Improved Modeling and Design of Microphones Using Radio Frequency Detection with Capacitive Micromachined Ultrasonic Transducers

Broadband acoustic sensing, over several decades of frequency, has traditionally been difficult to achieve. An alternative approach to conventional condenser microphones is to use capacitive micromachined ultrasonic transducer (CMUT) membranes with a sensitive radio frequency (RF) detection method. Since the resonant frequency of a typical CMUT membrane is several megahertz, the membrane response to acoustic frequencies below resonance, from DC to several hundred kilohertz, is constant. This paper presents the theory, modeling, and sensitivity predictions of the RF detection method. Electrical thermal noise is now incorporated in the model and ultimately limits the sensitivity. In addition, we present experimental results showing the flat frequency response, from 0.1 Hz to 100 kHz, of a microphone using RF detection. Present measurements demonstrate a sensitivity of 53 dB/Pa/Hz, though improvements to the design are expected to achieve sensitivities approaching 100 dB/Pa/Hz.