The smoothingTimeConstant property of the AnalyserNode interface is a double value representing the averaging constant with the last analysis frame. It's basically an average
between the current buffer and the last buffer the AnalyserNode processed, and results in a much smoother set of value changes over time.

The smoothingTimeConstant property's value defaults to 0.8; it must be in the range 0 to 1 (0 meaning no time averaging). If 0 is set, there is no averaging done, whereas a value of 1 means "overlap the previous and current buffer quite a lot while computing the value", which essentially smoothes the changes across AnalyserNode.getFloatFrequencyData/AnalyserNode.getByteFrequencyData calls.

In technical terms, we apply a Blackman window and smooth the values over time. The default value is good enough for most cases.

Note:  If a value outside the range 0–1 is set, an INDEX_SIZE_ERR exception is thrown.


var audioCtx = new AudioContext();
var analyser = audioCtx.createAnalyser();
analyser.smoothingTimeConstant = 1;


A double.


The following example shows basic usage of an AudioContext to create an AnalyserNode, then requestAnimationFrame and <canvas> to collect frequency data repeatedly and draw a "winamp bargraph style" output of the current audio input. For more complete applied examples/information, check out our Voice-change-O-matic demo (see app.js lines 128–205 for relevant code).

If you are curious about the effect the smoothingTimeConstant() has, try cloning the above example and setting analyser.smoothingTimeConstant = 0; instead. You'll notice that the value changes are much more jarring.

var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
var analyser = audioCtx.createAnalyser();
analyser.minDecibels = -90;
analyser.maxDecibels = -10;
analyser.smoothingTimeConstant = 0.85;


analyser.fftSize = 256;
var bufferLength = analyser.frequencyBinCount;
var dataArray = new Uint8Array(bufferLength);

canvasCtx.clearRect(0, 0, WIDTH, HEIGHT);

function draw() {
  drawVisual = requestAnimationFrame(draw);


  canvasCtx.fillStyle = 'rgb(0, 0, 0)';
  canvasCtx.fillRect(0, 0, WIDTH, HEIGHT);

  var barWidth = (WIDTH / bufferLength) * 2.5;
  var barHeight;
  var x = 0;

  for(var i = 0; i < bufferLength; i++) {
    barHeight = dataArray[i];

    canvasCtx.fillStyle = 'rgb(' + (barHeight+100) + ',50,50)';

    x += barWidth + 1;



Specification Status Comment
Web Audio API
The definition of 'smoothingTimeConstant' in that specification.
Working Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 10.0webkit 25.0 (25.0)  Not supported 15.0webkit
22 (unprefixed)
Feature Android Firefox Mobile (Gecko) Firefox OS IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support ? 26.0 1.2 ? ? ? 33.0

See also


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

AnalyserNode API Property Reference Référence smoothingTimeConstant Web Audio API