Recording Audio from the User

Paul Kinlan

Many browsers now have the ability to access video and audio input from the user. However, depending on the browser, it might be a full dynamic and inline experience, or it could be delegated to another app on the user's device.

Start simple and progressively

The easiest thing to do is simply ask the user for a pre-recorded file. Do this by creating a simple file input element and adding an accept filter that indicates we can only accept audio files, and a capture attribute that indicates we want to get it direct from the microphone.

<input type="file" accept="audio/*" capture />

This method works on all platforms. On desktop, it will prompt the user to upload a file from the file system (ignoring the capture attribute). In Safari on iOS, it will open up the microphone app, allowing you to record audio and then send it back to the web page; on Android, it will give the user the choice of which app to use record the audio in before sending it back to the web page.

Once the user has finished recording and they are back in the website, you need to somehow get ahold of the file data. You can get quick access by attaching an onchange event to the input element and then reading the files property of the event object.

<input type="file" accept="audio/*" capture id="recorder" />
<audio id="player" controls></audio>
  <script>
    const recorder = document.getElementById('recorder');
    const player = document.getElementById('player');

    recorder.addEventListener('change', function (e) {
      const file = e.target.files[0];
      const url = URL.createObjectURL(file);
      // Do something with the audio file.
      player.src = url;
    });
  </script>
</audio>

Once you have access to the file, you can do anything you want with it. For example, you can:

  • Attach it directly to an <audio> element so that you can play it
  • Download it to the user's device
  • Upload it to a server by attaching it to an XMLHttpRequest
  • Pass it through the Web Audio API and apply filters on to it

Whilst using the input element method of getting access to audio data is ubiquitous, it is the least appealing option. We really want to get access to the microphone and provide a nice experience directly in the page.

Access the microphone interactively

Modern browsers can have a direct line to the microphone allowing us to build experiences that are fully integrated with the web page and the user will never leave the browser.

Acquire access to the microphone

We can directly access the Microphone by using an API in the WebRTC specification called getUserMedia(). getUserMedia() will prompt the user for access to their connected microphones and cameras.

If successful the API will return a Stream that will contain the data from either the camera or the microphone, and we can then either attach it to an <audio> element, attach it to a WebRTC stream, attach it to a Web Audio AudioContext, or save it using the MediaRecorder API.

To get data from the microphone, we just set audio: true in the constraints object that is passed to the getUserMedia() API.

<audio id="player" controls></audio>
<script>
  const player = document.getElementById('player');

  const handleSuccess = function (stream) {
    if (window.URL) {
      player.srcObject = stream;
    } else {
      player.src = stream;
    }
  };

  navigator.mediaDevices
    .getUserMedia({audio: true, video: false})
    .then(handleSuccess);
</script>

If you want to choose a particular microphone, you can first enumerate the available microphones.

navigator.mediaDevices.enumerateDevices().then((devices) => {
  devices = devices.filter((d) => d.kind === 'audioinput');
});

You can then pass the deviceId that you wish to use when you call getUserMedia.

navigator.mediaDevices.getUserMedia({
  audio: {
    deviceId: devices[0].deviceId,
  },
});

By itself, this isn't that useful. All we can do is take the audio data and play it back.

Access the raw data from the microphone

To access the raw data from the microphone, we have to take the stream created by getUserMedia() and then use the Web Audio API to process the data. The Web Audio API is a simple API that takes input sources and connects those sources to nodes which can process the audio data (adjust Gain etc.) and ultimately to a speaker so that the user can hear it.

One of the nodes that you can connect is an AudioWorkletNode. This node gives you the low-level capability for custom audio processing. The actual audio processing happens in the process() callback method in the AudioWorkletProcessor. Call this function to feed inputs and parameters and fetch outputs.

Check out Enter Audio Worklet to learn more.

<script>
  const handleSuccess = async function(stream) {
    const context = new AudioContext();
    const source = context.createMediaStreamSource(stream);

    await context.audioWorklet.addModule("processor.js");
    const worklet = new AudioWorkletNode(context, "worklet-processor");

    source.connect(worklet);
    worklet.connect(context.destination);
  };

  navigator.mediaDevices.getUserMedia({ audio: true, video: false })
      .then(handleSuccess);
</script>

// processor.js
class WorkletProcessor extends AudioWorkletProcessor {
  process(inputs, outputs, parameters) {
    // Do something with the data, e.g. convert it to WAV
    console.log(inputs);
    return true;
  }
}

registerProcessor("worklet-processor", WorkletProcessor);

The data that is held in the buffers is the raw data from the microphone and you have a number of options with what you can do with that data:

  • Upload it straight to the server
  • Store it locally
  • Convert it to a dedicated file format, such as WAV, and then save it to your servers or locally

Save the data from the microphone

The easiest way to save the data from the microphone is to use the MediaRecorder API.

The MediaRecorder API will take the stream created by getUserMedia and then progressively save the data that is on the stream into your preferred destination.

<a id="download">Download</a>
<button id="stop">Stop</button>
<script>
  const downloadLink = document.getElementById('download');
  const stopButton = document.getElementById('stop');


  const handleSuccess = function(stream) {
    const options = {mimeType: 'audio/webm'};
    const recordedChunks = [];
    const mediaRecorder = new MediaRecorder(stream, options);

    mediaRecorder.addEventListener('dataavailable', function(e) {
      if (e.data.size > 0) recordedChunks.push(e.data);
    });

    mediaRecorder.addEventListener('stop', function() {
      downloadLink.href = URL.createObjectURL(new Blob(recordedChunks));
      downloadLink.download = 'acetest.wav';
    });

    stopButton.addEventListener('click', function() {
      mediaRecorder.stop();
    });

    mediaRecorder.start();
  };

  navigator.mediaDevices.getUserMedia({ audio: true, video: false })
      .then(handleSuccess);
</script>

In our case, we are saving the data directly into an array that we can later turn into a Blob, which can be then used to save the data to our Web Server or directly to the storage on the user's device.

Ask permission to use microphone responsibly

If the user has not previously granted your site access to the microphone, then the instant that you call getUserMedia, the browser will prompt the user to grant your site permission to the microphone.

Users hate getting prompted for access to powerful devices on their machine and they will frequently block the request, or they will ignore it if they don't understand the context of which the prompt has been created. It is best practice to only ask to access the microphone when first needed. Once the user has granted access, they won't be asked again, however, if they reject access, you can't ask the user for permission again.

Use the permissions API to check if you already have access

The getUserMedia API provides you with no knowledge of if you already have access to the microphone. This presents you with a problem, to provide a nice UI to get the user to grant you access to the microphone, you have to ask for access to the microphone.

This can be solved in some browsers by using the Permission API. The navigator.permission API allows you to query the state of the ability to access specific API's without having to prompt again.

To query if you have access to the user's microphone, you can pass in {name: 'microphone'} into the query method and it will return either:

  • granted — the user has previously given you access to the microphone;
  • prompt — the user has not given you access and will be prompted when you call getUserMedia;
  • denied — the system or the user has explicitly blocked access to the microphone and you won't be able to get access to it.

And you can now quickly check to see if you need to alter your user interface to accommodate the actions that the user needs to take.

navigator.permissions.query({name: 'microphone'}).then(function (result) {
  if (result.state == 'granted') {
  } else if (result.state == 'prompt') {
  } else if (result.state == 'denied') {
  }
  result.onchange = function () {};
});

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