Stereo → mono
Stereo to mono converter
The left and right channels are summed and halved — (L + R) ÷ 2 — producing one channel at the same perceived level. The file becomes about half the size.
…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
Mono is the right answer more often than people expect. A single-microphone interview recorded to a stereo file is carrying two copies of the same signal. A podcast played on a phone speaker is mono at the point of playback anyway. A voice recording where one channel was dead is unusable in stereo and fine in mono. And in every case the file is half the size.
The downmix is a straightforward average, which preserves perceived loudness without clipping. Sonic Batch also reports when the two channels are nearly identical (so the downmix is essentially free) and when they differ substantially (so you should listen before committing).
Does this change the audio quality?
Averaging two channels is lossless arithmetic on the PCM, but it is not reversible: once summed, the separate channels cannot be recovered. Any content that was out of phase between left and right will partially or completely cancel.
- Out-of-phase content cancels. Some “wide” stereo effects and mid-side recordings lose material entirely when summed — always listen to the result.
- The conversion cannot be undone. Keep the stereo original.
- If only one channel has usable audio, a downmix halves its level. Check the per-channel levels in the file panel first.
A worked example
Halving the size of an interview archive
- Drop the stereo recordings in. The file panel flags files whose channels are near-identical.
- Keep the default: Channels set to Mono, MP3 at 128 kbps.
- Optionally turn on LUFS normalisation at −16 to level the set at the same time.
- Convert all and download the ZIP — the manifest records the before and after sizes.
Check the channel correlation before you commit
If the file panel shows the channels are effectively identical, the downmix costs you nothing and halves your storage — do it. If they differ, listen to the mono result before deleting the original, particularly with music: phase-cancelled reverb and stereo-widened synths can disappear. For speech recorded on a single microphone, mono is always correct.
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 |
|---|---|---|---|---|
| MP3 | libmp3lame | 32–320 kbps | 8–48 kHz | Yes + artwork |
| WAV | pcm_s16le | 16, 24, 32-bit | 8–96 kHz | Yes |
| FLAC | flac | 16, 24-bit | 8–96 kHz | Yes + artwork |
| M4A | aac | 32–320 kbps | 8–48 kHz | Yes + artwork |
| Opus | libopus | 16–256 kbps | 8–48 kHz | Yes |
| OGG | libvorbis | 32–320 kbps | 8–96 kHz | Yes |
| 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
Part of the audio vanished after downmixing.
That is phase cancellation: content present in one channel and inverted in the other sums to silence. It is a property of the source, not a bug. Keep the stereo file for that material.
The mono file is quieter than the stereo one.
If the audio was only in one channel, averaging halves it (−6 dB). Turn on normalisation to bring it back up, or use a tool that selects a single channel rather than averaging.
The file size did not halve.
For lossy formats the encoder was already exploiting the similarity between channels through joint stereo, so a mono encode saves less than 50%. For WAV and FLAC the saving is close to half.
Frequently asked questions
Does converting stereo to mono lose quality?
It loses the stereo image, which is information you may or may not need. The remaining single channel is not degraded. For single-source recordings there is nothing to lose.
How much smaller will the file be?
About half for WAV and FLAC. For MP3, AAC and Opus, typically 30–45% — those codecs already share data between channels.
Can I convert mono back to stereo?
You can duplicate the channel, which is what the mono to stereo page does, but the original separation is gone permanently.
Is the processing done locally?
Yes. The downmix happens on decoded samples inside your browser, and the encode too.