EspAudioSensor
| Project ESP audio sensor | |
|---|---|
| File:NoPicture.jpg | |
| ESP-based audio sensor | |
| Status | Initializing |
| Contact | bertrik |
| Last Update | 2019-04-14 |
Introduction
This project is about using an ESP-32 together with an I2S digital microphone to create an audio sensor.
This could be a decibel meter, or perhaps an environmental noise meter.
Measuring audio as a citizen science project
Possibly a nice match for measuring stuff by citizens could be to measure audio:
- ESP8266/ESP32 have an I2S input, there are cheap microphones available that directly output I2S, saving on interface electronics
- ESP8266/ESP32 is probably powerful enough to do basic processing, like FFT, summing power in various frequency ranges, etc.
- ESP8266/ESP32 has a wifi connection so it can easily upload data
Questions/stuff to investigate:
- what kind of measurements can we do reliably?
- probably absolute measurements are very difficult
- possibly useful relative measurements
- calculate average over (say) 10 seconds, and compare it with the average over (say) 10 minutes
- compare averages over time, e.g. compare daytime minute average with each other
- determine peak-to-average ratios
Waag society uses the following microphone in their kit 2.1: Invensense ICS4342
On AliExpress you can find many INMP441, with claims of compatibility with ESP32, so I ordered those INMP441.
External things to investigate:
- https://www.rijksoverheid.nl/onderwerpen/geluidsoverlast/geluidsoverlast-in-de-wet
- INMP441 microphone datasheet
- https://github.com/maspetsberger/esp32-i2s-mems
Theory
The plan is to divide the audio spectrum up into octaves and calculate the total energy in each octave. To this we can then easily apply sensor/housing specific corrections, do A weighting, etc.
An octave is basically a factor of two in frequency. Coincidentally an FFT also calculates things in factors of two, for example when sampling at 44100 Hz, you get the following octaves
- 22050 - 11025 Hz
- 11025 - 5512 Hz
- 5512 - 2756 Hz
- 2756 - 1378 Hz
- 1378 - 689 Hz
- 689 - 344 Hz
- 344 - 172 Hz
- 172 - 86 Hz
- 86 - 43 Hz
A-weighting
Audio levels are generally measured in dB using the A-weighted scale. A-weighting calculates a subjective loudness level from a physical loudness.
Energy it calculated per octave and an A-weight factor is applied. In decibels this means adding a octave-specific number to the loudness.
Hardware
The physical device consists of:
- an ESP32 (or possibly an ESP8266), it has an I2S digital audio input.
- a digital I2S microphone, like the INMP441 (datasheet)
The microphone is connected as follows:
- INMP441 GND to ESP32 GND
- INMP441 VDD to ESP32 3.3V
- INMP441 SD to ESP32 A4/32
- INMP441 SCK to ESP32 A16/14
- INMP441 WS to ESP32 15
- INMP441 L/R to ESP32 GND
Software
Initial code can be found on github.
See also: