Turn a mono recording into a 2-channel stereo file. Duplicate the channel for a clean dual-mono file, or create real width with mono-safe comb widening or a Haas delay. A live goniometer and mono fold-down check show how the result will sound on any speaker.
Last updated:
Stereo method
Preview & mono check: demo clip
LMR
−1
+1
The preview uses the first 20 seconds of the selected file. Mono fold-down plays (L + R) ÷ 2, which is what a phone speaker, smart speaker or mono PA plays.
Advertisement
Advertisement
How to use the Mono to Stereo Converter
Pick a stereo method: Duplicate for a plain 2-channel copy, Comb widening for mono-safe width, or Haas delay for a wide, leaning image.
Set the width and delay while listening to the built-in demo clip, or add your own file and press Preview.
Check mono compatibility: compare Play stereo with Play mono fold-down and watch the correlation meter stay on the right half.
Choose WAV or MP3 and press Convert to stereo. All files in the list are processed with the same settings.
Download each stereo file or press Download all (ZIP).
Which stereo method should you use?
Method
How it works
Mono fold-down
Best for
Duplicate (dual mono)
L = R = mono
Identical to the original
Platforms or editors that only accept stereo files, voice-overs, podcasts
Comb widening (M/S)
L = M + S, R = M − S, with S = width × mono delayed by a few ms
Identical to the original (the side signal cancels)
Mono music stems, synths, guitars and ambience that should feel wider but must stay safe on phones and mono speakers
Haas delay
One channel is delayed by 1 to 40 ms
Comb filtering: notches at odd multiples of 1 ÷ (2 × delay)
Strong, wide effects for headphones and stereo speakers; check it in mono
Reading the mono-compatibility check
The correlation is r = Σ(L·R) ÷ √(ΣL² · ΣR²). Dual mono gives +1; comb widening at 60% width typically lands between +0.4 and +0.6. The side / mid energy compares S = (L − R) ÷ 2 with M = (L + R) ÷ 2 in decibels: −20 dB is subtle, −6 dB is wide, 0 dB is very wide.
The mono fold-down level compares the mono sum with the average level of one stereo channel. With comb widening the mono sum is the untouched original, so it is only slightly lower than the widened channels. With a Haas delay of d seconds the mono sum has its first notch at 1 ÷ (2d): a 10 ms delay puts notches at 50 Hz, 150 Hz, 250 Hz and so on. At 100% width the delayed channel contains only the delayed copy, so those notches cancel completely, which is why lower widths are safer.
Tips for natural-sounding width
Start with comb widening at 40 to 70% and a delay of 6 to 12 ms; go lower if the sound becomes metallic.
For the Haas method stay between about 5 and 30 ms. Shorter delays mostly shift the image to one side, longer ones turn into an audible echo.
Leave speech centred: for dialogue, podcasts and audiobooks use Duplicate.
Keep bass in mono at 120 to 200 Hz for music that will play on mono speakers, phones or in clubs.
Always compare Play stereo and Play mono fold-down before you export.
Frequently asked questions
Can you really turn mono into stereo?
Not into a true stereo recording: a mono file holds one signal, so the original positions of instruments cannot be recovered. What you can do is create pseudo-stereo, where small differences between the left and right channels (a short delay or complementary filtering) make the sound feel wider and less centred.
What is the difference between dual mono and pseudo-stereo?
Dual mono copies the same signal to both channels, so the file is technically stereo but sounds exactly like the mono original, centred between the speakers. Pseudo-stereo makes the two channels slightly different so the sound spreads across the stereo field.
What is the Haas effect?
When the same sound reaches the two ears with a delay of roughly 1 to 30 milliseconds, the brain hears one sound coming from the side that arrives first, and the image feels wider. This is called the precedence or Haas effect. With longer delays the copy starts to be heard as a separate echo.
Why is comb widening mono-compatible?
Comb widening adds a delayed copy of the signal as a side signal: left = mid + side and right = mid − side. When a player folds stereo to mono it computes (left + right) ÷ 2, the side signal cancels exactly, and the original mono sound comes back without any filtering.
What does the correlation meter show?
Correlation compares the two channels from −1 to +1. Identical channels read +1. Values between about +0.3 and +0.9 mean a wide but mono-compatible image. Values near 0 or below mean the channels are very different or out of phase, which can sound thin or hollow when played in mono.
Why keep the bass in mono?
Low frequencies carry most of the energy and are hard to localise. Widening them adds little but can make the bass weaker or uneven in mono and on vinyl or club systems, so the tool can leave everything below 120, 200 or 350 Hz identical in both channels.
Are my files uploaded?
No. Decoding, widening and encoding all happen in your browser with the Web Audio API and a JavaScript MP3 encoder, so your audio never leaves your device.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.