[sdiy] [SDIY] Measuring the sound levels from a microphone using an ESP32 board

Ben Stuyts ben at stuyts.nl
Mon Jul 6 12:30:49 CEST 2020

Hi Spiros,

Do you have to use an analog electret microphone? If not, you could look at one of those mems-based microphones (used in e.g. cell phones), which often have a digital output. It is usually PDM based I think, but you should be able to interface it to an ESP32 and decode it.

For example: https://www.st.com/content/st_com/en/products/mems-and-sensors/mems-microphones/imp34dt05.html <https://www.st.com/content/st_com/en/products/mems-and-sensors/mems-microphones/imp34dt05.html>

About your analog solution, do you have any idea what the sample rate is of the library that you used?


> On 6 Jul 2020, at 10:18, Spiros Makris <spirosmakris92 at gmail.com> wrote:
> Hello list,
> I have to develop a small metering application for a university project. The idea behind it is to demonstrate the internet of things and how it can be used to monitor the (audio) noise levels in a room, to be used in places like studios, production facilities or even bars. 
> The sensor used is a common electret type microphone, which is preamplified using a current to voltage opamp stage, based on the OPA172 amplifier. The supply voltage is 3V (same as the microcontroller), but I have the option of 5, 9 or even 12V if needed (only unipolar, though.) Then, this amplified signal needs to be converted to a measurement that will somehow relate to "how loud" the sounds are. Accurate SPL measurements are not required (although they would sure be nice if this was a commercial application). I may have access to a measuring microphone in order to calibrate this when I'm done. I'm using an ESP32 board and its onboard ADC.
> I first tried to use the RMS Arduino library, which measures an AC signal coming into any analogue pin (no external processing, other than prequalification). I couldn't get it to show any coherent readings - I'm not sure if this is due to the sample number or sampling period I'm using. Making an RMS calculation method from scratch wasn't much better either - the variations I could detect were (or seemed to be) minuscule - definitely not enough to be meaningful in this application.
> I'm now trying a more analogue approach: the preamp signal is passed through a Schottky diode, connected to a large capacitor. A transistor is placed parallel to the capacitor, to be used as a reset switch (controlled by the processor).
> <image.png>
> I have tried AC coupling and DC coupling the input - both methods work (DC coupled version is shown). When the input goes over the threshold of the diode, the capacitor is charged (with a pretty large current), and the droop rate is small enough to measure it with the controller, then reset it, to take another measurement. So far so good, but I need some way of eliminating that 0.3V threshold.
> <image.png>
> The graph is taken from the AC coupled version (but is identical to the DC-coupled one, save for some DC offset). The X-axis shows the input amplitude and the Y-axis shows the capacitor's final voltage.
> The ESP32 has adjustable attenuation for the ADC - the maximum range can be adjusted to ~1.3V, ~2.5V and ~3.3V (approximate values). The resolution is 12bits.
> I thought of using a rail to rail opamp to make an ideal diode - however, I only have a SOIC8 version of an OPA172 and no time to print a PCB. Perhaps I can source adapters quickly though - is there a single opamp circuit that could help me?
> I can also mess around with the preamplifier stage, adjusting the dc bias or gain. 
> I'm kinda stuck. Any ideas?
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