Wave Phasing

The first two drop-downs change the wave combination. The third drop-down adjusts the phase.

<!DOCTYPE html>
<html>
<head>
  <style>
    body {
      margin: 0px; 
      overflow:hidden;
    } 
    div {
      margin-left: 10px;
    }   
    path {
      fill: none;
      stroke: black;
      stroke-width: 1;
    }
    svg {
      border: 1px solid steelblue;
      margin: 10px;
    }
  </style>
  <script src="//d3js.org/d3.v4.min.js"></script>
</head>
<body>
  <svg></svg>
  <div>
    <select id="wave-select1" class="wave-select"></select>
    <select id="wave-select2" class="wave-select"></select>
    <select id="speed-select"></select>
    <button id="stop-start" onclick="playing ? stopSound() : phase()">start</button>
    Mute: <input type="checkbox" onchange="muteSound()" checked/>
  </div>
</body>
<script>

var width = Math.max(940, innerWidth-20),
        height = Math.max(450, innerHeight-60);

var playing = false;
  var source;
  var timer;

var audioCtx = new (window.AudioContext || window.webkitAudioContext)(),
      gainNode = audioCtx.createGain(),
      sampleRate = 44100,
      frequency = 440,
      waveCycles = 4,
      samples = sampleRate / frequency;

var eases = [
    "easeLinear", 
    "easePolyIn", "easePolyOut", "easePolyInOut", 
    "easeQuadIn", "easeQuadOut", "easeQuadInOut", 
    "easeCubicIn", "easeCubicOut", "easeCubicInOut", 
    "easeSinIn", "easeSinOut", "easeSinInOut", 
    "easeExpIn", "easeExpOut", "easeExpInOut", 
    "easeCircleIn", "easeCircleOut", "easeCircleInOut", 
    "easeElasticIn", "easeElasticOut", "easeElasticInOut", 
    "easeBackIn", "easeBackOut", "easeBackInOut", 
    "easeBounceIn", "easeBounceOut", "easeBounceInOut"
    ];

d3.selectAll(".wave-select").selectAll("option")
      .data(eases)
    .enter().append("option")
      .attr("value", function (d) {return d})
      .text(function (d) {return d});

d3.select("#speed-select").selectAll("option")
      .data(d3.range(1, 20))
    .enter().append("option")
      .attr("value", function (d) {return d})
      .text(function (d) {return d});

d3.selectAll("select")
      .on("change", function () {if (playing) phase()});

var buffer = audioCtx.createBuffer(2, samples * waveCycles, sampleRate);

var svg = d3.select("svg")
      .attr("width", width)
      .attr("height", height);

var x = d3.scaleLinear()
      .range([0, width])
      .domain([samples * .25, buffer.length - samples * .75]);

var y = d3.scaleLinear()
      .range([height - 60, 60])
      .domain([-1, 1]);

var wave = d3.line()
      .curve(d3.curveMonotoneX)
      .x(function (d, i) {return x(i)})
      .y(function (d) {return y(d)});

var waveShape1 = svg.append("path")
      .datum([])
      .attr("stroke-dasharray", [5, 5])
      .attr("stroke", "#e9e9e9")
      .attr("d", wave);

var waveShape2 = svg.append("path")
      .datum([])
      .attr("stroke-dasharray", [5, 5])
      .attr("stroke", "#e9e9e9")
      .attr("d", wave);

var mainWave = svg.append("path")
      .datum([])
      .style("stroke", "red")
      .style("stroke-width", 2)
      .attr("d", wave);

var position = svg.append("circle")
      .attr("r", 5);

phase();

function loadAudio() {
    var offset = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 0;

var ease1 = d3.select("#wave-select1").property("value");
    var ease2 = d3.select("#wave-select2").property("value");

var combined = new Float32Array(buffer.length),
        buffer1 = new Float32Array(buffer.length),
        buffer2 = new Float32Array(buffer.length);

for (i = 0; i < buffer.length; i++) {
      var thisVal = Math.abs((i + samples / 4) % samples / (samples / 2) - 1),
          thisVal2 = Math.abs((i + offset + samples / 4) % samples / (samples / 2) - 1);

var eased = (d3[ease1](thisVal) - .5) * 2,
          eased2 = (d3[ease2](thisVal2) - .5) * 2;

buffer.getChannelData(1)[i] = eased;
      buffer.getChannelData(0)[i] = eased2;
    }

combined = combined.map(function (d, i) {
      return (buffer.getChannelData(0)[i] + buffer.getChannelData(1)[i]) / 2;
    });

waveShape1.datum(buffer.getChannelData(0)).attr("d", wave);
    waveShape2.datum(buffer.getChannelData(1)).attr("d", wave);
    mainWave.datum(combined).attr("d", wave);

position.attr("cx", function (d) {return x(buffer.length >> 1)})
        .attr("cy", function (d) {return y(combined[buffer.length >> 1])});

if (playing == false) playSound();
  }

function phase() {
    var t = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 0;

if (playing) stopSound();
    d3.select("#stop-start").text("stop");
    var step = d3.select("#speed-select").property("value");

timer = d3.timer(function () {
        t = t >= samples ? 0 : t;
        loadAudio(t += +step);
      }, 1);
  }

function playSound() {
    if (playing) stopSound();
    playing = true;
    source = audioCtx.createBufferSource();
    source.buffer = buffer;
    source.connect(gainNode);
    gainNode.connect(audioCtx.destination);
    source.loop = true;
    muteSound();
    source.start();
  }

function stopSound() {
    timer.stop();
    playing = false;
    if (source) source.stop();
    muteSound();
    d3.select("#stop-start").text("start");
  }

function muteSound() {
    return gainNode.gain.value = d3.select("input").property("checked") ? 0 : 1;
  }
</script>

https://d3js.org/d3.v4.min.js