Arduino script to measure average distance using HC-SR04 ultrasonic sensor and buzzing as the distance gets less than a threshold (10cm) with increasing frequency as the distance gets shorter.

hc-sr04-with-buzzer.ino

#define echoPin 7
#define trigPin 8

long duration;
long distance;
int n = 0;
double accumulator = 0.0;

void setup() {
  Serial.begin(9600);
  pinMode(echoPin, INPUT);
  pinMode(trigPin, OUTPUT);
}

void loop() {
    digitalWrite(trigPin, LOW);
    delayMicroseconds(2);
  
    digitalWrite(trigPin, HIGH);
    delayMicroseconds(10);
  
    digitalWrite(trigPin, LOW);
  
    duration = pulseIn(echoPin, HIGH);
    distance = duration /58.2;
  
    if(distance < 10) {
      tone(11, (1.0/(distance*10.))*30000);
      delay((distance*1.0/2.0)*100);
      noTone(11);
    }
  
    n+=1;
    accumulator += distance;
    Serial.println(accumulator/n);
    //Serial.println(distance);
    delay(50);  
}