Sound Level Meter Build: dB Measurement with Arduino

Sound Level Meter Build: dB Measurement with Arduino

Building a sound level meter with Arduino that accurately measures sound in decibels (dB) is a rewarding project with real-world applications — workplace noise compliance, home studio monitoring, or city noise pollution mapping. This guide covers the electronics and firmware for a calibrated dB measurement system using an electret microphone module, Arduino, and OLED display. You’ll learn the physics of dB measurement, calibration techniques, and how to meet Indian occupational noise standards (IS 3483 and the Noise Pollution Control Rules, 2000).

Understanding Decibels and Sound Level Physics

The decibel (dB) is a logarithmic unit expressing the ratio of a measured sound pressure to a reference pressure. The reference is 20µPa — the threshold of human hearing. The formula:

// Sound Pressure Level (SPL) formula:
// SPL_dB = 20 × log10(P_measured / P_reference)
// Where P_reference = 20µPa (0.00002 Pa)
//
// Common reference levels:
// 0 dB   = Threshold of hearing (20µPa)
// 30 dB  = Library, quiet bedroom
// 60 dB  = Normal conversation
// 85 dB  = Prolonged exposure damage threshold (NIOSH)
// 90 dB  = Indian factory floor limit (8-hour TWA, IS 3483)
// 110 dB = Chainsaw, rock concert
// 120 dB = Pain threshold
// 140 dB = Jet engine at 30m

A key insight: every 6dB increase represents a doubling of sound pressure; every 10dB increase sounds roughly twice as loud to human perception. This logarithmic compression is why a whisper (30dB) and a jackhammer (100dB) represent a pressure difference of over 3,000×.

Components and Circuit Design

• Arduino Nano or Uno (₹250-350)
• Electret microphone module with op-amp (MAX4466 preferred for its AGC, or LM393 variant) (₹80-150)
• OLED 128×64 display (I2C, SSD1306) (₹180-250)
• 10-segment LED bar or NeoPixel strip for visual level display (₹50-100)
• 100µF + 10µF + 100nF decoupling capacitors (₹20)
• Optional: SD card module for data logging (₹80-120)
• Optional: RTC DS3231 for timestamped logging (₹120-180)
• Total build cost: ₹700-1200

Circuit Design Notes

/* Circuit: Arduino Sound Level Meter
 *
 * Microphone Module:
 * VCC → Arduino 3.3V (not 5V for lower noise!)
 * GND → Arduino GND
 * OUT → Arduino A0
 * Gain pot → Adjust for +/-2V swing maximum (avoid clipping)
 *
 * OLED Display (SSD1306 I2C):
 * VCC → Arduino 3.3V or 5V
 * GND → Arduino GND
 * SCL → Arduino A5 (SCL)
 * SDA → Arduino A4 (SDA)
 *
 * LED Bar Graph (optional visual indicator):
 * Pin 2-11 → LED anodes (via 220Ω to GND)
 */

Arduino Sound Level Meter Code

// Arduino Sound Level Meter
// Measures RMS sound pressure and converts to dB SPL
// Requires calibration for accurate absolute dB readings

#include <U8g2lib.h> // OLED library

// Display initialization (I2C OLED)
U8G2_SSD1306_128X64_NONAME_F_HW_I2C u8g2(U8G2_R0, U8X8_PIN_NONE);

#define MIC_PIN A0
#define SAMPLE_WINDOW 50  // Sample window in ms (50ms = 20 readings/sec)
#define NUM_SAMPLES 2400  // Samples in 50ms at Arduino's ~48kHz ADC speed

// Calibration constants (adjust after calibration against reference meter)
#define DB_OFFSET 48.0  // Offset to match reference SPL meter at 60dB
#define VREF 5.0        // Arduino reference voltage
#define ADC_BITS 1024.0 // Arduino 10-bit ADC

// A-weighting filter coefficients (approximate for 1kHz center)
// Full A-weighting requires IIR filter — simplified version here
float aWeightOffset = 0; // -2dB at 40kHz, +1dB at 1kHz, -6dB at 63Hz approx.

void setup() {
  analogReference(DEFAULT); // 5V reference for maximum dynamic range
  Serial.begin(115200);
  
  u8g2.begin();
  u8g2.setFont(u8g2_font_ncenB14_tr);
  
  Serial.println("dB Sound Level Meter Ready");
  Serial.println("WARNING: Uncalibrated - readings are relative");
}

void loop() {
  // Step 1: Sample audio for SAMPLE_WINDOW milliseconds
  unsigned long startTime = millis();
  unsigned long sampleCount = 0;
  double sumSquares = 0;
  
  while (millis() - startTime  0.0001) {
    db = 20.0 * log10(rms) + DB_OFFSET;
    db = max(db, 20.0);  // Clamp minimum to 20dB (our noise floor)
    db = min(db, 130.0); // Clamp maximum
  } else {
    db = 20.0; // Noise floor
  }
  
