An experimental investigation of electrical beam steering of the difference frequencywavegenerated by aparametric array in air is described. A16-channel phased array system, composed of apiezoelectric micromachined ultrasonic transducer array,ad igital signal processor,a nd ap ower amplifier,w as constructed and tested. The beam patterns of the primary and difference frequencywaves were measured while adjusting the steering angle overthe range of 0 • -40 • .Spurious signals due to intermodulation distortion of the driving and receiving systems were also considered. The experimental results demonstrate the potential use of parametric array beam steering for ultrasonic ranging sensors.

In a previous study [H. Lee etal., J. Acoust. Soc. Am. 125, 1879–1893 (2009)], a parametric array (PA) transducer in air using micromachined ultrasonic transducers (MUT) was examined as an ultrasonic ranging sensor. It was shown that the piezoelectric micromachined ultrasonic transducer (pMUT) may be effective for PA ultrasonic ranging sensors. Since the pMUT are usually used in their array, PA source using pMUT can steer its directional sound beam by manipulating phases of radiator in the array. The beam steering capability of the PA source can provide very useful function for parametric loudspeakers as well as directional ultrasonic range sensors. In this work, 16-channel pMUT array for digital beam-steering of parametric array was designed and fabricated. Each channel of the pMUT array consisted of 9 PZT unimorph elements. The two resonance frequency-type pMUT units are designed for generating two primary beams. Beam-steering of each primary beam was controlled separately by applying complex weighting to each channel. An acoustic experiment verified the PA beam-steering of the pMUT. The result shows that the proposed pMUT is suitable for digital beam-steering of parametric array and that it is also useful for the parametric loudspeaker and the PA ultrasonic range sensor.