Research on The Low Frequency Curved Hydrophone

In order to receive low frequency sound waves with high sensitivity, a double sided three-lamination flexural hydrophone was studied, applying the finite element software COMSOL to the simulation and optimization design of the curved hydrophone. The influence of every part on the receiving sensitivity grade of the hydrophone was analyzed to provide the optimal scheme. Finally, we produced a hydrophone prototype and tested it in the water. The maximum size of the hydrophone prototype was 45 mm. The experiment results show that in the receiving frequency range 500 Hz−2.5 kHz, the maximum receiving pressure sensitivity grade was −178 dB, undulating less than 4 dB. The experiment result is the same as that of simulation.

As an underwater acoustic signal receiving device, a sound pressure hydrophone can be used to capture subtle changes in the underwater sound pressure signals, generating a voltage output proportional to the sound pressure, and convert sound energy into electrical signals that are easy to observe, The key equipment for the normal operation of the sonar system is an indispensable and necessary equipment in the underwater acoustic research. However, the existing low-frequency, high-sensitivity hydrophones often have a relatively large size. The three-stacked disc structure of the transducer, the bending vibration mode dominates the vibration, has the characteristics of low resonant frequency, small size, simple structure and so on. However, in the application of the three-stack disc, it is more used on the transmitting transducer or vector hydrophone, and less on the acoustic pressure hydrophone. The disadvantage of low-frequency bending hydrophones is that the working frequency band is very narrow, but like commercially available hydrophones, the bandwidth is very wide, but the sensitivity level is not high. If there is a need to receive sound waves only in a specific low frequency band, the laminations are bent The hydrophone with the structured structure has the advantage of high sensitivity level and has its use value. This paper intends to design a three-lamination curved hydrophone, which takes advantage of the small size and low resonance point of the three-lamination disc, and adopts the design form of connecting two upper and lower three-lamination discs in parallel, and adjusts the fundamental frequency through size optimization. The position of the resonance point is used to realize a small-sized hydrophone with high sensitivity response in low frequency band.


1 The design of the three-lamination curved hydrophone
Three-lamination bending hydrophone, the middle part is a metal ring, the metal ring symmetrically bonds two three-lamination discs up and down, the piezoelectric ceramics of the three-lamination discs are connected in series, and the upper and lower two three-lamination discs are connected in series. Through parallel connection, this structure can make the hydrophone vibrate symmetrically, and is easy to assemble and manufacture.

2 Finite element simulation of hydrophone
COMSOL multiphysics simulation finite element software, with acoustic-piezoelectric interaction module, can be used to analyze multi-physics problems such as fluid-structure coupling in plane wave or spherical wave sound field, and can directly simulate the working scene of hydrophone receiving sound waves in water. And can extract the corresponding voltage of the piezoelectric ceramic surface of the hydrophone to calculate the receiving sensitivity. This article uses COMSOL software to analyze and design the curved hydrophone.

2.1 Finite element simulation model of hydrophone
Use COMSOL multiphysics simulation software to perform finite element analysis on the designed hydrophone. First, establish the finite element model of the hydrophone, and ignore the bonding layer between the piezoelectric ceramic and the metal, the bonding layer between the metals, and the polyurethane rubber potted in the outermost layer in the modeling.Establishing a three-dimensional model of the hydrophone with glue and welded electrode wires, choosing PZT-5 as the piezoelectric ceramic material, choose duralumin, copper or steel as the material for the middle metal disc, and choose copper as the material for the middle metal ring.

2.2 Research on the vibration mode of hydrophone
Using COMSOL software to analyze the characteristic frequency of the hydrophone, you can intuitively obtain the characteristic frequency and vibration displacement of the different vibration modes of the hydrophone. The schematic diagram includes the relative position of each part of the hydrophone in each vibration mode. These analysis results help to better understand the working principle of the hydrophone. The vibration of the first-order vibration mode of a hydrophone of a certain size. This vibration mode is the mode when the hydrophone receives sound waves.


2.3 Structural optimization design of hydrophone
Using COMSOL software to simulate and analyze the working performance of the hydrophone in the water. You can directly establish a water area with a radius of 0.05 m around the hydrophone, and then set a plane sound wave background field with a sound pressure of 1 Pa in the water area to simulate The actual working scenario of the hydrophone in water, the established underwater model of the hydrophone is shown in Figure 4. In the COMSOL analysis setting, the research step selects the frequency domain, so that the response of the entire linear system when subjected to simple harmonic excitation can be analyzed, and the voltage excited by the hydrophone under the action of sound waves of different frequencies can be calculated. Then extract the voltage on the piezoelectric ceramic surface of the hydrophone, and calculate the corresponding receiving sensitivity level of the hydrophone through a formula. Since the hydrophone works in an open-circuit state, the peak of the hydrophone's receiving sensitivity is at its anti-resonance frequency, and the receiving sensitivity level of a hydrophone of a certain size is simulated.


It can be seen from the simulation results that the receiving sensitivity level curve of the hydrophone with this structure is relatively flat in the low frequency band. Next, we will study the dimensional changes of each part of the hydrophone, and the effect of the anti-resonance frequency and the low frequency receiving sensitivity level of the hydrophone Influence. Taking the geometric parameters of the PZT and the metal discs in the tri-stack, and the type of metal materials as variables , the size and fluctuation degree of the designed hydrophone's sound pressure receiving sensitivity level in the low frequency band are taken as the goal, and the hydrophone is carried out. The optimized design of the hydrophone strives to make the sound pressure receiving sensitivity level of the hydrophone in the low frequency band as high as possible and the fluctuations as small as possible. The variables used in the simulation analysis of the controlled variable method are: 1) the material properties of the three laminated metal discs; 2) the ratio of the PZT radius to the metal sheet radius; 3) the ratio of the PZT thickness to the thickness of the metal sheet; 4) the thickness of the three laminated sheets of equal thickness compared with the radius.

2.3.1 Types of PZT and types of metal sheets
Change the type of metal disc in the middle of the three laminations, and obtain the anti-resonance frequency and receiving sensitivity level curve of the hydrophone in water by simulation calculation. The results are shown in Table 1 and Figure 6.


It can be seen from Table 1 that as the Young's modulus of the selected metal gradually increases, the anti-resonant frequency of the hydrophone gradually increases. It can be seen from Fig. 6 that as the Young's modulus of the metal sheet gradually increases, the receiving sensitivity level of the low frequency band of the hydrophone gradually decreases.

2.3.2 Ratio of PZT radius to metal sheet radius
Keep the thickness of the PZT and the intermediate metal sheet unchanged, and take the radius of the intermediate metal sheet as 20 mm. When only the PZT radius is changed, the hydrophone anti-resonance frequency and receiving sensitivity level curves in the water are shown in Figures 7 and 8.