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

[Ian Fritz]

Ha! I just last week reinvented fig. 10 — except with the stages in reverse order. Easy to add a pot for continuous 2Q to 4Q tuning.

> Sent from my Xperia SP on O2
> 
> ---- Spiros Makris wrote ----
> 
> I have been studying some of the circuits there. I'm leaning towards those on figure 6-6A, but I will use the lm324 opamp instead (to get an output down to 0V), followed by a simple LPF (so basically increase C1 on the schematic shown below), making a very simple envelope follower. Taking the average of that is an easy task and doesn't need much speed. 
> The peak detection circuit will catch sudden noises better, though. Perhaps I could employ both, for sustained and impulsive noises, respectively.
> <image.png>
> 
> 
>> On Mon, Jul 6, 2020 at 2:58 PM Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>> Do the calculations digitally on the audio data as Tom said, if your micro is up to the job.
>> 
>> If you have to use an analogue circuit approach because the micro isn't fast enough or draws too much current, then take a look here... 
>> 
>> https://sound-au.com/appnotes/an001.htm
>> 
>> This excellent web page shows many precision rectifier circuits that go a long way towards eliminating the dead band that you otherwise get using just a single rectifier diode.
>> 
>> Good luck with your project! 
>> 
>> -Richie,
>> 
>> 
>> Sent from my Xperia SP on O2
>> 
>> ---- Tom Wiltshire wrote ----
>> 
>> >I’d sample the incoming audio directly at a good rate (48KHz maybe) and then do any processing on the data. Much easier to do math inside the uP, rather than outside in analog circuitry.
>> >
>> >As to what algorithm you use to measure sound level, there are an absolute ton of them, as I discovered when I looked into VU meters.
>> >
>> >Tom
>> >
>> >==================
>> >       Electric Druid
>> >Synth & Stompbox DIY
>> >==================
>> >
>> >
>> >
>> >> On 6 Jul 2020, at 09:18, 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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> <image.png>
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