Exporting intensity of audio in Web Audio API

Question 1

If I understand correctly, you want a number that is high when the sound is loud, and low when the sound is quiet. You can use the "Sound Pressure Level" for that.

Getting this number from the Web Audio API is rather straightforward, and you had guessed correctly that we will use the AnalyserNode to achieve this. Here is a example code that shows you how to do it:

var ac = new AudioContext();
/* create the Web Audio graph, let's assume we have sound coming out of the
 * node `source` */
var an = ac.createAnalyser();
source.connect(an);
/* Get an array that will hold our values */
var buffer = new Uint8Array(an.fftSize);

function f() {
  /* note that getFloatTimeDomainData will be available in the near future,
   * if needed. */
  an.getByteTimeDomainData(buffer);
  /* RMS stands for Root Mean Square, basically the root square of the
  * average of the square of each value. */
  var rms = 0;
  for (var i = 0; i < buffer.length; i++) {
    rms += buffer[i] * buffer[i];
  }
  rms /= buffer.length;
  rms = Math.sqrt(rms);
  /* rms now has the value we want. */
  requestAnimationFrame(f);
}

requestAnimationFrame(f);
/* start our hypothetical source. */
source.start(0);

Question 2

I wanted to thank you for this answer some 4 years later. I just did a quick POC and got it to work with the following code. I hope it might help somebody else too.

In this example, I am taking the live audio from my Mic and logging the results to the console - in my case, under the chrome dev tools.

<html>
  <head>
    <title>Intensity test</title>
  </head>
  <body>
    <script>
      var ac = new AudioContext();
      var an = ac.createAnalyser();
      var source = "";
      var buffer = new Uint8Array(an.fftSize);
      var scriptProcessorNode = ac.createScriptProcessor(16384, 1, 1);
      if (!navigator.getUserMedia)
        navigator.getUserMedia = navigator.getUserMedia || 
          navigator.webkitGetUserMedia || navigator.mozGetUserMedia || 
          navigator.msGetUserMedia;
      if (navigator.getUserMedia) {
        navigator.getUserMedia(
          {audio:true},
          function(stream) {
            source = ac.createMediaStreamSource(stream);
            source.connect(an);
            requestAnimationFrame(f);
          },
          function(e) {
            alert('Error capturing audio.');
          }
        );
      }

      function f() {
        an.getByteTimeDomainData(buffer);
        var rms = 0;
        for (var i = 0; i < buffer.length; i++)
          rms += buffer[i] * buffer[i];
        rms /= buffer.length;
        rms = Math.sqrt(rms);
        requestAnimationFrame(f);
        console.log(rms);
      }
    </script>
  </body>
</html>