How to use the mono and stereo converter
- Add the audio file: MP3, M4A, WAV, OGG, FLAC, WebM, or the audio of a video.
- Under Convert to, choose Mono or Stereo.
- For stereo, set Balance (left −100 … right +100). 0 keeps the sound centred.
- Pick Save as and, for lossy formats, a Bitrate. Press Convert and download the file.
What it does and when to use it
Common jobs:
- A voice in one ear only. Some lapel or camera microphone setups record onto one channel only. Converting to mono puts the voice in both ears. If the other channel is pure noise, it is better to keep just the good one with the audio channel splitter.
- Smaller files for speech. A mono WAV is half the size of the same stereo WAV.
- Checking a mix. Phone speakers and smart speakers often play in mono. A mono version reveals parts that vanish when the channels are combined.
- Meeting a stereo requirement. Some editors or upload forms expect two channels; Stereo mode duplicates a mono track.
To change format or sample rate at the same time, the audio converter has a Channels option as well.
How it works
After decoding in a Web Worker on your device:
- Mono: every output sample is the average of all input channels, (L + R) ÷ 2 for stereo. The tool measures the RMS level of both versions; if they differ by more than 3 dB, a phase warning appears.
- Stereo from mono: the balance b (−1 to +1) sets a pan angle a = (b + 1) × 45°. Left gain = cos a × √2 and right gain = sin a × √2, each capped at 1. In the centre both are exactly 1, so the voice keeps its original level in each speaker.
- Stereo from stereo: the channels are kept and the balance turns the opposite side down in a straight line.
Worked examples
From the tool’s output on 5-second test files:
- Stereo interview, identical left and right → Mono: Channels 1 (mono), File size 430.7 KB as WAV.
- Stereo file with the right channel inverted → Mono: the channels cancel, and the result warns that the mono mix is noticeably quieter because of out-of-phase content.
- Mono voice → Stereo, Balance −30: Channels 2 (stereo), File size 861.4 KB. By the formula above the left channel stays at full level and the right is about 2.6 dB quieter.
Limits and tips
- Files with more than two channels (5.1) are averaged into mono equally. Stereo mode keeps only the first two channels.
- Mono mode on a file that is already mono simply reports “Already mono”.
- Formats other than WAV are decoded with WebCodecs in your browser. MP3 output and AIFF, AMR, WMA or CAF input need the ffmpeg fallback, a single download of about 30 MB from cdn.jsdelivr.net.
Frequently asked questions
Does mono to stereo create a real stereo image?
No. Both channels carry the same sound, so it plays from both speakers (or wherever you set the balance) but has no width. Real stereo needs two different signals, such as two microphones.
Why does my mono version sound quieter or hollow?
If the left and right channels are out of phase, averaging them makes parts cancel out. The tool compares levels before and after and warns when the mono mix is noticeably quieter, which points to a phase problem in the recording.
When is mono better than stereo?
For speech: podcasts, voice notes, phone systems and audiobooks rarely gain anything from stereo, and a mono file is half the size as WAV. Mono also fixes recordings where a voice is stuck in one earphone.
What does the balance slider do on a stereo file?
It turns down the opposite side. At −30 the right channel is reduced to 70% and the left stays as it is. On a mono file it pans the single voice with a constant-power curve instead.
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