Free · no login · nothing uploaded
Mono to stereo converter.
You cannot add width that was never there.
What you can do is put the same signal in both channels, which every player treats as stereo and which loses nothing at all. That is the default here. The widening trick is offered as well, because it genuinely works on headphones — and the tool measures exactly how much low end it costs you on a system.
Drop mono files to convert
WAV, AIFF, FLAC, MP3 or M4A. Drop a whole folder’s worth — one set of settings is applied to all of them and they come back as a single archive. Nothing is uploaded; the page makes no network request.
What the category sells you
Widening is not recovery.
Every mono-to-stereo converter on the web implies it is putting something back. Nothing is being put back.
A mono recording contains one signal. There is no second channel that got lost, no hidden width waiting to be extracted — the microphone or the bounce captured one stream of samples and that is the entirety of what exists. Any tool offering to turn it into stereo is either copying it, which is honest, or generating a second version of it that differs somehow, which is invention. Both are legitimate; only one of them is what the marketing implies.
The invention usually takes one of two forms. A delay, which is what is offered here, produces the most convincing result and comb-filters the mono sum. An all-pass or phase-rotation approach avoids some of the comb filtering and produces a vaguer, less convincing width. Neither adds information; both trade mono compatibility for an impression. On a podcast or a headphone-only listen, that trade is often worth making. On something going to a club, it very rarely is.
So this tool defaults to duplication and puts widening behind a deliberate choice, with the mono-sum cost measured on the output. If you widen a file and the report says it loses two decibels when summed, that is two decibels a sound system will take out — and you now know before it happens rather than afterwards.
Duplication is lossless, and is almost always what you want
Copying the mono signal into two channels produces a file that every piece of software and hardware treats as stereo, that sounds exactly like the original, and that sums back to mono with nothing lost. It fixes the actual problem people arrive with, which is a piece of software refusing a mono file or an upload spec demanding two channels. It is not a compromise — for a genuinely mono recording it is the correct answer and there is no better one.
Widening is a trick, and it is worth knowing what kind
Delay one channel by a few milliseconds and the ear reads the pair as wide. That is the Haas effect and it is real psychoacoustics rather than snake oil — under about 30 milliseconds the brain fuses the two into one sound with a position rather than hearing an echo. On headphones it is convincing. It is also inventing spatial information that was never in the recording, so what you get is an impression of width rather than a stereo image, and it behaves badly the moment anything sums it.
And the cost is measured rather than described
A delay is a comb filter once the two channels are added together: certain frequencies reinforce and others cancel, spaced evenly up the spectrum. A club rig sums everything under its crossover unconditionally, so a widened file arrives with notches cut through its bass that were not there before. The tool measures exactly how many decibels the sum costs and prints it next to the result, so the trade is a number you can look at rather than a warning you can ignore.
The two methods
Duplicate or widen
One is lossless and reversible. The other is a trade, and the trade is specific.
| Method | What it does | What it costs |
|---|---|---|
| Duplicate | The identical signal in both channels. | Nothing. It sums perfectly back to mono, sounds exactly like the original, and doubles the file size. This is the right answer for almost every reason people come here. |
| Haas, 5 ms | One channel delayed by five milliseconds. | A subtle sense of width, and mild comb filtering when summed. The gentlest version of the trick, and the safest if you are going to use it at all. |
| Haas, 12 ms | The usual setting. | Clearly wide on headphones. The comb filtering is now real and the mono sum measurably loses energy. Fine for something that will never see a system. |
| Haas, 20 ms | Strongly wide. | Obvious width, and the image is starting to feel detached from the centre. Mono compatibility is meaningfully damaged. |
| Haas, 30 ms | The edge of fusion. | Around here the ear starts hearing an echo rather than a position, and different listeners cross that line at different points. Rarely the right choice. |
If you drop a file that is already stereo, the tool says so rather than doing something pointless to it. To go the other way, use the stereo to mono converter — which measures what the sum costs, and is worth running on any master before it goes to a club.
Frequently asked
Common questions
Can I convert a whole folder at once?+
Yes. Drop as many mono files as you like, pick the method once, and they come back as a single archive. Anything already stereo in the queue is reported as such rather than being pointlessly processed.
Does my file get uploaded?+
No. Decoding, converting and writing all happen inside your browser tab. The page makes no network request once it has loaded.
Which method should I pick?+
Duplicate, unless you have a specific reason not to. It is lossless, it produces a file every player accepts as stereo, and it costs nothing. Reach for widening only when the material is going somewhere that will never sum it — a headphone mix, a podcast, a video — and even then start at five milliseconds.
Why does my software reject a mono file?+
Some tools, upload specs and hardware simply require two channels and refuse anything else rather than handling it. That is the most common reason people need this conversion and duplication solves it completely — the resulting file is genuinely stereo as far as anything downstream is concerned, and the audio is bit-for-bit the original in both channels.
Does duplicating to stereo hurt quality?+
Not in any way. Both channels carry exactly the same samples as the mono original, so nothing is filtered, delayed or recalculated. The file is twice the size and that is the whole difference.
What is the Haas effect?+
Below about 30 milliseconds, the ear does not hear a delayed copy of a sound as a separate event — it fuses the two and uses the delay to decide where the sound is coming from. So delaying one channel makes the pair read as wide rather than as an echo. It is genuine psychoacoustics and it works. What it does not do is create real stereo information, and it stops working the moment anything sums the two channels back together.
Why does widening cost me bass?+
Because a delay becomes a comb filter when summed. Adding a signal to a delayed copy of itself makes some frequencies reinforce and others cancel, evenly spaced up the spectrum, with the spacing set by the delay time. A club system sums everything below its crossover regardless of what you want, so those notches land in your low end. The tool measures the total mono-sum loss on the widened output so you can see the size of it.
Can I undo the widening later?+
Not cleanly. You can throw one channel away and get back to mono, which discards the delayed copy, but you cannot recover the file as though nothing happened if you have already re-encoded or processed it further. Keep the mono original.
Is a duplicated file the same as a dual-mono file?+
Yes — dual mono is exactly this: a stereo container carrying two identical channels. Some software distinguishes them and most does not. If you ever want to go back, the stereo to mono converter will detect that the channels are identical and tell you nothing is being lost.
What about old mono recordings?+
Duplicate them and leave them alone. A mono recording from before stereo existed is a complete document of what was captured, and widening it invents a spatial impression the performance never had. If you want it to sit differently in a mix, do that in the mix with reverb and placement rather than baking a comb filter into the file.
Explore
Stereo to mono · Audio quality checker · Audio normalizer · Spectrogram analyzer · Waveform generator · Mono sum and phase · Sub bass · All tools
Build a set from tracks that fit
DJ terms on this page: Mono sum and phase · Sub-bass · Waveform · Headroom · WAV and AIFF · Sample rate · Soundcheck · Ground loop · Full glossary