Digital time delay network for an active underwater acoustic coating

Abstract

A digital time delay system has been developed for control of an active underwater coating. The coating contains piezoelectric polymer sensors and a piezocomposite actuator encapsulated within a host polymer. Its application is to be layered onto submerged objects that desire reduction of an underwater sound reflection. The control system featured considers a time delay network as opposed to an analog phase delay approach featured in past studies [Howarth et al., J. Wave Mat. Interact. 4, 83–88 (1989); Howarth et al., J. Acoust. Soc. Am. 91, 823–831 (1991)]. The concept of the active coating control is that of an open-loop velocity feedback between sensors and actuator. This scheme prevents feedback instability because the actuator output is acoustically decoupled from the sensors. The digital time delay controller has the added benefit of parallel multichannel processing to characterize both the incident acoustic pressure and the reflected pressure. The reflected pressure is monitored at the sensors for near-field determination of the effectiveness of the active coating system prior to far-field sound radiation. Experimental results were obtained for normal incidence in a water-filled acoustic pulse tube terminated at its boundary with an active coating and air backing. Echo reduction comparisons as functions of sound reduction and bandwidth are presented.