LEGO® MINDSTORMS® Sound Sensor (9845)
Your robot's "ears!"  Using the NXT Brick (sold separately), the Sound Sensor allows your LEGO® MINDSTORMS® NXT robot to hear! The Sound Sensor is able to measure noise levels in both dB (decibels) and dBA (frequencies around 3¿6 kHz where the human ear is most sensitive), as well as recognize sound patterns and identify tone differences.