Acoustic Sensing with Capacitive Micromachined Membranes and Radio Frequency Detection

Abstract

Broadband acoustic sensing over several decades of frequency has traditionally been difficult to achieve. Conventional condenser and electret microphones typically depend on membrane or cavity resonances to achieve their maximum sensitivity, which inherently limits the frequency range of the acoustic sensor. In addition, the large, thin diaphragms typically used to sense low-frequency sound are extremely fragile and are not suited for hostile outdoor environments as might be encountered on the battlefield. New microphones using capacitive micromachined ultrasonic transducer (CMUT) technology and radio frequency (RF) detection overcome many of the problems associated with conventional microphones. These micromachined membranes are small and robust enough that they can be vacuum-sealed and still withstand atmospheric pressure and submersion in water. The sealing process not only seals out moisture, but also suppresses the mechanical noise associated with squeezefilm damping of air behind the membrane. Because the ultrasonic membrane’s resonance is well above the frequencies of interest, the membrane mechanical response is very flat from DC up to several hundred kilohertz. Although the mechanical signal-to-noise ratio is very high, the changes in membrane capacitance are very small, so a sensitive RF detection scheme is necessary to recover the acoustic signal. In this paper, we present the theory and modeling of RF detection with CMUT membranes. Previous experimental results already demonstrate the flat frequency response of the acoustic sensor, with responses varying only 1 dB over a range of 0.1 Hz to 100 kHz. Several strategies, most notably for reducing transmission line loss, are described to improve the sensitivity of the microphone from its measured value of 53 dB/Pa/Hz on recent devices. With the improvements, simulation results predict that device sensitivities greater than 100 dB/Pa/Hz are possible with a device that is only 4mm in size. Report Documentation Page Report Date 01OCT2001 Report Type N/A Dates Covered (from... to) Title and Subtitle Acoustic Sensing with Capacitive Micromachined Membranes and Radio Frequency Detection Contract Number