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Stereo to mono converter.
Sum it, and find out what disappeared.
Every stereo-to-mono converter adds the channels together and hands you a file. This one measures what that removed — overall and, more importantly, in the sub band. That second number answers a question you may not have asked: a club system sums your low end to mono whether you do or not.
Drop stereo files to sum
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.
The thing headphones hide
Your low end may already be gone.
A phase problem in the sub is inaudible on headphones, decisive on a system, and completely measurable.
On headphones each ear hears one channel and nothing ever sums, so a track whose kick and sub are out of phase between the left and right sounds enormous — arguably better than it should, because the width reads as size. Play the same file on a rig and the low end is thin or simply absent, because the subwoofers received the sum of two things that cancel.
This is one of the few genuinely catastrophic mix problems that gives you no warning at all in the place most people work. It survives every check you would normally do. It survives a peak meter, it survives a spectrum analyser on the stereo file, and it survives every listening session that does not happen on a large system. The first time you find out is when a room does not move.
Running your master through this tool answers it in a few seconds: the sub row is the amount of low end the mono sum destroys, and a club rig performs that sum for you. Anything worse than about a decibel down there is worth going back into the mix for. The fix is almost always the same one — make everything below the crossover mono in the arrangement, keep the width for the parts that live above it.
Summing is not free, and how much it costs varies enormously
When two channels become one, anything they share adds up and anything they disagree about partly cancels. On a narrow mix that costs a fraction of a decibel and nobody would notice. On a wide, heavily processed master it can take several decibels out — and it does not take them evenly, it takes them from wherever the stereo information lives. So the honest version of this tool tells you the number rather than handing you a file and letting you find out by ear.
The sub figure is the one that matters, for a different reason
A large sound system feeds its subwoofers a mono sum by design. The wavelength at 60 Hz is close to six metres, which is wider than the space between the audience and the speakers, so a stereo image down there is physically impossible and every rig sums it. That means the sub-band loss measured here is not just what this conversion costs — it is what the room already does to your track. A bad number means the low end is not reaching the dancefloor on a big system, and you would never hear it on headphones.
Level is preserved rather than naively added
Simply adding two channels makes a correlated signal about 6 dB hotter, which clips instantly on anything modern. The sum here is averaged, so the result sits at the same level the original did and can be dropped straight into a set next to the stereo version without a gain change. If the two channels turn out to be sample-identical — a mono file in a stereo container, which is more common than you would think — the tool says so, because in that case nothing whatsoever was lost.
Reading the numbers
What the correlation and the loss are telling you
Correlation is how much the two channels agree. Loss is what that agreement is worth in decibels.
| Reading | What it means | What to do |
|---|---|---|
| Correlation near +1 | The channels are nearly identical. The mix is narrow, or the material is mono in a stereo file. | Nothing. Summing costs you almost nothing and mono compatibility is perfect. |
| Correlation 0.6 to 0.9 | A normal, moderately wide mix. | Nothing. This is where most well-made records sit and a sum takes a fraction of a decibel. |
| Correlation 0.2 to 0.6 | A wide mix with a lot of decorrelated content. | Fine above the crossover, worth checking below it. Look at the sub row rather than the overall number. |
| Correlation below 0 | The channels are actively fighting. Something is out of phase. | Investigate. A polarity-flipped layer, a stereo widener pushed too far, or a sample that was already broken. |
| Sub loss worse than -1 dB | A meaningful amount of the low end vanishes when summed. | Go back to the mix. Make everything under about 120 Hz mono. This is what a club system will do to it regardless. |
| Sub loss worse than -3 dB | Half the low end disappears on a system. | This is a serious problem and it is the single most common reason a track that sounds huge on headphones does nothing in a room. |
If the tool reports the file was already dual mono — two channels carrying identical samples — nothing was lost and the conversion is purely a container change. That is worth doing anyway: it halves the file size and removes any ambiguity for software that treats the two cases differently.
Frequently asked
Common questions
Can I check a whole folder at once?+
Yes, and used that way it stops being a converter and becomes a survey. Drop a night's worth of tracks and every one comes back with its own mono-sum and sub-band loss, so you can see at a glance which records in your set are going to lose low end on a big system. The summed files come back as one archive; the numbers are the point.
Does my file get uploaded?+
No. Decoding, summing, measuring and writing all happen in your browser tab, and the page makes no network request once loaded. An unreleased master is safe to drop on it.
Why would I want a mono file at all?+
A few real reasons. Some venue and broadcast systems want mono delivery. Bass and sub layers in a production are usually made mono deliberately. Voice recordings and interview material are mono by nature and keeping them stereo doubles the file for nothing. And halving the size of a large file matters when a USB stick is full. But the most common reason people arrive here is the diagnostic one — finding out what a sum costs.
What does the correlation number mean?+
It measures how much the two channels agree, from +1 to -1. At +1 they are identical, so summing them changes nothing. At 0 they are completely unrelated, and summing loses roughly 3 dB of the decorrelated part. Below 0 they are actively opposed and summing cancels them — at exactly -1 the file would sum to silence. Most well-made stereo records sit between 0.4 and 0.9.
Why is the sub band measured separately?+
Because it is the only band where mono compatibility is not optional. A large sound system feeds its subs a mono sum — the wavelengths are longer than the distances involved, so a stereo image down there cannot physically exist. Whatever the sub row says is lost is lost in every club you play, whether or not you ever convert the file. Width in the top two bands is a creative choice; width in the sub is almost never one.
Will summing to mono make my track quieter?+
Slightly, by the amount shown in the report, and it varies with the material. The channels are averaged rather than added so the overall level is preserved — what changes is that decorrelated content partly cancels, which is a genuine loss of energy rather than a gain change. If you need the result at a specific level afterwards, run it through the normaliser.
Can I get the stereo back afterwards?+
No. Summing is a one-way operation — once the two channels are one, the information about what differed between them is gone. Keep your original. Anything that claims to turn mono back into stereo is inventing width rather than recovering it.
My file says it is already dual mono. What does that mean?+
Both channels contain byte-identical samples, so it is a mono recording stored in a stereo container. It happens with older transfers, some exports, and a lot of sample packs. Converting it properly to mono loses literally nothing and halves the file size.
How do I fix a bad sub-band number?+
In the mix, not in the file. The usual causes are a stereo widener applied across the whole master, a sub layered with a detuned or delayed copy of itself, or a sample that arrived already broken. The standard fix is to make everything below your crossover — usually around 120 Hz — mono in the arrangement, which most mixing chains have a control for. Keep the stereo information for the parts that live above it, where a system can actually reproduce it.
Is this the same thing the audio quality checker does?+
The quality checker measures the same property across four bands and reports it as a finding among several, without changing your file. This tool measures it and then gives you the summed file. Reach for the checker when you want to know; reach for this when you want the mono version too.
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Audio quality checker · Mono to stereo · LUFS meter · Audio normalizer · Spectrogram analyzer · Mono sum and phase · Sub bass · All tools
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