const int buzzer  = 16;
    const int trigPin = 17;
    const int echoPin = 18;
    
    //define sound velocity in cm/uS
    #define SOUND_VELOCITY 0.034
    
    long duration;
    float distanceCm;
    
    void setup() {
      Serial.begin(9600); // Starts the serial communication
      pinMode(trigPin, OUTPUT); // Sets the trigPin as an Output
      pinMode(echoPin, INPUT); // Sets the echoPin as an Input
      pinMode(buzzer, OUTPUT); // Sets the buzzer as an Output
    }
    
    void loop() {
      // Clears the trigPin
      digitalWrite(trigPin, LOW);
      delayMicroseconds(2);
      // Sets the trigPin on HIGH state for 10 micro seconds
      digitalWrite(trigPin, HIGH);
      delayMicroseconds(10);
      digitalWrite(trigPin, LOW);
      // Reads the echoPin, returns the sound wave travel time in microseconds
      duration = pulseIn(echoPin, HIGH);
    
      // Calculate the distance
      distanceCm = duration * SOUND_VELOCITY / 2;
    
      distanceCm = 5.50;
      // Prints the distance on the Serial Monitor
      Serial.print("Distance (cm): ");  Serial.println(distanceCm);
      
      digitalWrite(buzzer, HIGH);
      delay(100);
      digitalWrite(buzzer, LOW);
      delay(distanceCm * 100);
    }