processing4/java/libraries/sound/examples/Demos/Spectrum/Spectrum.pde

/*
This example shows how to use the Fast Fourier Transform function to get the spectrum
of a sound. This function calculates the FFT of a signal and returns the positive normalized 
magnitude spectrum. This means we pass it the number of bands we want (the actual FFT size is 
two times that size) and a float array with the same size. 
*/

import processing.sound.*;

SoundFile sample;
FFT fft;

int scale=1;
int bands=512;
float[] spec = new float[bands];

public void setup() {
    size(bands,360);
    background(255);

    //Load and play a soundfile and loop it. This has to be called 
    // before the FFT is created.
    sample = new SoundFile(this, "beat.aiff");
    sample.loop();
    
    // Create and patch the rms tracker
    fft = new FFT(this);
    fft.input(sample, bands);
}      
  
public void draw() {
    background(255);
    
    fft.analyze(spec);
    
    for(int i = 0; i < bands; i++)
    {
      // The result of the FFT is normalized
      // draw the line for frequency band i scaling it up by 5 to get more amplitude.
      line( i, height, i, height - spec[i]*height*5 );
    } 
}