  // Step 4: Slow peak hold (decays 0.5dB per cycle)
  static double peakDb = 20.0;
  if (db >= peakDb) {
    peakDb = db;
  } else {
    peakDb -= 0.5;
    peakDb = max(peakDb, db);
  }
  
  // Step 5: Leq accumulation (time-averaged dB over 1 minute)
  static double sumLinear = 0;
  static int leqCount = 0;
  static unsigned long leqStart = millis();
  sumLinear += pow(10, db / 10.0);
  leqCount++;
  double leq = 0;
  if (leqCount > 0) {
    leq = 10.0 * log10(sumLinear / leqCount);
  }
  
  // Reset Leq every 60 seconds
  if (millis() - leqStart > 60000) {
    sumLinear = 0;
    leqCount = 0;
    leqStart = millis();
  }
  
  // Output to Serial
  Serial.print("dB: "); Serial.print(db, 1);
  Serial.print(" | Peak: "); Serial.print(peakDb, 1);
  Serial.print(" | Leq(1min): "); Serial.println(leq, 1);
  
  // Update OLED
  updateDisplay(db, peakDb, leq);
}

void updateDisplay(double db, double peak, double leq) {
  u8g2.clearBuffer();
  
  // Main dB reading (large)
  u8g2.setFont(u8g2_font_logisoso28_tr);
  char dbStr[10];
  sprintf(dbStr, "%.0f", db);
  u8g2.drawStr(0, 35, dbStr);
  u8g2.setFont(u8g2_font_6x10_tr);
  u8g2.drawStr(60, 35, "dB SPL");
  
  // Bar graph (20-110dB mapped to 128 pixels)
  int barWidth = map((int)db, 20, 110, 0, 128);
  u8g2.drawBox(0, 40, barWidth, 10);
  u8g2.drawFrame(0, 40, 128, 10);
  
  // Peak and Leq
  u8g2.setFont(u8g2_font_6x10_tr);
  char info[30];
  sprintf(info, "Pk:%.0f Leq:%.0f", peak, leq);
  u8g2.drawStr(0, 63, info);
  
  u8g2.sendBuffer();
}

Calibration Methods

Without calibration, your meter measures relative loudness, not true dB SPL. Three calibration approaches:

Method 1: Reference Sound Source (Best)

Use a phone app showing calibrated dB (like NIOSH SLM) alongside your Arduino meter. Adjust DB_OFFSET until readings match at 60-70dB. Best accuracy: ±2-3dB.

Method 2: Known Tone at Known Distance

// Calibration using pink noise
// 1. Generate pink noise at 70dB from phone speaker at 30cm
// 2. Note your Arduino's raw dB reading
// 3. DB_OFFSET = 70 - raw_reading
//
// Example: if raw_reading = 22 when reference = 70dB
// DB_OFFSET = 70 - 22 = 48 (as in code above)

Method 3: Commercial SPL Meter

The most reliable approach — borrow a Class 2 SPL meter (available at electronics rental shops, or buy for ₹1500-3000) and cross-calibrate. Calibrate at 3-4 different levels (50, 65, 80, 95dB) for linearity verification.

Indian Noise Standards and Compliance

India’s noise regulations are governed by the Noise Pollution (Regulation and Control) Rules, 2000 under the Environment Protection Act, 1986. Ambient noise standards by zone:

For occupational noise in factories, IS 3483:1965 (revised) sets the maximum 8-hour TWA at 90dB(A), aligned with ILO standards. Exposures above 115dB(A) are prohibited even for short durations.

Data Logger with SD Card

// Add SD card logging to sound level meter
#include <SD.h>
#include <RTClib.h>

#define SD_CS_PIN 10
RTC_DS3231 rtc;

void initLogger() {
  if (!SD.begin(SD_CS_PIN)) {
    Serial.println("SD init failed!");
    return;
  }
  if (!rtc.begin()) {
    Serial.println("RTC not found!");
    return;
  }
  // Create/open log file with today's date
  DateTime now = rtc.now();
  char filename[20];
  sprintf(filename, "%02d%02d%04d.CSV", now.day(), now.month(), now.year());
  File logFile = SD.open(filename, FILE_WRITE);
  if (logFile) {
    logFile.println("Timestamp,dB_SPL,Peak_dB,Leq");
    logFile.close();
  }
}

void logReading(double db, double peak, double leq) {
  DateTime now = rtc.now();
  char filename[20];
  sprintf(filename, "%02d%02d%04d.CSV", now.day(), now.month(), now.year());
  
  File logFile = SD.open(filename, FILE_WRITE);
  if (logFile) {
    char entry[100];
    sprintf(entry, "%04d-%02d-%02d %02d:%02d:%02d,%.1f,%.1f,%.1f",
            now.year(), now.month(), now.day(),
            now.hour(), now.minute(), now.second(),
            db, peak, leq);
    logFile.println(entry);
    logFile.close();
  }
}
// Log every 1 second — generates ~86,400 rows/day
// 1GB SD card holds ~3 years of continuous logging

Frequently Asked Questions