Fast Fourier Transforms (FFT) are used to visualize the frequency spectrum of an audio file. The Minim library has a class that handles the math for you, so you can focus on drawing the spectrum. If you want to learn more about Fast Fourier Transforms, you should read this paper by Paul Bour ke, available at http://paulbourke.net/miscellaneous/dft/. While you are on his website, check out the rest of the things Paul has done. You'll find some really amazing geometry algorithms.
The first part of this sketch is similar to the previous ones; the only thing we'll add is an FFT object with the same buffer size and sample rate as the song we'll load.
import ddf.minim.*;
import ddf.minim.signals.*;
import ddf.minim.analysis.*;
import ddf.minim.effects.*;
Minim minim;
AudioPlayer player;
FFT fft;
void setup()
{
size( 1024, 480 );
minim = new Minim( this );
player = minim.loadFile("song.mp3", 512 );
player.loop();
fft = new...