Research On the Low Frequency Curved Hydrophone（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.

It can be seen from Fig. 7 that as the radius of the PZT increases, the anti-resonant frequency of the hydrophone in the water gradually increases, and when it approaches 20 mm, the anti-resonant frequency hardly increases. Figure 8 shows that as the PZT radius becomes larger, the receiving sensitivity level of the hydrophone in the low frequency band gradually decreases, but the degree of decrease is not large, and the fluctuations are more flat. 2.3.3 The ratio of PZT thickness to metal thickness keeps PZT and the radius of the middle metal sheet unchanged. The thickness of the middle metal sheet is 1 mm, and only the PZT thickness is changed. The anti-resonance frequency and receiving sensitivity level curve of the hydrophone in water are shown in Figure 9 and 10.

It can be seen from Figure 11 that as the thickness of the PZT (metal sheet) increases, the anti-resonant frequency in the water of the hydrophone gradually increases. In Figure 12, as the thickness of the PZT (metal sheet) gradually increases, the receiving sensitivity level of the hydrophone in the low frequency band gradually decreases, and the fluctuations gradually become smaller.

2.3.5 Regularity analysis

The response change law obtained in the above optimization process can be summarized as follows: 1) As the Young's modulus of the middle metal disc gradually increases, the anti-resonant frequency of the hydrophone gradually becomes larger, and the receiving sensitivity level of the low frequency band becomes smaller and fluctuates. 2) As the ratio of the PZT to the metal sheet radius becomes larger, the anti-resonant frequency of the hydrophone in water becomes larger, the receiving sensitivity level of the low frequency band decreases, and the fluctuation becomes smaller; 3) As the ratio of the PZT thickness to the metal sheet thickness becomes larger , The anti-resonant frequency of the hydrophone in the water first increases and then decreases, reaching the peak value at a ratio of 1, and the low-frequency receiving sensitivity level first increases and then decreases, reaching the peak at a ratio of about 0.5, and the low-frequency fluctuations gradually decrease; 4) etc. In the thick triple laminate, as the ratio of the thickness to the radius of the PZT (metal sheet) becomes larger, the anti-resonant frequency of the hydrophone in water becomes larger, the receiving sensitivity level in the low frequency band becomes smaller, and the fluctuation becomes smaller. In general, the larger the transducer size, the smaller its resonance frequency, and the fundamental resonance frequency of the hydrophone increases with the increase of the PZT radius or thickness. This is because the hydrophone uses three The flexural vibration mode of the laminated sheet. The main influencing factor of this vibration mode is the stiffness of the triplex. When the PZT radius or thickness increases, the stiffness of the entire triplex becomes greater, so the resonance of the triplex flexural vibration mode The frequency will become larger, making the resonance frequency of the hydrophone larger. The height of the metal ring clamped in the middle of the hydrophone is much smaller than the diameter of the three-layered sheet, and it does not participate in the bending vibration of the three-layered sheet, so the impact on the hydrophone is small.

2.4 Final result

According to the above-mentioned influence law through structural optimization, and taking into account the difficulty of the actual production process of the various parts of the hydrophone, the size parameters of the various parts of the hydrophone are finally determined as shown in Table 2. Use COMSOL software to simulate and calculate the impedance curve of the hydrophone in water. The anti-resonant frequency is 5.2 kHz, as shown in Figure 13.

Use COMSOL software to simulate and calculate the receiving sensitivity level of the hydrophone in the frequency range of 100 Hz to 6 kHz, as shown in Figure 14.

Using COMSOL software to simulate and calculate the receiving sensitivity level of the hydrophone in the frequency range of 100 Hz to 6 kHz, as shown in Figure 14.

In the low frequency band 100 Hz~2.5 kHz, the receiving sensitivity level of the hydrophone is about −178 dB, and the fluctuation is less than 3 dB, as shown in Figure 15. When the wavelength of the sound wave is much larger than the maximum linear scale of the transducer, the transducer has no directivity. In the working frequency band of the hydrophone, the minimum wavelength when the sound wave frequency is 2.5 kHz is 0.6 m, which is larger than the maximum size of the hydrophone by 0.045 m, it can be considered that the hydrophone has no directivity when receiving sound waves.

3 Production and testing of hydrophone

According to the final structural parameters of the hydrophone optimized by COMSOL, the structural components were processed and the hydrophone prototype was made, as shown in Figure 16. After potting, the diameter of the hydrophone is 45 mm and the thickness is 12 mm.

The performance test of hydrophone was carried out in an anechoic pool, the size of the pool was 25 m × 16 m × 10 m, and the comparison method was used for measurement, and a standard hydrophone (B&K 8105) was used for comparison measurement. Pulse signal transmission is adopted, and the distance between the transmitting transducer and the standard hydrophone is 1.5 m (satisfying the far-field condition), and it is placed along the length of the pool with a hanging depth of 4 m. The admittance curve in the water of the prototype hydrophone is finally measured as shown in Figure 17.

It can be seen from Figure 17 that the anti-resonant frequency of the hydrophone prototype is 3.3 kHz. Due to the limitation of the lower limit of the sound wave frequency that the transmitting transducer used can only transmit 500 Hz sound wave, the lowest frequency of the measuring water receiving sensitivity level curve is 500 Hz, as shown in Figure 18.

It can be seen from Figure 18 that in the 500 Hz ~ 2.5 kHz frequency band, the receiver sensitivity level of hydrophone is at most −178 dB, and the fluctuation is less than 4 dB. The difference between the measuring and simulated results of the anti-resonant frequency of the hydrophone is mainly due to the fact that the surface of the hydrophone prototype is potted with a layer of watertight polyurethane rubber with a thickness of 2 mm, which will increase the equivalent vibration quality of the hydrophone. It is difficult to simulate this viscoelastic material on the COMSOL simulation software. The assembly accuracy of the structural parts and the bonding process will also have a certain impact on the performance of the hydrophone. The above two factors cause the difference between the measured data and the finite element simulation value. . Compare the measured data of the receiving sensitivity level in the 500 Hz~2.5 kHz frequency band with the simulation results, as shown in Figure 19. In this frequency band, the measured maximum receiving sensitivity level is −178 dB, and the fluctuation is less than 4 dB. The measured data and the simulated value The trend is the same, and the measured data fluctuates slightly larger than the simulated value.

4 Conclusion

1) Designing and producing a low-frequency bending hydrophone. The measuring hydrophone has a receiving sensitivity level of −178 dB in the frequency band 500Hz−2.5 kHz, and the fluctuation is less than 4 dB. 2.The small-size low-frequency bending hydrophone has realized the characteristics of receiving sound waves with higher sensitivity, which has guiding significance for the application of the bending disc structure in the hydrophone.