Difference between revisions of "EspAudioSensor"

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(Theory)
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An octave is basically a factor of two in frequency.
 
An octave is basically a factor of two in frequency.
Coincidentally
+
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
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* 689 - 344 Hz
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* 344 - 172 Hz
 +
* 172 - 86 Hz
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* 86 - 43 Hz
  
 
=== A-weighting ===
 
=== A-weighting ===

Revision as of 17:32, 14 April 2019

Project ESP audio sensor
350px
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:

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: