function setup() {
  createCanvas(windowWidth, windowHeight);
  noFill();

  source = new p5.AudioIn();
  source.start();

  fft = new p5.FFT();
  fft.setInput(source);
}
Auto Correlation multiples each sample in a buffer by all
 of the other samples. This emphasizes the fundamental
 frequency. Auto Correlation is useful for pitch detection,
 as well as for visualization
 
 This example is a Correlogram which is a plot
 of the autocorrelations.
 
 Example by Jason Sigal and Golan Levin.