The LED parking senors emit ultrasonic waves that are reflected by the obstacle, to cause a visual-acoustic signal. The precise distance of the obstacles is detected to facilitate parking manoeuvres even in the most restricted spaces. The frequency of the ultrasonic waves has been chosen in order to minimise disturbance or damage to people and pets. Because it has a small LED monitor so we call it LED parking sensors, there are 2,4,6,8 sensors optional for all models. Just choose the best parking sensors for your car.