Arduino sensor de distncia

double
t, dist;

void setup(){
  pinMode(3,OUTPUT);
  pinMode(4, INPUT);
  Serial.begin(9600);
}

void loop(){
  digitalWrite(3, LOW);
  delayMicroseconds(2);
  digitalWrite(3, HIGH);
  delayMicroseconds(10);
  digitalWrite(3, LOW);
  t = pulseIn(4, HIGH);

  if (t>1000) t=1000;
  Serial.println(t);

  tone(9, 4000, 100);
  delay(t/2);

}