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Underwater detection is accomplished using an underwater ultrasonic sensor, sound navigation and ranging (SONAR). Stealth to avoid detection by SONAR plays a major role in modern underwater warfare. In this study, we propose a smart skin that avoids detection by SONAR via controlling the signal reflected from an unmanned underwater vehicle (UUV). The smart skin is a multilayer transducer composed of an acoustic window, a double-layer receiver, and a single-layer transmitter. It separates the incident signal from the reflected signal from outside through the time-delay separation method and cancels the reflected wave from the phase-shifted transmission sound. The characteristics of the receiving and transmitting sensors were analyzed using a finite element analysis. Three types of devices were compared in the design of the sensors. Polyvinylidene fluoride (PVDF), which had little effect on the transmitted sound, was selected as the receiving sensor. A stacked piezoelectric transducer with high sensitivity compared to a cymbal transducer was used as the transmitter. The active reflection control system was modeled and verified using 2D 360° reflection experiments. The stealth effect that could be achieved by applying a smart skin to a UUV was presented through an active reflection–control omnidirectional reflection model. [ABSTRACT FROM AUTHOR]

In this paper, we propose an acoustic block for widening the bandwidth and modulating center frequency of airborne transducers to improve the ultrasound imaging or metrology system. When acoustic block is attached, the characteristics of the ultrasonic transducer are changed by the squeeze film effect. Extra elastic and damping force are generated by squeeze film damping and can be changed according to parameters of acoustic block. For impedance matching, a CMUT vibrating in bending mode was used. The CMUT's FBW is 3.18 % and the center frequency is 103.5 kHz. We changed the area ratio of the acoustic block and attached it to the CMUT. The FBW was changed to 7.14 to 9.14 % and the center frequency to 108.7 to 112.1 kHz. With acoustic block, conventional transducers can be transformed into transducers with better performance in areas such as non-destructive testing and ultrasonic imaging. [ABSTRACT FROM AUTHOR]