Sound Detector Sparkfun

// The following link contains the original work. https://github.com/sparkfun/Sound_Detector

 //GND → GND
 //VCC → 5V
 //Gate → Pin 2
 //Envelope → A1
 
#define PIN_GATE_IN 2
#define IRQ_GATE_IN  0
#define PIN_LED_OUT 13
#define PIN_ANALOG_IN A1

// soundISR()
// This function is installed as an interrupt service routine for the pin
// change interrupt.  When digital input 2 changes state, this routine
// is called.
// It queries the state of that pin, and sets the onboard LED to reflect that 
// pin's state.
void soundISR()
{
  int pin_val;

  pin_val = digitalRead(PIN_GATE_IN);
  digitalWrite(PIN_LED_OUT, pin_val);   
}

void setup()
{
  Serial.begin(9600);

  //  Configure LED pin as output
  pinMode(PIN_LED_OUT, OUTPUT);

  // configure input to interrupt
  pinMode(PIN_GATE_IN, INPUT);
  attachInterrupt(IRQ_GATE_IN, soundISR, CHANGE);

  // Display status
  Serial.println("Initialized");
}

void loop()
{
  int value;

  // Check the envelope input
  value = analogRead(PIN_ANALOG_IN);

    // Convert envelope value into a message
  //Was considered as minimum at 21 dB. Then we proceeded to write down the Voltage and the
  //following equation was solved. "8" is the registred value in 21 dB.
  //21=20*log(8/x) x~2.79
    Serial.print(20*log(value/2.8)); 
    Serial.print(" dB");
    Serial.print('\n');
    // pause for 1 second
    delay(100);
}