How do car parking sensors work?

Small round sensors set into a car's bumper emit bursts of ultrasound, typically around 40 kilohertz, which is far too high-pitched for humans to hear. When the sound hits an object such as a wall or another car, part of it reflects back to the sensor. Because sound travels through air at about 343 metres per second, the car's computer can use the echo's return time to work out the distance. As the gap shrinks, the warning beeps get faster and usually become a continuous tone when the car is very close. This is the same echolocation principle bats and dolphins use, and some cars add cameras or radar sensors for extra help.