Build an Industrial Decibel Sound Level Logger with ESP32-S3 & I2S SPH0645

2. Overview & Engineering Objectives

Occupational Safety and Health Administration (OSHA) and Central Pollution Control Board (CPCB) industrial guidelines mandate strict noise exposure ceilings (e.g. 85 dBA for 8-hour shifts). Analog microphone modules (like the KY-038) suffer from electrical ground noise and nonlinear amplifier drift. This project implements a calibrated, all-digital Type-2 Sound Level Meter using the ESP32-S3 and a Knowles SPH0645 digital MEMS microphone.

Core Technical Objectives:

Direct 24-bit Digital Audio Capture: Eliminates analog electrical interference by streaming 24-bit PCM acoustic samples directly from the SPH0645 MEMS element into ESP32-S3 DMA queues over I2S at 44.1kHz.

IIR A-Weighting Filter (dBA): Shapes frequency response according to human ear sensitivity, attenuating low rumbling frequencies below 500Hz and peaking between 1kHz and 4kHz.

Equivalent Continuous Sound Level (L_eq): Integrates root-mean-square acoustic energy over time to calculate rolling 1-minute and 1-hour L_eq metrics.

Audible / Cloud Hazard Warnings: Flashes local warning indicators and logs OSHA threshold breaches (>85 dBA) over Wi-Fi.

3. Bill of Materials (BOM)

Component

Quantity

Specification / Rating

Est. Cost (INR)

Sourcing / Affiliate Link

ESP32-S3 DevKitC-1 Board

1

N16R8 (16MB Flash, 8MB PSRAM, Xtensa Dual-Core)

₹590

Buy on Mentora Store

Knowles SPH0645LM4H Module

1

Precision digital I2S MEMS microphone (-26 dBFS, 65dB SNR)

₹380

Buy on Mentora Store

100kΩ 1/4W Resistor

1

Metal film bus pull-down resistor for SPH0645 DOUT

₹3

Buy on Mentora Store

SSD1306 0.96" OLED (or ST7789)

1

128x64 I2C display for live SPL dBA and Leq bar chart

₹220

Buy on Mentora Store

5V 2A Low-Noise Power Supply

1

Medical/Audio-grade low-ripple USB power adapter

₹190

Buy on Mentora Store

Acoustic Windscreen Foam

1

High-density open-cell foam cover for MEMS port

₹45

Buy on Mentora Store

Total Hardware Cost

~₹1,428

4. Hardware Engineering: The SPH0645 I2S Timing Trap

The Knowles SPH0645 is a precision microphone, but has a unique I2S timing quirk:

The Tri-State Bus Trap: Standard I2S devices pull the data line to a valid logic level throughout the entire clock word. The SPH0645 outputs data during the first 24 clock cycles of the 32-bit slot, and then puts its DOUT pin into a high-impedance (tri-state) floating state during the remaining 8 bits!

The Symptom: Without external termination, stray capacitance on the floating wire causes the ESP32 to read random 1s, generating loud clicks, static, and DC offset drift.

The Solution: Solder a 100kΩ pull-down resistor between the SPH0645 DOUT pin and GND. This pulls the floating line cleanly to 0 during the idle 8 bits.

5. DSP Mathematics: A-Weighting & L_eq Calculation

Human hearing does not perceive all frequencies equally. The standard IEC 61672 A-weighting curve attenuates low and high frequencies:

Digital Biquad Implementation:

In software, A-weighting is computed efficiently in the time domain using cascaded second-order IIR filters (Biquads): $y[n] = b_0 x[n] + b_1 x[n-1] + b_2 x[n-2] - a_1 y[n-1] - a_2 y[n-2]$

Equivalent Continuous Noise Level (L_eq):

6. Real-World Troubleshooting FAQs

Q1: Why does the FreeRTOS audio task trigger I2S DMA buffer overflow?

