Ultra-low frequency underwater acoustic projectors: Present status and future trends

Ultra-low frequency (ULF) underwater transducers, used in the 10–400 Hz frequency range, have usually radiating surfaces the dimensions of which are small with respect to the acoustic wavelength. To radiate a high acoustic power with a monopolar ULF transducer, a large volume velocity is required to counterbalance the low radiation resistance. Three transduction technologies are available to realize compact high power ULF transducers: hydroacoustic, electromagnetic, and active material-based. In the latter case, piezoelectric ceramics and magnetostrictive rare-earth alloys are often associated to flexural vibration such as found in flextensional transducers. Compared to these materials, piezoelectric single crystals, which exhibit lower stiffnesses and produce higher strains together with higher energy densities, are potential active materials for future ULF underwater transducers. In this work, ULF transducers are analyzed in terms of their working frequencies, acoustic powers, and masses. Thirty-two ULF underwater projectors build during the last 25 years are considered. For single crystal transducers, prototypes working at higher frequencies as well as transducers modeled with finite element method are taken into account. Using these data and classical scaling laws, abacuses displaying acoustic power-frequency curves for given masses are constructed for each technology. They show that single crystals transducers could provide more compact and powerful solutions for frequencies above 40 Hz.