Mono → stereo
Mono to stereo converter
The single channel is written to both the left and right outputs. This produces a valid stereo file, but the two channels are identical: there is no stereo separation, because there is none in the source.
…or drag files and folders here. Add as many as you like — everything is processed on this device.
.3g2 .3gp .aac .aif .aifc .aiff .amr .caf .flac .m4a .m4r .m4v .mkv .mov .mp3 .mp4 .oga .ogg .opus .qt .wav .wave .weba .webm
In one paragraph
This conversion exists for compatibility, not for sound. Some video editors, broadcast systems, CD mastering tools, DAW templates and hardware players require a two-channel file and reject mono outright. Duplicating the channel satisfies them.
What it does not do — and what a lot of tools imply — is create a stereo image. Stereo separation comes from two microphones or a mix decision. It cannot be derived from a single channel. Anything that widens, delays or phase-shifts one side to fake it is adding an effect, usually one that sounds hollow on headphones and collapses when summed back to mono. Sonic Batch offers a plain duplicate and a power-matched duplicate, and nothing that pretends otherwise.
Does this change the audio quality?
The audio is unchanged. Duplicate mode copies the samples to both channels verbatim. Power-matched mode applies −3.01 dB to each side so the summed energy matches the mono original, which keeps the perceived level identical on mono playback systems.
- No process can create real stereo separation from a mono source. This duplicates; it does not widen.
- The file will be roughly twice the size in WAV or FLAC, and modestly larger in lossy formats.
- If the destination accepts mono, converting is pure overhead — check first.
A worked example
Meeting a broadcast delivery spec that requires two channels
- Drop the mono files in.
- Choose WAV output, 24-bit, and Power-matched duplicate so the level matches the mono master.
- Confirm Channels is set to Stereo.
- Convert all and download the ZIP for delivery.
Plain duplicate for players, power-matched for delivery specs
Plain duplicate is right when something simply needs two channels and will play them back as-is — video editors, DAW imports, hardware players. Power-matched (−3.01 dB per side) is right when a delivery spec measures loudness, because a plain duplicate raises the summed level by 3 dB on mono-fold-down systems. If you do not know which, plain duplicate is the safer default.
Supported formats, limits and browser notes
Sonic Batch decodes with the browser when it can (faster, no engine download) and falls back to the self-hosted WebAssembly engine when it cannot. Every combination below works in every listed browser; the column only tells you which decoder does the work.
| Input | What it is | Chrome | Firefox | Safari | iOS |
|---|---|---|---|---|---|
| MP3 | The universal lossy format. Plays on essentially every device made since 1998. | Native | Native | Native | Native |
| WAV | Raw PCM samples with a small header. Perfect fidelity, roughly 10 MB per stereo minute. | Native | Native | Native | Native |
| M4A | Apple’s default AAC container. Better quality than MP3 at the same bitrate. | Native | Native | Native | Native |
| AAC | Bare AAC frames with no MP4 wrapper — used by broadcast streams and some hardware. | Native | Partial | Native | Native |
| OGG | Royalty-free lossy codec. Standard in game engines, Godot, Unity and Wikimedia. | Native | Native | Engine | Engine |
| Opus | The best-sounding lossy codec below 128 kbps. Used by WhatsApp, Discord and YouTube. | Native | Native | Partial | Partial |
| FLAC | Lossless compression, typically 50–60% of WAV size, bit-identical when decoded. | Native | Native | Native | Native |
| AIFF | Apple’s uncompressed PCM format — the WAV equivalent for Logic and Pro Tools. | Partial | Partial | Native | Native |
| M4R | An M4A file with a .m4r extension. iOS only lists it as a ringtone if it is ≤ 40 seconds. | Partial | Partial | Native | Native |
| AMR | Phone-call speech codec: 8 kHz mono, 4.75–12.2 kbps. Decode only — nothing should be created in it. | Engine | Engine | Engine | Engine |
| CAF | Apple container used by older iOS Voice Memos. Usually holds ALAC, IMA4 or raw PCM. | Engine | Engine | Partial | Partial |
| 3GP | Legacy phone recording container, normally AMR-NB or low-bitrate AAC audio. | Engine | Engine | Partial | Partial |
| MOV | QuickTime video from iPhone and DSLR cameras. Audio is normally AAC or PCM. | Partial | Engine | Native | Native |
| MP4 | The standard video container. Its audio track is almost always AAC. | Native | Partial | Native | Native |
| WebM | Open container used by browser recorders and YouTube downloads. Audio is Opus or Vorbis. | Native | Native | Partial | Partial |
| Output | Encoder | Bitrates / depth | Sample rates | Tags |
|---|---|---|---|---|
| WAV | pcm_s16le | 16, 24, 32-bit | 8–96 kHz | Yes |
| MP3 | libmp3lame | 32–320 kbps | 8–48 kHz | Yes + artwork |
| FLAC | flac | 16, 24-bit | 8–96 kHz | Yes + artwork |
| M4A | aac | 32–320 kbps | 8–48 kHz | Yes + artwork |
| AIFF | pcm_s16be | 16, 24-bit | 8–96 kHz | Yes |
- Max per file
- 400.0 MB
- Max per queue
- 4.00 GB
- Max duration
- 4:00:00
- Max archive
- 1.50 GB
These are enforced ceilings, checked before any decoding starts, not measured maximums — a file over the limit is rejected rather than allowed to exhaust the tab’s memory. Real-world capacity is lower on older phones, which is why the scheduler runs one file at a time on iOS and Safari.
Where your audio goes
Nowhere. Your file is read from disk into this browser tab with the File API, decoded and encoded by code running in that same tab, and handed back to you as a download. It is never transmitted.
That is a structural property of the application, not a policy promise: there are no API routes, no server actions and no third-party processing services in this project, so there is nothing capable of receiving a file. The page’s Content-Security-Policy also restricts connections to this origin, and the automated test suite includes a browser test that fails if any network request during import, preview, conversion or download carries media bytes.
The trade-off, stated plainly:results live in your tab’s memory until you download them. Closing or reloading the page discards them. There is nothing on a server to recover, because nothing was ever put there.
Troubleshooting
It sounds exactly the same as the mono file.
It should. Both channels contain the same signal. This conversion is about container and channel-count compatibility, not sound.
The level changed after conversion.
Power-matched mode intentionally applies −3.01 dB per channel. Switch to plain duplicate if you want the sample values preserved exactly.
Can I get a wider sound?
Not honestly from a mono source. Widening effects add artificial delay or phase differences and generally make things worse, so this tool does not offer them.
Frequently asked questions
Can mono be converted to real stereo?
No. Stereo separation is recorded, not computed. Duplicating the channel makes a two-channel file; it does not create a stereo image.
Why would I need this then?
Because a lot of software and hardware requires two channels: broadcast delivery specs, some video editors, CD mastering tools and certain hardware players.
Which layout should I pick?
Plain duplicate for general compatibility. Power-matched when a loudness specification will be measured on the result.
Does the file get bigger?
Roughly double in WAV and FLAC. In MP3, AAC and Opus, less than double, because those codecs recognise that the channels are identical.