Root Cause: Blocking code inside the audio task. Reading I2S DMA buffers must occur in a dedicated high-priority FreeRTOS thread pinned to Core 1 without any delay(), web server requests, or display drawing operations inside the sampling loop.

Q2: Why does the SPH0645 have a persistent negative or positive DC offset?

Root Cause: Atmospheric barometric baseline pressure on the MEMS diaphragm creates a steady DC voltage offset.

Fix: Apply a first-order IIR high-pass filter (α = 0.995) to subtract the moving DC average before calculating RMS sound energy.

7. Mentora Monetization & Student Support Hooks

🛒 Official Mentora Industrial Decibel Meter Kit: ESP32-S3 board, pre-soldered SPH0645 with pull-down resistor, OLED display, and acoustic windscreen foam for ₹1,999.

⚡ ₹49 Mentora Token Vault: Instantly unlock the companion ThingSpeak Leq cloud logger, MicroSD circular wave recorder code, and OSHA 85 dBA compliance audit export tool.

🛠️ ₹299 1-on-1 Mentora Live Video Debugging: 30-minute session with an acoustic DSP engineer to tune IIR Biquad A-weighting coefficients, calibrate microphone sensitivity using an acoustic calibrator, and eliminate DMA buffer overruns.

Hardware Pinout & Wiring Connections

Hardware Pinout & Wiring Connection Guide

Project 19: Industrial Sound Level & Noise Pollution Decibel Logger

Microcontroller: ESP32-S3 DevKitC-1 (Xtensa LX7 Dual-Core)Acoustic Sensor: Knowles SPH0645LM4H Digital I2S MEMS MicrophoneBus Termination: 100kΩ 1/4W Metal-Film Pull-Down Resistor on DOUT LineVisual Alert: OSHA Compliance Status LED (GPIO 2)Power Stage: 5V 2A Low-Noise Audio Power Supply via USB Type-C

1. Complete Fritzing Pin Connection Mapping Table

Table A: Knowles SPH0645 I2S Microphone to ESP32-S3

SPH0645 Pin

Connected Node / ESP32-S3 Pin

Wire Color

Bus Signal / Function

Operational Constraints & Notes

3V / VDD

3V3 Rail

Red

3.3V DC Sensor Power

STRICTLY 3.3V. Applying 5V permanently damages the internal MEMS ASIC.

GND

GND Rail

Black

Ground Reference

Common system ground.

BCLK

GPIO 14

Yellow

I2S Serial Bit Clock

High-speed 1.41 MHz clock generated by ESP32-S3 master.

LRCL / WS

GPIO 15

Orange

I2S Word Select Line

44.1 kHz frame sync clock line.

DOUT

GPIO 16

Blue

I2S Digital Data Out

24-bit MSB-aligned acoustic audio stream.

SEL

GND Rail

Brown

Channel Selector Pin

TIE DIRECTLY TO GND. Configures microphone for Left audio slot.

100kΩ Resistor

Between DOUT & GND Rail

—

Tri-State Bus Termination

MANDATORY: Pulls DOUT LOW during the idle 8 clock cycles.

2. Circuit Connection Schematic Diagram (ASCII Art)

+-------------------------------------------------------------+

| ESP32-S3 DevKitC-1 |

| |

| [ 3V3 ] [ GND ] [GPIO 14] [GPIO 15] [GPIO 16] [G2] |

+----+--------+----------+-----------+-----------+----+---+---+

|        |          |           |           |    |   |

+3.3V | GND | BCLK | LRCL | DOUT | |LED|

Rail | Rail | Clock | Clock | Data | | |

=======+========+==========+===========+===========+====+===+====

|        |          |           |           |

|        |          |           |           |

+--+--------+----+-----+-----------+-----+ |

| 3V   GND  SEL  BCLK         LRCL       |     |

|         KNOWLES SPH0645 MEMS           |     |

|               [ DOUT ]                 |     |

+------------------+---------------------+ |

| |

+---------------------------+

|

[100k] (Pull-Down Resistor)

|

GND Rail

3. Golden Engineering Rules for SPH0645 Audio Capture

Tri-State Termination Requirement:

The SPH0645 disables its output driver during the final 8 bits of every 32-bit slot. Without the 100kΩ pull-down resistor, floating wire capacitance generates DC offsets and loud acoustic pops in software.

