antennas generate EM waves by driving permanent magnetswith external motors [14; 15; 16]. Meanwhile, piezoelectric mechanical antennas generate self-resonance through the piezoelectric effect and oscillate their internal dipole moment, thus radiating EM waves [17; 18; 19].  
Despite the potential benefits, current mechanical antenna-based communication systems still face significant challenges. As an emerging communication composite, existing systems of mechanical antennas are rudimentary and still in the starting phase without complete communication protocol design and software support. They are often developed independently for specific mechanical antennas or applications, leading to a high degree of coupling and poor system compatibility. In addition, the working methods of existing systems are inconvenient to use, and the operating procedures for these systems are often cumbersome, requiring specialized knowledge and even direct operation of underlying hardware devices for information transmission and reception [14; 15; 16; 17; 18; 19].  
To solve these challenges, a novel speech communication system based on the PZT-42 piezoelectric electret mechanical antenna is proposed. The working mechanism and performance of the PZT-42 mechanical antenna are briefly introduced, and the components of the speech communication system are well explained. This speech communication system is proposed with the following key contributions:  
* A mechanical antenna communication hardware system is designed;
* A three layer hierarchical mechanical antenna communication protocol is proposed;
* A software system with an extensible modular architecture is proposed.  
Additionally, the system proposed a human-computer interaction that provides speech input for messages to be transmitted in order to achieve a humanized design. As a supplement, a case study is conducted to demonstrate the overall workflow of the system and the specific responsibilities of each component. By integrating the system with mechanical antenna-based communication, data transmission and message processing are effectively decoupled, and compatibility and scalability are greatly enhanced.