Every listener hears a re-encoded version of your master. Understanding what codecs do explains which mixing decisions survive that process and which don't.
Lossy codecs exploit auditory masking — the fact that a loud sound hides quieter sounds near it in frequency and time.
If a loud snare hits at 3kHz, quieter content around 3kHz becomes inaudible for a few milliseconds. The codec knows this and simply doesn't store the masked content. You can't hear what was removed because you couldn't hear it in the original either.
That's the honest basis of lossy compression. It's not "lower quality" so much as "storing only what you can perceive." The problems come when the model is wrong.
Cymbals and hi-hats. Broadband noise with sharp transients is the worst case — it doesn't compress efficiently, and errors are audible as swirling or splashiness.
Sibilance. Vocal "s" sounds are high-frequency noise bursts. Codecs often make them lispy or metallic.
Reverb tails. Low-level, diffuse content spread across the spectrum. Frequently the first thing sacrificed at lower bitrates.
Pre-echo. A sharp transient after silence can smear backwards, producing a faint sound before the hit. Most noticeable on solo percussion and plucked strings.
Very wide stereo. Codecs often use joint stereo, encoding mid and side signals. Extreme width is expensive and can collapse.
| Codec | Strength | Typical use |
|---|
| MP3 | Universal compatibility | Legacy, references |
| AAC | Better than MP3 at the same bitrate | Apple Music, YouTube |
| Opus | Best quality at low bitrates | Spotify (low tiers), voice, streaming |
| Vorbis | Open, good quality | Spotify (some tiers), games |
Leave true-peak headroom. Encoding can push peaks above 0dBFS. Master to -1.0 dBTP so the decoded output doesn't distort.
Don't over-brighten. Excessive high end costs the codec data and is where artifacts appear. A mix that's harsh before encoding is worse after.
Be careful with extreme stereo width. Check your mix in mono; if it collapses badly, joint stereo encoding will hurt it.
Never upload lossy. Uploading an MP3 means double encoding — your artifacts, plus the platform's. Always upload WAV or FLAC.
Check your master encoded. Export a 256kbps AAC and listen. It's what your audience actually hears, and it takes two minutes.
Related reading: audio file formats explained, LUFS and loudness explained, and how to export for streaming.
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