This is a simple coding that display the distance of the obstacle to the sensor. When the sensor is approaching the obstacle, the buzzer will sound at a higher rate.

HC-SR04 Ultrasonic Sensor

#include <NewPing.h>

#define TRIGGER_PIN  12  // Arduino pin tied to trigger pin on the ultrasonic sensor.
#define ECHO_PIN     11  // Arduino pin tied to echo pin on the ultrasonic sensor.
#define MAX_DISTANCE 200 // Maximum distance we want to ping for (in centimeters). Maximum sensor distance is rated at 400-500cm.

NewPing sonar(TRIGGER_PIN, ECHO_PIN, MAX_DISTANCE); // NewPing setup of pins and maximum distance.

void setup() {
  Serial.begin(115200); // Open serial monitor at 115200 baud to see ping results.
  pinMode(7,OUTPUT);
}

void loop() {
  delay(50);                      // Wait 50ms between pings (about 20 pings/sec). 29ms should be the shortest delay between pings.
  int distance = sonar.ping_cm(); // Send ping, get ping time in microseconds (uS).
  Serial.print("Ping: ");
  Serial.print(distance); // Convert ping time to distance in cm and print result (0 = outside set distance range)
  Serial.println("cm");
  if (distance < 35)
  {
    digitalWrite(7, HIGH);
    delay(distance*10);
    digitalWrite(7, LOW);
    delay(distance*10);
  }
}