I will be using the HC-SR04 ultrasonic sensor to measure sensor. I am using Arduino MEGA2560.

The speed of sound is 340.29 m/s then convert this to 29.38 us / cm.

340.29 m/s then 34029/1000000 cm/us then inverting 29.38 us/cm.

/*
HC-SR04 Ping distance sensor]
VCC to arduino 5v GND to arduino GND
Echo to Arduino pin 13 Trig to Arduino pin 12
Some code and wiring inspired by http://en.wikiversity.org/wiki/User:Dstaub/robotcar
*/

#define t 4
#define e 5
#define led_2 52


void setup() {
  Serial.begin (9600);
  pinMode(t, OUTPUT);
  pinMode(e, INPUT);
  pinMode ( led_2 ,  OUTPUT);
  digitalWrite(led_2, LOW);

}


void loop() {
  long duration, distance;
  // The speed of sound is 340.29 m/s or 29.38 microseconds per centimeter.
  // The ping travels out and back, so to find the distance of the
  // object we take half of the distance travelled
  
  // Trigger Pulse 10us
  digitalWrite(t, LOW);  
  delayMicroseconds(2); 
  digitalWrite(t, HIGH);
  delayMicroseconds(10); 
  digitalWrite(t, LOW);
  
  // Reads a pulse (either HIGH or LOW) on a pin. if value is HIGH, pulseIn() waits for the pin to go HIGH, 
  // starts timing, then waits for the pin to go LOW and stops timing. Returns the length of the pulse in microseconds.
  // it returns an unsigned long datatype
  duration = pulseIn(e, HIGH);
  distance = (duration/2) / 29.38;

  if (distance >= 200 || distance <= 0){
    Serial.println("Out of range");
  }
  else {
    Serial.print(distance);
    Serial.println(" cm");
    if ( distance < 25 ){
      digitalWrite( led_2 , HIGH);
    } else {
      digitalWrite ( led_2 , LOW );
    }
  }
  
  
  
  
  delay(500);
}