Thermoacoustic Coupling as the Dominant RF Interference Mechanism in MEMS Microphones

This paper investigates radio-frequency (RF) interference mechanisms in micro-electro-mechanical systems (MEMS) microphones, identifying thermoacoustic (periodic heating of the ASIC resulting in pressure changes in the acoustic back volume of the microphone) coupling as the dominant mechanism responsible for generating audible artifacts. A TDK T5838 digital MEMS microphone was used as the test vehicle for this investigation. A simulation chain combining Ansys HFSS electromagnetic modeling, circuit simulation to identify ASIC power loss, and Ansys Icepak transient thermal simulation predicts back volume temperature oscillations and sound pressure via the ideal gas law. A conducted-immunity test bench was built following IEC 62132-4, i.e., direct power injection on I/O pins, sweeping the RF carrier from 500 MHz to 7 GHz with 80% amplitude-modulated (AM) interference. The interferer (hereafter referred to as noise) shows a trend toward higher amplitudes with increasing carrier frequency and lower amplitudes with increasing modulation frequency, consistent with thermoacoustic rather than electrical demodulation. Simulated and measured noise levels agree well across all four evaluated pins. Measurements under reduced pressure confirm the thermoacoustic origin: lowering the back volume air pressure reduces interference significantly and eliminates the modulation-frequency roll-off, exposing electrical demodulation close to baseline (without RF interferer) noise. As a mitigation strategy, we encapsulated bond wires, identified as heat exchangers into the back volume, with a thermally insulating silicone material, achieving 20 to 25 dB noise reduction. These results confirm thermoacoustic coupling as the primary RF interference path and point to practical strategies for improving RF robustness in next-generation MEMS microphone designs.