Acoustic projectors for AUV and UUV applications in shallow water regions

For acoustic identification of objects in a littoral environment, there are generally three frequency bands of interest; 1 kHz to 10 kHz, 10 kHz to 100 kHz and 100 kHz to 1 MHz, where the selection of these bands is dependent upon the specific Navy mission. This paper will discuss the progress of the Naval Research Laboratory in developing acoustic projector prototypes to address the lower two frequency bands for unmanned underwater vehicle (UUV) and/or autonomous underwater vehicle (AUV) applications. The band of 1 kHz to 10 kHz is currently being addressed using cymbal flextensional vibrator elements sandwiched into thin panels. In-air data has shown that high levels of acoustic displacement at low frequencies are possible with these devices while more recent in-water data has verified these expectations. This success has led to modelling and prototyping of similar devices for shallow water regions. The frequency range of 10 kHz to 100 kHz has been investigated for several years where the acoustic projector was originally reported during AeroSense 1998. The results of integrating the NRL broadband projector into the NSWC/Coastal Systems Station (CSS) synthetic aperture sonar (SAS) UUV will be presented. This system integration considers the projector as a constant source level over the 10 kHz to 100 kHz band by driving the 100 lcHz resonant transducer with an inversely shaped transformer. The presentation will conclude with a discussion of the future development trends in shallow water transducers for AUV and UUV missions.