Arduino sound meter

sound-meter.ino

/*
 * sound-meter.ino: Arduino sound meter.
 *
 * This program continuously samples a sound signal on analog input 0
 * and outputs the computed sound intensity through the serial port.
 *
 * The analog-to-digital converter is set to "free running mode" and
 * takes one sample every 104 us. After subtracting a DC offset
 * (constant dc_offset below), the samples are squared and low-pass
 * filtered with a time constant of 256 sample periods (26.6 ms). They
 * are then decimated by a factor 256 and transmitted as an ASCII stream
 * at 9600/8N1 through the serial port.
 *
 * The program is intended for an Arduino Uno, and is likely to work on
 * any AVR-based Arduino having an ADC and clocked at 16 MHz.
 *
 * Copyright (c) 2016 Edgar Bonet Orozco.
 * Released under the terms of the MIT license:
 * https://opensource.org/licenses/MIT
 */

// Analog input connected to the microphone (between 0 and 5).
const uint8_t mic_pin = 0;

// DC offset, i.e. ADC reading of silence.
const int dc_offset = 512;

// Intensity readings provided by the ADC interrupt service routine.
volatile uint32_t intensity_reading;
volatile bool intensity_reading_ready;

// Low-pass filter for the sound intensity.
// Time constant = 256 * sampling period = 26.624 ms
static uint32_t low_pass(uint32_t x)
{
    static uint32_t integral;
    integral += x - integral / 256;
    return integral / 256;
}

// ADC interrupt service routine.
// Called each time a conversion result is available.
ISR(ADC_vect)
{
    // Read the ADC.
    int32_t sample = ADC;

    // Compute the average sound intensity.
    uint32_t intensity = sq(sample - dc_offset);
    uint32_t avg_intensity = low_pass(intensity);

    // Decimate by a factor 256.
    static uint8_t sample_count;
    if (++sample_count == 0) {
        intensity_reading = avg_intensity;
        intensity_reading_ready = true;
    }
}

void setup()
{
    // Configure the ADC in "free running mode".
    ADMUX  = _BV(REFS0)  // ref = AVCC
           | mic_pin;    // input channel
    ADCSRB = 0;          // free running mode
    ADCSRA = _BV(ADEN)   // enable
           | _BV(ADSC)   // start conversion
           | _BV(ADATE)  // auto trigger enable
           | _BV(ADIF)   // clear interrupt flag
           | _BV(ADIE)   // interrupt enable
           | 7;          // prescaler = 128

    // Configure the serial port.
    Serial.begin(9600);
}

void loop()
{
    // Wait for a reading to be available.
    while (!intensity_reading_ready) /* wait */;

    // Get a copy.
    noInterrupts();
    uint32_t intensity_reading_copy = intensity_reading;
    intensity_reading_ready = false;
    interrupts();

    // Transmit to the host computer.
    Serial.println(intensity_reading_copy);
}