Miniature low-pass mechanical filter for improved frequency response with MEMS microphones & low-pressure transducers

Abstract

We present an acoustic low-pass mechanical filter for high intensity, high temperature silicon piezoresistive microelectromechanical system (MEMS) microphones and low-pressure (<170kPa) differential transducers that has been tuned such that only static pressure and low frequency (DC to 1–10Hz) dynamic pressures are passed to the backside of the pressure sensitive diaphragm. We present an analytical modelof the frequency response of the low-pass mechanical filter that is based on 1D wave propagation with viscous dissipation. We compare the model to experimental results obtained using a spinning valve dynamic pressure generator to demonstrate improved performance of the transducer in the frequency range of 1–2800Hz. Experimental test results yield a −3dB point at 5±4Hz for the low-pass mechanical filter. The addition of this filter enables the use of a MEMS microphone without a controlled static pressure supplied to the transducer's differential tube or the addition of several meters of tubing to prevent the transmission of low frequency content to the backside of the pressure sensitive diaphragm. The spinning valve dynamic pressure tests demonstrate a flat frequency response (±2dB, 25% deviation from the accepted input signal) from 20Hz to 2.8kHz (test limitation) for a close-coupled measurement at the front of the microphone, which is expected to remain constant over the operating temperature range of the transducer and at various static ambient pressures both above and below atmospheric pressure.