Motion sensing alarm system

const int TRIG_PIN   = 6; // Arduino pin connected to Ultrasonic Sensor's TRIG pin
const int ECHO_PIN   = 7; // Arduino pin connected to Ultrasonic Sensor's ECHO pin
const int BUZZER_PIN = 3; // Arduino pin connected to Piezo Buzzer's pin
const int DISTANCE_THRESHOLD = 50; // centimeters

float duration_us, distance_cm;

// notes in the melody:
int melody[] = {
  659, 659, 659,
};

// note durations: 4 = quarter note, 8 = eighth note, etc, also called tempo:
int noteDurations[] = {
  8, 8, 4,
};

void setup() {
  pinMode(TRIG_PIN, OUTPUT);   // set arduino pin to output mode
  pinMode(ECHO_PIN, INPUT);    // set arduino pin to input mode
  pinMode(BUZZER_PIN, OUTPUT); // set arduino pin to output mode
}

void loop() {
  // generate 10-microsecond pulse to TRIG pin
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // measure duration of pulse from ECHO pin
  duration_us = pulseIn(ECHO_PIN, HIGH);
  // calculate the distance
  distance_cm = 0.017 * duration_us;

  if(distance_cm < DISTANCE_THRESHOLD)
    buzzer(); // play a song

  delay(500);
}

void buzzer() {
  // iterate over the notes of the melody:
  int size = sizeof(noteDurations) / sizeof(int);

  for (int thisNote = 0; thisNote < size; thisNote++) {
    // to calculate the note duration, take one second divided by the note type.
    //e.g. quarter note = 1000 / 4, eighth note = 1000/8, etc.
    int noteDuration = 1000 / noteDurations[thisNote];

    // turn on Piezo Buzzer for the note's duration
    digitalWrite(BUZZER_PIN, HIGH);
    delay(noteDuration);

    // turn off Piezo Buzzer to distinguish the notes
    digitalWrite(BUZZER_PIN, LOW);
    // to distinguish the notes, set a minimum time between them.
    // the note's duration + 30% seems to work well:
    int pauseBetweenNotes = noteDuration * 1.30;
    delay(pauseBetweenNotes);
  }
  digitalWrite(BUZZER_PIN, LOW);
}