_renderWaveform

_renderWaveform: function(buffer){
              var samples = buffer.getChannelData(0);
              var container = this.$$('paper-item-body');
              var canvas = this.$$('#waveform');
              var width = container.offsetWidth;
              var height = Math.min(container.offsetHeight, 76);


              var ratio = this._getBackingScale(canvas);
              canvas.width = Math.floor(width * ratio);
              canvas.height = Math.floor(height * ratio);
              canvas.style.width = Math.floor(width) + 'px';
              canvas.style.height = Math.floor(height) + 'px';
              canvas.getContext('2d').setTransform(ratio, 0, 0, ratio, 0, 0);

              var samplesPerPixel = Math.floor(samples.length/width);

              var ctx = canvas.getContext('2d');

              ctx.clearRect(0, 0, width, height);
              ctx.strokeStyle = this.waveformColor || 'rgba(0,0,0,0.4)';
              ctx.translate(0, height/2);
              for (var pixelIndex = 0;pixelIndex < width; pixelIndex++){
                var sumSq = 0;
                var sampleCount = 0;
                var dataIndex = pixelIndex*samplesPerPixel;
                for (var sampleIndex = 0; sampleIndex < samplesPerPixel; sampleIndex++){
                  var thisSampleIndex = dataIndex + sampleIndex;
                  if (thisSampleIndex < samples.length){
                    sumSq += samples[thisSampleIndex] * samples[thisSampleIndex];
                    sampleCount++;
                  }
                }
                var x = pixelIndex;
                var y = Math.sqrt(sumSq/sampleCount) * (height/2);
                ctx.beginPath();
                ctx.moveTo(x, -y);
                ctx.lineTo(x, y);
                ctx.stroke();
              }