Web Audio playbackRate explained

Let's starting by looking at a brief example of playbackRate usage:

const audio = document.createElement("audio");
audio.setAttribute("src", "audio-file.mp3");
audio.playbackRate = 0.5;

Here we create an <audio> element, and set its src to a file of our choice. Next we set playbackRate to 0.5, which represents half normal speed (the playbackRate is a multiplier applied to the original rate).

Let's create a <video> element first, and set up video and playback rate controls in HTML:

<video id="myVideo" controls>
  <source
    src="http://jplayer.org/video/m4v/Big_Buck_Bunny_Trailer.m4v"
    type="video/mp4" />
  <source
    src="http://jplayer.org/video/webm/Big_Buck_Bunny_Trailer.webm"
    type="video/webm" />
</video>

<form>
  <input id="pbr" type="range" value="1" min="0.5" max="4" step="0.1" />
  <p>Playback Rate <span id="currentPbr">1</span></p>
</form>

And apply some JavaScript to it:

window.onload = () => {
  const v = document.getElementById("myVideo");
  const p = document.getElementById("pbr");
  const c = document.getElementById("currentPbr");

  p.addEventListener(
    "input",
    () => {
      c.textContent = p.value;
      v.playbackRate = p.value;
    },
    false,
  );
};

Finally, we listen for the input event firing on the <input> element, allowing us to react to the playback rate control being changed.

In addition to playbackRate, we also have a defaultPlaybackRate property available, which lets us set the default playback rate: the playback rate to which the media resets; for example, if we change the source of the video, or (in some browsers) when an ended event is generated.

So defaultPlaybackRate allows us to set the playback rate before playing the media, while playbackRate allows us to change it during media playback.

There is also an event available called ratechange, which fires every time the playbackRate changes.

• Most browsers stop playing audio outside playbackRate bounds of 0.5 and 4, leaving the video playing silently. For most applications, it's recommended that you limit the range to between 0.5 and 4.
• The pitch of the audio track does not change when playBackRate is altered.
• Negative values indicating the media should play in reverse is not currently supported by most browsers.

• Hyperaudio's Playback Rate Test
• HTMLMediaElement: interface which defines the playbackRate property.