The Web Audio API

The Web Audio API for hackers

(or, how to properly make elaborate noise on the web)

High-five at the Mozilla Festival!

Outline of the talk

  • Overview
  • Nodes presentation
  • AudioParam
  • Demos/Livecoding
  • Known shortcomings
  • Firefox Devtool
  • Gecko/Blink/Webkit Compatibility

What is Web Audio, in one sentence

What does the spec tell us?

[...] A high-level JavaScript API for processing and synthesizing audio in web applications. The primary paradigm is of an audio routing graph, where a number of AudioNode objects are connected together to define the overall audio rendering.

What is Web Audio, in one picture (and some code)

Web Audio Graph
var ac = new AudioContext(); 
ac.decodeAudioData(ogg_arraybuffer, function(audiobuffer) { 
  var source = ac.createBufferSource();
  source.buffer = audiobuffer; 
  var d = ac.createDelay()
  var osc = ac.createOscillator();
  osc.type = "square";
  osc.frequency.value = 100; // Hz
  var g = ac.createGain();
}, function error() { alert("This is broken"); });

Use cases

  • Games
  • Audio visualization
  • Web Application audio feedback
  • Musical applications
  • Fancy streaming music player (until we have MediaSource Extension widely available Firefox bug, spec)
  • (Ideally) Anything on the web that makes non-trivial noises
  • (including offline rendering)

Feature Overview

  • Asynchronous compressed audio decoding to ArrayBuffers (formats: same as <audio>)
  • Precise timing for sample playback
  • Arbitrary audio routing/mixing
  • Effect transition, scheduling (automation)
  • Ability to analyse audio (e.g. FFT)
  • Integration with the web platform (MediaRecorder, WebRTC, <audio>)
  • Low-latency playback

Internal Processing model

  • Can't share AudioNodes between AudioContext
  • Can share AudioBuffers, though
  • Dedicated thread for audio processing
  • Main thread for control (shared with everything else: <canvas>, WegGL, etc.)
  • Message passing (between main thread and audio thread): no locking
  • Float32 everywhere
  • Block processing (128 sample-frames)
  • Feedback loops allowed (iff at least one DelayNode in the cycle)
  • Low latency (not low enough yet for Firefox but coming)

Processing model


  • Message passing for everything to avoid blocking the audio thread (that would result in hearable glitches)
  • You schedule things to happen on the audio thread, they do not happen immediatly, but at the next audio callback
  • You can't retrieve values from the audio thread (they would be meaningless, because late)


  • Input nodes: produce audio -- audio sources
  • Output nodes: receive audio -- audio sinks
  • Processing nodes: process incoming audio, output processed signal

Source Nodes

  • AudioBufferSourceNode, sample player, plays AudioBuffer. One shot, but cheap to create
  • OscillatorNode, sine, square, saw, triangle, custom from FFT. One shot (use a GainNode)
  • ScriptProcessorNode (with no input) to generate arbitrary waveforms using JavaScript (but beware, it's broken!)
  • MediaElementAudioSourceNode to pipe the audio from <audio> or <video> in an AudioContext
  • MediaStreamAudioSourceNode, from PeerConnection, getUserMedia.

Processing Nodes

  • GainNode: Change the volume
  • DelayNode: Delays the input, in time. Needed for cycles.
  • ScriptProcessorNode (with both input and output connected): arbitrary processing (but beware, it's broken)
  • PannerNode: Position a source and a listener in 3D and get the sound panned accordingly
  • Channel{Splitter,Merger}Node: Merge or Split multi-channel audio from/to mono
  • ConvolverNode: Perform one-dimensional convolution between an AudioBuffer and the input (e.g. reverb)

Processing Nodes (moar)

  • WaveShaperNode: Non-linear wave shaping (e.g. guitar distortion)
  • BiquadFilter: low-pass, high-pass, band-pass, all-pass, etc.
  • DynamicsCompressorNode: adjust audio dynamics

Output Nodes

  • MediaStreamAudioDestinationNode: Outputs to a MediaStream(send to e.g. a WebRTC PeerConnection or a MediaRecorder to encode)
  • AudioDestinationNode: Outputs to speakers/headphones
  • ScriptProcessorNode: arbitrary processing on input audio
  • AnalyserNode: Get time-domain or frequency-domain audio data, in ArrayBuffers

AudioParam for automation

  • Almost every AudioNode parameter is an AudioParam
  • Set directly: g.gain.value = 0.5;
  • Use automation curve functions:
    • g.gain.setValueAtTime(0.5, ctx.currentTime + 1.0);
    • g.gain.linearRampToValueAtTime(0.5, ct + 1.0);
    • g.gain.exponentialRampToValueAtTime(0.0, ct + 1.0);
    • g.gain.setTargetAtTime(0.0, ct + 1.0, 0.66 /*exp constant*/);
    • g.gain.setValueCurveAtTime(arrayBuffer, ct + 1.0, arrayBuffer.length / ctx.sampleRate);
    • g.gain.cancelScheduledValues (ct);
  • De-ziperring

AudioParam + OscillatorNode = LFO automation

                 var osc = ctx.createOscillator(); // default: sine 
                 osc.frequency.value = 4; // Hz 
                 var lpf = ctx.createBiquadFilter(); // default: low-pass 
                 var gain = ctx.createGain() 
                 gain.gain.value = 1000; // [0; 1.0] to [0; 1000] 
                 osc.connect(gain.gain); // osc -> AudioParam 
                 // dubstep 

Demos !

A fancy jsbin to play with the Web Audio API.

Example code at the bottom of the "output" panel, a drum loop and a reverb impulse are included, feel free to add your own files!

(Fork it before using it, to keep your changes)

Known shortcomings

  • The ScriptProcessorNode is broken by design
  • Sometimes Float32 are too memory-heavy (OfflineAudioContext trick to same memory)
  • No FFT/IFFT exposed other than in the AnalyserNode
  • Does not work (yet) in WebWorkers
  • No way to enqueue buffers and have them play (but you have MediaSource for that)


  • It still exists !
  • It can play a Blob (from IndexedDB, XMLHttpRequest, local file, ArrayBuffer)
  • For when you don't care about perf, scheduling effects
  • For when you don't want to have the data loaded in memory
  • Think about using MediaElementAudioSourceNode, best of both worlds!

Cheat sheet

Cheat sheet for the Music Hack Day: Here !

Don't be afraid to go read the spec! (or to ask me question, I'll be around writing code)

Firefox Web Audio Editor

Open the devtools, cog icon, enable it, reload the page that has an AudioContext.

In heavy development (Made by Jordan Santell (@jsantell)), available in Nightly builds. (The canvas editor can also be useful for the MHD)

Possible demo page (but any page should work, or that's a bug).

Questions ?