A sensor is placed on the rear bumper of the car, which is capable of both emitting and detecting ultrasound. As it pulses, a computer measures the time it takes for the sound it emits to be heard again. Parking sensor operate at a close range and some sensors have a minimum operating range. When the user set distance threshold is passed, the sensor will emit an audible sound alerting the driver that it is getting close to an object. the distance thresold is useful for those who may want to leave more room behind their vehicles, and those who like to squeeze in as tight as possible. Some more expensive models may also have an extra unit inside the car which can also visually alert the driver.