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The fftSize property of the AnalyserNode interface is an unsigned long value and represents the window size in samples that is used when performing a Fast Fourier Transform (FFT) to get frequency domain data.


var curValue = analyserNode.fftSize;
analyserNode.fftSize = newValue;


An unsigned integer, representing the window size of the FFT, given in number of samples. A higher value will result in more details in the frequency domain but fewer details in the time domain.

Must be a power of 2 between 252^5 and 2152^15, so one of: 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, and 32768. Defaults to 2048.

Note: If its value is not a power of 2, or it is outside the specified range, a DOMException with the name IndexSizeError is thrown.


The following example shows basic usage of an AudioContext to create an AnalyserNode, then requestAnimationFrame and <canvas> to collect time domain data repeatedly and draw an "oscilloscope 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).

var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
var analyser = audioCtx.createAnalyser();


analyser.fftSize = 2048;
var bufferLength = analyser.fftSize;
var dataArray = new Uint8Array(bufferLength);

// draw an oscilloscope of the current audio source

function draw() {

      drawVisual = requestAnimationFrame(draw);


      canvasCtx.fillStyle = 'rgb(200, 200, 200)';
      canvasCtx.fillRect(0, 0, WIDTH, HEIGHT);

      canvasCtx.lineWidth = 2;
      canvasCtx.strokeStyle = 'rgb(0, 0, 0)';


      var sliceWidth = WIDTH * 1.0 / bufferLength;
      var x = 0;

      for(var i = 0; i < bufferLength; i++) {
        var v = dataArray[i] / 128.0;
        var y = v * HEIGHT/2;

        if(i === 0) {
          canvasCtx.moveTo(x, y);
        } else {
          canvasCtx.lineTo(x, y);

        x += sliceWidth;

      canvasCtx.lineTo(canvas.width, canvas.height/2);



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

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support14 Yes25 No156
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support Yes14 Yes26 No15 ?

See also

Document Tags and Contributors

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