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:

• https://github.com/maspetsberger/esp32-i2s-mems