int speakerPin = 9;
 unsigned long echo = 0;
 int ultraSoundSignal = 8; // Ultrasound signal pin
 unsigned long ultrasoundValue = 0;

 void setup()
 {
 Serial.begin(9600);
 pinMode(ultraSoundSignal,OUTPUT);
 pinMode(speakerPin, OUTPUT);
 }

 unsigned long ping(){
 pinMode(ultraSoundSignal, OUTPUT); // Switch signalpin to output
 digitalWrite(ultraSoundSignal, LOW); // Send low pulse
 delayMicroseconds(2); // Wait for 2 microseconds
 digitalWrite(ultraSoundSignal, HIGH); // Send high pulse
 delayMicroseconds(5); // Wait for 5 microseconds
 digitalWrite(ultraSoundSignal, LOW); // Holdoff
 pinMode(ultraSoundSignal, INPUT); // Switch signalpin to input
 digitalWrite(ultraSoundSignal, HIGH); // Turn on pullup resistor
  echo = pulseIn(ultraSoundSignal, HIGH); //Listen for echo
  ultrasoundValue = (echo / 58.138); //convert to CM
 return ultrasoundValue;
 }

 void loop()
 {
 int x = 0;
 x = ping();
 Serial.print(x);
 Serial.println(" cm");
 delay(250); //delay 1/4 seconds.
 if (x<=20)
   {
     digitalWrite(speakerPin, HIGH); delayMicroseconds(1000);
     digitalWrite(speakerPin, LOW); delayMicroseconds(10);
   }
 }