Sample of usage of MediaStream as an input to an AnalyserNode in WebAudio API

analyserNode.js

var analyserAudioNode, sourceAudioNode, micStream;
var streamReceived = function(stream) {
  micStream = stream;

  analyserAudioNode = audioContext.createAnalyser();
  analyserAudioNode.fftSize = 2048;

  sourceAudioNode = audioContext.createMediaStreamSource(micStream);
  sourceAudioNode.connect(analyserAudioNode);

  /* This is our pitch detection algorithm.
     You can find its implementation in the Autocorrelation section of this demo. */
  detectPitch();
};

navigator.getUserMedia({audio: true}, streamReceived);