duty2.js

function pulse(destination, playbackTime, opts) {
  var t0 = playbackTime;
  var audioContext = destination.context;
  var oscillator = audioContext.createOscillator();
  var gain = audioContext.createGain();
  var frequency = opts.frequency;
  var volume = opts.volume;
  
  function createPulseWave(audioContext, duty, length) {
    var a = new Float32Array(length);
    var b = new Float32Array(length);
    
    for (var i = 1; i < length; i++) {
      a[i] = (2 / (i * Math.PI)) * Math.sin(duty * i * Math.PI);
    }
    
    return audioContext.createPeriodicWave(a, b);
  }
  
  
  var wave = createPulseWave(audioContext, opts.duty, 2048);

  oscillator.setPeriodicWave(wave);
  oscillator.frequency.value = frequency;
  oscillator.start(t0);
  oscillator.connect(gain);

  gain.gain.value = volume;
  gain.connect(destination);
}

example.js

function example01(audioContext, pulse) {
  function mtof(midi) {
    return 440 * Math.pow(2, (midi - 69) / 12);
  }

  var destination = audioContext.destination;
  var t0 = audioContext.currentTime;

  pulse(destination, t0, { frequency: 440, volume: 0.4, duty: 0.9 });
}