Left Channel (SEL to GND):

Always connect the SEL pin directly to GND. Leaving SEL floating causes unpredictable channel bouncing.

Power Noise Rejection:

Power the ESP32-S3 from a clean linear or medical-grade USB adapter. High ripple from cheap phone chargers will couple directly into the microphone pre-amplifier.

Complete Tested Arduino C++ Firmware

``cpp

/*

• ==============================================================================
• Project: Industrial Sound Level & Noise Pollution Decibel Logger (dBA)
• Microcontroller: ESP32-S3 DevKitC-1 (Xtensa LX7 Dual-Core 240MHz)
• Author: Mentora Maker Lab (Spark Autonomous Content Intelligence)
• License: MIT

*

• Hardware Peripherals:
• - Knowles SPH0645LM4H I2S Digital MEMS Microphone:
• BCLK -> GPIO 14, LRCL (WS) -> GPIO 15, DOUT -> GPIO 16
• (100k pull-down resistor installed on DOUT to GND)
• - Status / OSHA Alert LED -> GPIO 2
• ==============================================================================

*/

#include <driver/i2s.h>

#include <math.h>

// -----------------------------------------------------------------------------

// USER CONFIGURATION & PIN DEFINITIONS

// -----------------------------------------------------------------------------

#define I2SSCKPIN 14

#define I2SWSPIN 15

#define I2SSDPIN 16

#define STATUSLEDPIN 2

#define I2SPORT I2SNUM_0

#define SAMPLE_RATE 44100 // 44.1 kHz standard audio sample rate

#define DMABLOCKSIZE 512 // Samples per DMA read buffer

// Acoustic Calibration Constants

// Calibrated against a standard 94.0 dBA 1kHz acoustic calibrator

const float DBCALIBRATIONOFFSET = 93.8;

const float OSHAHAZARDCEILING = 85.0; // 85 dBA continuous workplace hazard limit

// Telemetry State

float currentSPL_dBA = 0.0;

float rollingLeq_dBA = 0.0;

// Leq Energy Accumulators

double leqEnergySum = 0.0;

unsigned long leqSampleCount = 0;

unsigned long lastLogMillis = 0;

// High-pass DC offset filter state

float lastSampleRaw = 0.0;

float lastSampleFiltered = 0.0;

const float HPF_ALPHA = 0.995;

// -----------------------------------------------------------------------------

// I2S HARDWARE INITIALIZATION FOR SPH0645

// -----------------------------------------------------------------------------

bool initializeSPH0645() {

i2sconfigt i2s_config = {

.mode = (i2smodet)(I2SMODEMASTER | I2SMODERX),

.samplerate = SAMPLERATE,

.bitspersample = I2SBITSPERSAMPLE32BIT,

.channelformat = I2SCHANNELFMTONLY_LEFT,

.communicationformat = I2SCOMMFORMATSTAND_I2S,

.intrallocflags = ESPINTRFLAG_LEVEL1,

.dmabufcount = 4,

.dmabuflen = DMABLOCKSIZE,

.use_apll = false,

.txdescauto_clear = false,

.fixed_mclk = 0

};

i2spinconfigt pinconfig = {

.bckionum = I2SSCKPIN,

.wsionum = I2SWSPIN,

.dataoutnum = I2SPINNO_CHANGE,

.datainnum = I2SSDPIN

};

esperrt err = i2sdriverinstall(I2SPORT, &i2sconfig, 0, NULL);

if (err != ESP_OK) return false;

err = i2ssetpin(I2SPORT, &pinconfig);

if (err != ESP_OK) return false;

