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A guest who is only in your left ear, a stereo file that is really mono, a recording with left and right reversed: all of these are channel problems. Load the file, check its channel levels, hear the fix in a live preview and export it, all without uploading anything.
Processing locally on your device
Overview
Stereo audio has two channels, left and right; mono has one. Stereo-to-mono conversion, or downmixing, normally averages the two as (L + R) ÷ 2, which this tool does. When only one side has useful sound, for example an interview where each microphone was recorded to its own channel and one side is empty or noisy, you can instead keep only the left or only the right channel. The result is a mono file that plays in both ears on every device.
Before you choose, the channel check measures each side’s level and the correlation between them. A correlation near +1 means the sides are almost identical, so a stereo file can become mono with no loss. If the tool finds left and right are exactly identical, it tells you the file is mono stored as stereo. Values near 0 mean wide or unrelated sides, and negative values mean the channels are out of phase: averaging them would cancel bass and vocals. In that case invert one channel’s polarity first, then mix. A side more than 40 dB quieter than the other is flagged as silent.
Mono to stereo copies the single channel to both sides. That is what some platforms and editors require, and it plays centered everywhere. An optional pseudo-stereo control delays one side by up to 30 ms. This uses the Haas effect, where the ear perceives a very short delay as width rather than echo. Use it sparingly: it can sound hollow when the stereo version is later played back in mono. Swap left/right fixes recordings with crossed cables or an upside-down camera, and split saves each side as its own mono file for separate editing.
All of this happens with the Web Audio API, using the same channel splitter, merger, gain and delay nodes for the live preview and the export, so what you hear is exactly what you download. Rendering takes seconds, and WAV export needs no extra download. MP3, M4A, OGG, Opus and FLAC are encoded with a cached 31 MB FFmpeg engine. The whole file is decoded into memory, so recordings longer than about 30 minutes may be slow on phones.
| Problem | Operation | Output |
|---|---|---|
| Voice only in one ear | Mono from left or right only | Mono |
| Stereo speech file too big | Stereo → mono (mix) | Mono |
| Platform requires stereo | Mono → stereo | Stereo |
| Left and right reversed | Swap left / right | Stereo |
| Mono mix sounds thin | Invert one channel’s phase | Stereo |
| Need each mic separately | Split into two files | Two mono files |
Open an audio file, or a video to work on its soundtrack.
Check the channel levels and correlation in the sidebar.
Choose an operation: mix to mono, left or right only, mono to stereo, swap, invert phase or split.
Press Preview to hear the result, and adjust the width or the inverted side if needed.
Pick an output format and export the file (two files when splitting).
Use cases
Recorders and USB interfaces often put each microphone on its own channel. Keep only the side with the voice, or mix both, so listeners hear everyone in both ears instead of one.
Speech recorded with a single microphone gains nothing from two channels. Converting to mono halves the size of a WAV and lets lossy formats spend all their bits on one channel.
Swap channels when sounds appear on the wrong side of the video, or invert one channel when the channel check shows a negative correlation and the mono mix sounds thin.
Split a two-microphone recording into separate left and right files, so each speaker can be cleaned up, leveled or muted on its own track in an editor.
Pro tips
Answers to common questions about the mono ↔ stereo.
You lose the left-right placement of sounds, but not tonal quality. For speech recorded with one microphone nothing meaningful is lost, and the file gets smaller.
Parts of the signal are out of phase between the channels and cancel when averaged. Stereo widening effects and miswired microphones cause this. The channel check shows it as a low or negative correlation; inverting one side before mixing often fixes it.
No. It delays one side by a few milliseconds so the brain perceives width. It can make a mono recording feel more spacious on headphones, but it is not a true stereo recording and may sound slightly hollow if summed back to mono.
Two mono files, named with -left and -right, each containing one side of the original. They are downloaded together as a ZIP.
You can load a video and the tool works on its soundtrack, exporting an audio file. To put the fixed audio back into the video, use the Add Music tool afterwards.
No. ToolEnhancer processes files on your own device inside the browser. They are never sent to, stored on or seen by our servers.
Yes. The mono ↔ stereo is free to use with no sign-up, no watermarks and no daily limits.