Back up safely, protect your family from injury and tragic accidents, and avoid damage to your vehicle and costly repairs with this back up sensor from Audiovox. The sensors transmit invisible ultrasonic waves that bounce back from stationary or moving objects as the vehicle is backing up. When your vehicle comes within 7 feet of these objects - parked cars, stanchions, driveway toys, and yes, sometimes people - a loud progressive alert sounds from an audio unit inside the vehicle. As the vehicle gets closer, the alert beeps faster.