Measuring Sound Level with a MEMS Microphone

The INMP441 I2S MEMS microphone gives digital audio samples directly over I2S without needing an ADC. For a sound level meter, this simplifies the analog front end to nothing.

Reading INMP441 on ESP32: configure I2S in PDM or standard I2S mode (the INMP441 is standard I2S), read 1024 samples, compute the RMS value. The RMS correlates to dB SPL with a calibration factor derived from the microphone's sensitivity spec (-26 dBFS at 94 dB SPL).

Frequency weighting: a simple dB(A) approximation uses a digital FIR filter with weights derived from the A-weighting curve. Implementing a full 100-tap FIR in software at 44.1kHz on an ESP32 is feasible but uses significant CPU. A 20-tap approximation adequate for ±2dB accuracy above 100Hz can run at 1kHz block rate with minimal load.