# Frequency Response Extension Method of MET Vector Hydrophone Based on Dynamic Feedback Network
## 2 Principles and Background  
### Structure and Acoustic Theory of Hydrophone  
As a hydrophone using new transduction technology, compared with traditional electromechanical transducer hydrophones, such as vibrating hydrophones and piezoelectric hydrophones, the biggest feature of electrochemical vector hydrophones is the use of a high-sensitivity electrochemical transducer as the sensing element. The electrochemical transducer detects the migration of ionic charges involved in redox reactions in the solution by measuring the cathodic current output from electrode reactions, thereby characterizing the motion state of the liquid. The external sound pressure signal is derived from the motion state of the electrolyte inside the reaction chamber.  
The overall structure of the electrochemical hydrophone is shown in Figure 1. The electrochemical hydrophone consists of a cover, a sound window embedded with permanent magnetic material, a protective case, coils, coil mounting slots, an electrochemical cavity, a circuit cabin, and electrochemical transducers. The sound window is made of a material with low acoustic impedance and a thin thickness to minimize the energy loss of sound waves transmitted from the outside to the internal sensitive elements. It is embedded with permanent magnetic material, allowing the excitation current of the coils to generate feedback force through electromagnetic induction. The protective case is made of a metal material with excellent pressure resistance and corrosion resistance. The electrochemical cavity contains an electrolyte and electrochemical transducers, which play a crucial role in signal conversion. The circuit cabin includes signal processing and power supply components. During operation, sound wave signals enter the electrochemical cavity through the sound window, causing vibrations in the electrolyte. The electrochemical transducers convert these vibrations into electrical signals, which are used for monitoring sound pressure signals. The structure also includes coil mounting slots, where coils are installed above and below the electrochemical cavity, ensuring proper electromagnetic interaction with the sound window.  
The circuit system is located in the circuit compartment, including the digital signal processing circuit and analog conditioning circuit. The analog conditioning circuit can be divided into two parts according to the main functions, signal processing circuit and power supply circuit. The analog conditioning circuit is externally connected to the cable line for power supply and the signal line is led out through the cable line. The signal line can be directly connected to the oscilloscope for data observation or connected to the acquisition interface for data recording.  
In the working process, the sound wave signal enters the electrochemical reaction chamber through the sound window with small acoustic impedance, causing the internal electrolyte solution to vibrate with the sound wave signal. The change of the ion concentration gradient near the electrode causes the output current difference of the cathode to be non-zero and positively correlated with the external sound pressure signal. After the sound pressure signal is processed by the analog conditioning circuit, it is output to the digital signal processor for signal processing, and the digital signal processor outputs a feedback signal, which generates feedback force through the coil, so that both the frequency response of the system and the low-frequency detection capability of the hydrophone are improved.