How do you get the decibel level of an audio in Javascript

I am currently making a dbmeter visuilaizer using javascript, html and css.

I have gone through several web audio api tutorials, but nothing on there is close to being specific to what i want to do.

This is what i have so far:

window.onload = init;

function init(){

  var ctx = new webkitAudioContext()
    , url = 'http://ift.tt/1Ed6cKO'  
    , audio = new Audio(url)
    // 2048 sample buffer, 1 channel in, 1 channel out  
    , processor = ctx.createJavaScriptNode(2048, 1, 1)
    , meter = document.getElementById('meter')
    , source;

  audio.addEventListener('canplaythrough', function(){
    source = ctx.createMediaElementSource(audio);
    source.connect(processor);
    source.connect(ctx.destination);
    processor.connect(ctx.destination);
    audio.play();
  }, false);

  // loop through PCM data and calculate average
  // volume for a given 2048 sample buffer
  processor.onaudioprocess = function(evt){
    var input = evt.inputBuffer.getChannelData(0)
      , len = input.length   
      , total = i = 0
      , rms;
    while ( i < len ) total += Math.abs( input[i++] );
    rms = Math.sqrt( total / len );
    meter.style.width = ( rms * 100 ) + '%';
  };

}

can someone please explain what I need to do or point me in the right direction as this doesn't seem to be working.

Regards