function setup() {
  createCanvas(800,400);
  noFill();

  osc1 = new p5.SinOsc();
  osc2 = new p5.SinOsc();
  fft = new p5.FFT();
  osc1.phase(.5);
  osc2.phase(0);
  osc1.amp(1);
  osc2.amp(1);
  osc1.start(); osc2.start();

  phaseSlider = createSlider(0, 100, 50);
}
Tell two sine wave oscillators to start at the same time,
 50% out of phase. Phase Cancellation results!
 Change the phase with the slider.