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This paper reports on the development of a micro electro mechanical system microphone consisting of a piezoresistive cantilever and a micro HR (Helmholtz resonator) for use as an ultrasonic proximity sensor. The resonance frequency of the HR was designed to be equal to that of the cantilever. Therefore, the HR increases the sensitivity of the microphone to ultrasonic waves. The dimensions of the fabricated HR and the cantilever are 1.2 mm × 1.2 mm × 0.6 mm and 130 &mgr;m × 40&mgr;m × 0.3 &mgr;m, respectively. At a resonance frequency of 22.6 kHz, the cantilever with the HR detected ultrasonic waves with 14 times greater sensitivity than that of the cantilever without the HR. We demonstrated that the fabricated sensor is capable of measuring distances between 0.5 and 6.0 m. [ABSTRACT FROM AUTHOR]