9846 NXT Ultrasonic Sensor is a Mindstorms set released in 2006. It contains an Ultrasonic Sensor for the Mindstorms NXT. The Ultrasonic sensor allows Mindstorms robots to sense objects in front of them, up to 250 cm (98 in)
This is a description taken from LEGO.com. Do not modify it.
Your robot's "eyes!"
The Ultrasonic Sensor helps your LEGO® MINDSTORMS® NXTrobot judge distances and "see" where objects are! Using the NXT Brick (sold separately), the Ultrasonic Sensor is able to detect an object and measure its proximity in inches or centimeters.