Abstract: An ultra-low frequency flextensional transducer with multiple elliptical flextensional shells mechanically cascaded in the direction of the short axis is studied,and each flextensional shell is driven by giant magnetostrictive materials.The equivalent circuit of multiple cascaded shells is deduced,and the frequency and impedance equations of the transducer obtained by using the branch impedance method.Because of the structural complexity of the multiple cascaded shells,the finite element method is used to calculate the vibrating displacements of multiple structural points of the transducer.Through the translational displacement distribution,the structural parameters of the piston are optimized.The resonant frequencies of the transducer in the air and water are compared with the results calculated by the finite element method and the equivalent circuit method,and the calculated results are in good agreement.A prototype of ultra-low frequency flextensional transducer with 6 cascaded shells has been developed.The external dimension of the transducer isΦ230 x630 mm and the weight is 39 kg.The test result shows that the resonance frequency of the transducer in water is 130 Hz and the maximum transmitting current response is 161.1 dB.The maximum sound source level is 180.4 dB.The results described above show that the transducer has a good capability of transmitting ultra-low frequency underwater sound with small size.