The LEGO Arduino Cookbook Chapter 10: The LEGO Spectrum Analyzer
Here are couple videos of the LEGO Spectrun Analyzer built in Chapter 10 of The LEGO Arduino Cookbook. The spectral content of music is shown in two ways: an LED matrix and in three motorized mechanisms. The LED matrix is from mindsensors.com, with each column of the matrix displaying signal strength at 7 frequencies. Three frequency signals also drive motorized mechanisms, adapted from designs in Yoshihito Isogawa's "The LEGO MINDSTORMS EV3 Idea Book". In one video, electronic drums are being fed into the spectrum analyzer, which have a wide spectral content. In the bottom video is a classical guitar, which has midrange spectral content.