This sound sensor for Lego Mindstorms NXT is capable of measuring the loudness of the robot's environment. Using this sound sensor, your Lego Mindstorms NXT robot can move or flee to areas of higher or lower loudness depending on the NXT-G code that you have done.
The Lego sound sensor is capable of measuring the loudness of a piece in decibels (dB) and dBA (frequencies around 3-6 kHz, which the human ear is most sensitive).
This sound sensor was included by default in the previous version of Lego Mindstorms NXT kit (the version V2 of the kit has now replaced the old kit). The Lego sound sensor was replaced by a second touch sensor in version 2 of the kit and is no longer present by default. However, it is fully compatible with version 2 of the kit and NXT-G block is included by default in NXT-G.
The cable for the Lego Sound Sensor is sold separately.
Lego reference: 9845