Hello,
I’ve recently tested the SparkFun RedBoard ATP’s PDM microphone and I noticed an unusual frequency response.
My test setup includes a calibrated microphone in an enclosed box with sound isolation padding on the interior and exterior. I am playing a logarithmic chirp from 10Hz to 20kHz at 75dB SPL. For data collection, I am using the SparkFun RedBoard Artemis ATP, testing the PDM output at both 8kHz and 44.1kHz sampling rates.
According to the datasheet, I expect a near-flat frequency response. However, the output appears to be band-passed. I am aware that my microcontroller applies high-pass and low-pass filters to the PDM output, but I would expect the roll-off to be closer to the Nyquist frequency edges. Specifically, when sampling at 44.1kHz, the signal peaks around 2kHz, and the roll-off starts at about 5kHz, which seems incorrect.
I have verified that my speaker setup is not the issue by recording the same chirp sound with a Sony audio recorder, which shows a significantly flatter frequency response consistent with my calibration microphone.
Interestingly, I purchased a second PDM microphone from Adafruit, the “Adafruit PDM Microphone Breakout with JST SH Connector (PID: 4346),” and encountered a nearly identical frequency response despite using completely different chips. Is there something in the PDM library that could be attenuating the PDM microphone? Is there a way to work around this issue?
Any insights or recommendations on what might be causing this behavior would be greatly appreciated!
Thank you!
8kHz Spectrum
44.1kHz Spectrum