| Advances in the technology of silent submarine made the detection methods of sonar a major change,and active sonar occupied an important position,thus the technique of low-frequency,high-power acoustic transmitting also been in a rapid development in recent yeas.Flextensional transducers have been widely used in low-frequency transmitter area because of their outstanding performance in power-weight ratio,but they generally have a narrow-band and non-directivity in low-frequency.Four-sided type flextensional transducer is a special transducer with its compact internal structure,high utilization of the active material volume,large radiation area,thus it is more suitable as a low-frequency,high-power,small size sound source.The main content of this article is the study of wide-band characteristics and directivity in low-frequency of flextensional transducer.Firstly,the basic structural features,way and principle of four-sided type flextensional transducer were introduced when it works,and finite element software ANSYS was selected to assist design.On this basis,a new structure of symmetric four-sided type flextensional transducer was introduced,and finite element model was established.Main vibration modes,electro-acoustic properties in air and water,vibration displacement characteristic,and prestressed and hydrostatic conditions were analyzed.We got the conclusion that four-sided type flextensional transducer has higher radiant power by calculating its figure of merit.After,according to its vibration characteristics,and the impact of the structural parameters to the transmitting voltage response,we summed up the working methods to broaden the band of single resonance peak,and the prototype showed that it really has a good broadband emission characteristic.Based on previous analysis,a new method to form cardioid directivity was proposed,and a directional four-sided type flextensional transducer which has cardioid directivity in low-frequency was designed.Utilizing asymmetric radiating surfaces and asymmetric vibratiors,vibration displacements between front and back radiating surfaces for four-sided type flextensional transducer generate obvious contrast.Superposition effect of monopole and dipole was got and cardioid directivity was constructed,avoiding complex electrical design of conventional approach.The impact of the length difference between the longer and shorter oscillators was later analyzed,and then a specific conclusion was obtained from the relationship between the structure parameters of the shell and directivity.According to the results we redesigned the transducer,and the final prototype showed that we can achieve the directivity with cardioid directivity.Then,a hybrid broadband four-sided type flextensional transducer which owns broadband characteristic through multi-mode coupling was proposed.Magnetic circuit type was determined after static analysis together with dynamic anslysis of magnetic circuit structure of dual rods type,and finite element model was set up.Electro-acoustic properties of transducer with piezoelectric materials and rare earth materials separately driving were discussed,and transmitting voltage response curve was calculated when the two materials drive at the same time,and the final results get after optimization.Finally,symmetric four-sided type flextensional transducer and directional four-sided type flextensional transducer were made and tested both in air and water.The results showed that symmetric four-sided type flextensional transducer worked within 2.4kHz-5kHz,and the maximum transmitting voltage response is 140dB,the fluctuation of response is less than 4dB,which mean that symmetric four-sided type flextensional transducer realized the requirment of broadband emission.Besides,the resonant frequency of directional four-sided type flextensional transducer is 1.8kHz,it showed directivity with cardioid polar pattern at resonant frequency,and the difference between maximum directivity and minimum directivity is 13dB.The directional four-sided type flextensional transducer also had a good performance in broadband characteristic within 1.6kHz-3kHz. |
