What do the four coefficients mean?

They are the standard two by two mixing matrix. The first is the left channel's contribution to output left, the second is the right channel's contribution to output left, the third is the left channel's contribution to output right and the fourth is the right channel's contribution to output right. Reading them that way, both the formulas in the footer and every preset follow directly.

Why does the mono preset average instead of summing?

Because a plain sum of two loud channels clips easily. Summing with a half coefficient on each gives the average of the two, which keeps the mono version at a sensible level. This is verified behaviour: the preset averages the channels, and it is why a downmix of loud material does not come back distorted.

What is the difference preset used for?

It writes the left channel minus the right channel into both outputs, which cancels everything panned dead centre. That is the classic way to remove a centred vocal or to hear only the ambient and side content of a mix. It also removes centred drums and bass, so treat the result as an analysis rather than a finished master.

Can I boost a quiet side with the matrix?

Yes, up to a point. A coefficient above one raises that contribution, and the clamp allows values up to two. The limit is headroom rather than the matrix, since the tool has no metering and a boosted sum can exceed full scale silently. If you only need to bring a quiet side up, the balance tool is a much safer way to do it.

Does a mono file work with this tool?

It loads and processes, but there is no second channel for the matrix to mix with, so the right hand coefficients have nothing to act on. For anything that is genuinely mono, the mono to stereo converter is the appropriate tool instead, since it creates a second channel rather than combining two.

Channel Mixer

You Might Also Need

Left/Right Channel Swapper

Swap left and right channels

Mid/Side Processor

Process mid and side separately

Panning Tool

Adjust stereo pan position

Mono-to-Stereo Converter

Convert mono audio to stereo

Channel Mixer applies an arbitrary two by two matrix to a stereo file, so any left and right combination you can describe in four numbers becomes the output. Six presets cover the everyday cases and the coefficient boxes take you past them.

What is Channel Mixer?

Drop a stereo file, choose a preset or type your own four coefficients, and the whole file is mixed and returned as a WAV. The footer spells out the arithmetic before it runs, so there is no guessing about which coefficient feeds which output.

What the presets do:

Identity, which passes left to left and right to right with no change at all.
Swap L/R, which exchanges the two channels exactly.
Mono averaging, which sums the two channels with a half coefficient into a mono downmix written to both outputs.
Left to stereo and right to stereo, which duplicate one side across the pair.
Difference, which writes left minus right into both outputs.

The four coefficient boxes are labelled a for left to left, b for right to left, c for left to right and d for right to right, and each accepts a number between minus two and two in steps of one tenth. Output left is a times left plus b times right, and output right is c times left plus d times right, which is the standard two by two channel matrix and covers everything from a plain swap to a difference signal. Verified behaviour is that the identity matrix passes the signal through untouched, that the swap is exact, and that the mono preset averages the channels rather than summing them, so a mono downmix of two loud channels does not clip the way a plain sum would.

Where the matrix runs out:

It works on two channels only, so a mono input has nothing to combine with on the other side.
Coefficients are clamped between minus two and two, ruling out very large gain or near total attenuation in a single step.
There is no metering, so positive coefficients can push the output past full scale with nothing to warn you.
The difference preset cancels anything panned dead centre, which removes a centred vocal but takes a centred kick, bass and snare with it.
The matrix is static for the whole file, with no automation and no way to change coefficients part way through.
Nothing analyses the recording, so the tool cannot suggest which preset suits the material you loaded.
The output is always a stereo WAV, even when both outputs end up carrying identical content.

The matrix is the right frame for problems about how the two channels relate rather than about what each one contains. A compatibility check before publishing is best done with the averaging preset rather than by ear on one speaker, and a side only listen is simply the difference preset played back. Because the footer shows the arithmetic before the pass runs, you can reason about the result instead of guessing at it, which is more than most one click stereo fixes allow. Reach for it when you know the relationship you want between the two channels rather than merely wanting to correct one side; if the problem is simply that one side is louder than the other, the balance tool beside it does less and is much harder to get wrong.

How to use Channel Mixer

Drop a stereo audio file onto the panel; mono files have nothing for the matrix to combine.
Press one of the six presets to load its matrix, watching the footer show the resulting arithmetic.
Type your own a, b, c and d coefficients between minus two and two if none of the presets fits.
Download the mixed WAV, which is named after the source with a mixed suffix.

When to use Channel Mixer vs related tools

Channel Mixer is the general tool for redefining how left and right relate, so it covers every case between exchange and combination. Left/Right Channel Swapper is the one job version when a capture is simply flipped, and Channel Isolator discards a side entirely rather than mixing it. When the aim is to control how much centre and how much difference the file contains, Mid/Side Processor expresses the same arithmetic in friendlier terms, and Panning Tool is the choice when the image needs to move rather than be rebuilt.

Privacy & Security

This tool runs entirely in your browser — no data ever leaves your device. There is no server round-trip, no upload, no logging, and no account required. Your input is processed locally using client-side JavaScript and is never stored, transmitted, or accessible to anyone else. When you close the tab, everything disappears.