The OfflineAudioContext()
constructor—part of the Web Audio API—creates and returns a new OfflineAudioContext
object instance, which can then be used to render audio to an AudioBuffer
rather than to an audio output device.
Syntax
var offlineAudioCtx = new OfflineAudioContext(numberOfChannels, length, sampleRate); var offlineAudioCtx = new OfflineAudioContext(options);
Parameters
You can specify the parameters for the OfflineAudioContext()
constructor as either the same set of parameters as are inputs into the AudioContext.createBuffer()
method, or by passing those parameters in an options
object. Either way, the individual parameters are the same.
numberOfChannels
- An integer specifying the number of channels the resulting
AudioBuffer
should have. length
- An integer specifying the size of the buffer to create for the audio context, in sample-frames, where one sample-frame is a unit that can contain a single sample of audio data for every channel in the audio data. For example, a 512 sample-frame buffer for 8 channels of sound would be
512 * ((sizeof(float32) * 8) * 32)
or 524,288 bytes. sampleRate
- The sample-rate of the linear audio data in sample-frames per second. All user agents are required to support a range of 22050kHz to 96000kHz, and may support a wider range than that. The most commonly-used rate is 44100kHz, which is the sample rate used by CD audio.
It is important to note that, whereas you can create a new AudioContext
using the new AudioContext()
constructor with no arguments, the OfflineAudioContext()
constructor requires three arguments, since it needs to create an AudioBuffer
. This works in exactly the same way as when you create a new AudioBuffer
with the AudioContext.createBuffer()
method. For more detail, read Audio buffers: frames, samples and channels from our Basic concepts guide.
Return value
A new OfflineAudioContext
object whose associated AudioBuffer
is configured as requested.
Like a regular AudioContext
, an OfflineAudioContext
can be the target of events, therefore it implements the EventTarget
interface.
Example
const offlineCtx = new OfflineAudioContext({ numberOfChannels: 2, length: 44100 * 40, sampleRate: 44100, }); const source = offlineCtx.createBufferSource(); // etc...
For a full working example, see our offline-audio-context-promise Github repo (see the source code too.)
Specifications
Specification | Status | Comment |
---|---|---|
Web Audio API The definition of 'OfflineAudioContext' in that specification. |
Working Draft |
Browser compatibility
Feature | Chrome | Edge | Firefox | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | 551 | ? | 53 | No | 42 | ? |
Parameters accepted as a single object, as well as being passed in individually | 62 | ? | 57 | No | 49 | No |
Feature | Android webview | Chrome for Android | Edge mobile | Firefox for Android | IE mobile | Opera Android | iOS Safari |
---|---|---|---|---|---|---|---|
Basic support | 551 | 551 | ? | 53 | No | 42 | ? |
Parameters accepted as a single object, as well as being passed in individually | 62 | 62 | ? | 57 | No | 49 | No |
1. Before Chrome 59, the default values were not supported.