Recent low-frequency sonar transducers and arrays

Research in the field of high-power, low-frequency underwater transducers has been sustained by advances in submarine noise reduction and the promise of long-range detection at low frequencies. The earliest idea came up to use the already available passive towed array sonars as receive arrays in conjunction with additionally towed sound sources. The frequency of the active adjunct idea was of course given by the existing passive arrays. Very promising trials at sea, even in the shallow water environment, soon led to the consideration of full low-frequency active sonar systems (LFASS). Those have now been investigated for nearly two decades. With the emergence of new high-power transduction materials and the rapid evolution of fast computers and powerful modeling techniques, this research field has been fertile ground for transduction innovation. Specifically, high-power transducer developments include performance improvements to conventional piezoelectric ceramic projectors, progress in the exploitation of new active driver materials, and advances in transduction design techniques. While high power, low frequency, wide bandwidth, and high efficiency remain the primary performance objectives behind these developments; low weight, small size as well as medium to deep submergence are desirable physical attributes that require equal attention by the designer. This paper emphasizes the key characteristics of two sea-proven concepts for LFASS: Janus Helmholtz transducers and multihead arrays (international patents). The attention will be particularly focused on measured values of the sound sources. Finally, this paper addresses future trends in terms of sonar array designs for LFASS.