Design and realization of actively tunable sonar transducers

Active sonar systems increasingly use frequencies below 1 kHz because, due to their low absorption coefficient, these frequencies enable detection ranges of several hundred kilometers. For maximum detection range and range resolution, they require acoustic projectors with high sensitivity, i.e. high peak acoustic power, and high bandwidth. In this research, the use of actively tunable transducers as high bandwidth-sensitivity product, low frequency projectors, is investigated.; In low frequency active sonar, the most common signals used are frequency modulated (FM) pulses. The wider the frequency modulation range of the FM pulses, the better the range resolution. The first objective of this research was to establish the form of a tuning law that enables an actively tunable transducer to transmit FM pulses of frequency modulation bandwidth equal to their tunable range. Using numerical simulations of the transient velocity response of a hypothetical low frequency tunable transducer, and the Wigner time-frequency distribution to analyze input and output signals, this tuning law is shown to be tuning the resonance frequency to the instantaneous frequency of the FM pulse.; The second objective of this research was to design a low frequency actively tunable transducer having a high effective bandwidth-sensitivity product. The proposed design is a trilaminar disk that exploits the change in flexural rigidity occurring in circular plates when subjected to in-plane loads. Two trilaminar disks are designed using finite element models. A PZT 5H-based disk and a PZT 8-based disk are predicted to have effective bandwidth-sensitivity products 188% and 133% higher, respectively, than those of typical non-tunable trilaminar disks made of the same materials. Prototype transducers made of both materials are built and tested. Experimental results show that PZT 5H experiences property changes under DC electric field and static stress, and is therefore unsuitable for quasi-static tuning applications. PZT 8 is stable and provides a linear, instantaneous, and reversible frequency change. However, due to domain clamping, the experimental bandwidth-sensitivity product is about 65% of the predicted value, or 50% higher than that of a non-tunable trilaminar disk made of the same material.