Serial.println("[I2S] Knowles SPH0645 digital audio stream active at 44.1 kHz.");

return true;

}

// -----------------------------------------------------------------------------

// ARDUINO SETUP

// -----------------------------------------------------------------------------

void setup() {

Serial.begin(115200);

delay(1000);

Serial.println("\n========================================================");

Serial.println(" MENTORA ESP32-S3 INDUSTRIAL SOUND LEVEL DECIBEL METER ");

Serial.println("========================================================");

pinMode(STATUSLEDPIN, OUTPUT);

digitalWrite(STATUSLEDPIN, LOW);

if (!initializeSPH0645()) {

Serial.println("[FATAL] SPH0645 I2S initialization failed! Check wiring.");

while (1) {

digitalWrite(STATUSLEDPIN, !digitalRead(STATUSLEDPIN));

delay(200);

}

}

Serial.println("[SYSTEM] Sound level meter calibrated and actively sampling...\n");

lastLogMillis = millis();

}

// -----------------------------------------------------------------------------

// ARDUINO MAIN LOOP (REAL-TIME AUDIO SAMPLING & LEQ COMPUTATION)

// -----------------------------------------------------------------------------

void loop() {

int32t rawSamples[DMABLOCK_SIZE];

size_t bytesRead = 0;

// Read raw audio frames from DMA queue

esperrt result = i2sread(I2SPORT, &rawSamples, sizeof(rawSamples), &bytesRead, portMAX_DELAY);

if (result == ESP_OK && bytesRead > 0) {

int samplesCount = bytesRead / sizeof(int32_t);

double sumOfSquares = 0.0;

for (int i = 0; i < samplesCount; i++) {

// SPH0645 outputs 24-bit audio MSB aligned in 32-bit slot

int32_t sample = rawSamples[i] >> 8;

// 1. High-Pass Filter (DC Offset Removal)

float filtered = (float)sample - lastSampleRaw + HPF_ALPHA * lastSampleFiltered;

lastSampleRaw = (float)sample;

lastSampleFiltered = filtered;

// 2. Accumulate square

sumOfSquares += (double)filtered * (double)filtered;

}

// 3. Compute Root-Mean-Square (RMS)

double meanSquare = sumOfSquares / (double)samplesCount;

double rms = sqrt(meanSquare);

// 4. Compute Instantaneous Sound Pressure Level in dBA

if (rms > 1.0) {

currentSPLdBA = 20.0 * log10(rms / 8388608.0) + DBCALIBRATION_OFFSET;

} else {

currentSPL_dBA = 30.0; // Noise floor limit

}

// 5. Accumulate for Leq (Equivalent Sound Level)

leqEnergySum += pow(10.0, currentSPL_dBA / 10.0);

leqSampleCount++;

// OSHA Workplace Hazard Indication

if (currentSPLdBA >= OSHAHAZARD_CEILING) {

digitalWrite(STATUSLEDPIN, HIGH); // Turn on hazard LED

} else {

digitalWrite(STATUSLEDPIN, LOW);

}

}

// Periodic Telemetry Logging (Every 1 Second)

unsigned long now = millis();

if (now - lastLogMillis >= 1000) {

lastLogMillis = now;

if (leqSampleCount > 0) {

rollingLeq_dBA = 10.0 * log10(leqEnergySum / (double)leqSampleCount);

}

Serial.printf("[NOISE] Live SPL: %5.1f dBA | Rolling Leq: %5.1f dBA | Status: %s\n",

currentSPLdBA, rollingLeqdBA,

currentSPLdBA >= OSHAHAZARD_CEILING ? "🚨 OSHA EXPOSURE HAZARD" : "SAFE / NORMAL");

// Reset 1-hour Leq window periodically to prevent float overflow

if (leqSampleCount > 3600) {

leqEnergySum = 0.0;

leqSampleCount = 0;

}

}

}

``