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Piezoelectric microcantilever sensors (PEMSs) are very highly sensitive label-free sensors, and silicon-based PEMSs have parasitic elements such as parasitic capacitances and resistances between the electrodes through their substrate; they affect the coupling coefficient and leakage current and thereby the accuracy of the sensor. In this study, we have experimentally analyzed the parasitic effects that affect these PEMSs. Further, we have developed a method to reduce the parasitic effects and fabricated a PEMS on this basis; we have successfully demonstrated that its electrical properties improved. The parasitic effects were reduced by employing a deep trench structure near the electrodes and a highly resistive (>10 kΩ cm) silicon-on-insulator (SOI) wafer and by reducing the electrode pad areas. The coupling coefficient and leakage current were analyzed from the experimental data of admittance spectra and velocity spectra. The coupling coefficient increased by approximately 42%, the leakage current at resonance decreased by approximately 76%, the parasitic conductance decreased and the transformation factor increased. Moreover, the electrical power loss decreased as the leakage current decreased, whereas the motional conductance increased with the coupling coefficient. In this manner, we could enable more accurate electrical readouts of the PEMS by reducing the parasitic effects.

Parametric array (PA) loudspeakers generate directional audible sound via the PA effect, which can make private listening possible. The practical applications of PA loudspeakers include information technology devices that require large power efficiency transducers with a wide frequency bandwidth. Piezoelectric micromachined ultrasonic transducers (PMUTs) are compact and efficient units for PA sources [Je, Lee, and Moon, Ultrasonics 53, 1124-1134 (2013)]. This study investigated the use of an array of PMUTs to make a PA loudspeaker with high power efficiency and wide bandwidth. The achievable maximum radiation bandwidth of the driver was calculated, and an array of PMUTs with two distinct resonance frequencies (f1 = 100 kHz, f2 = 110 kHz) was designed. Out-of-phase driving was used with the dual-resonance transducer array to increase the bandwidth. The fabricated PMUT array exhibited an efficiency of up to 71%, together with a ±3-dB bandwidth of 17 kHz for directly radiated primary waves, and 19.5 kHz (500 Hz to 20 kHz) for the difference frequency waves (with equalization).

An ultrasonic radiator is developed to generate a difference frequency sound from two frequencies of ultrasound in air with a parametric array. A design method is proposed for an ultrasonic radiator capable of generating highly directive, high-amplitude ultrasonic sound beams at two different frequencies in air based on a modification of the stepped-plate ultrasonic radiator. The stepped-plate ultrasonic radiator was introduced by Gallego-Juarez et al. [Ultrasonics 16, 267-271 (1978)] in their previous study and can effectively generate highly directive, large-amplitude ultrasonic sounds in air, but only at a single frequency. Because parametric array sources must be able to generate sounds at more than one frequency, a design modification is crucial to the application of a stepped-plate ultrasonic radiator as a parametric array source in air. The aforementioned method was employed to design a parametric radiator for use in air. A prototype of this design was constructed and tested to determine whether it could successfully generate a difference frequency sound with a parametric array. The results confirmed that the proposed single small-area transducer was suitable as a parametric radiator in air.