The noise an MP3 places before the hit

Why is there sometimes a rustle in an MP3 just before a sharp hit, a rustle that was not in the recording? The codec does not walk through the sound one point at a time. It takes a chunk as a whole. If a loud click sits inside that chunk, the error from it spreads over the whole chunk, including the silence before the click.

The rustle arrives before the click

Take a spot that was quiet, and then a finger snap, a snare, or a plucked string comes in at once. In the original the silence breaks on the hit. In the MP3 a soft hiss shows up slightly before the hit. It is not the tail of the previous note. The hit has not happened yet, and the noise is already there.

The loud click sets the noise for the whole chunk the codec took on. While nothing else is playing next to it, there is nothing to cover that noise, and you hear it before the hit itself. In a dense passage the same noise sinks into the music.

On a low bitrate the chunk is longer, and the rustle is easier to catch. On a high one the codec more often manages to take a shorter chunk, and the dirt before the hit almost disappears.

While it is silent before the hit, the rustle sticks out. Once the music is already playing, you cannot pick that rustle out of it.

Where to catch it

Look for the first hit after a pause. A dry click, the start of a word, an acoustic guitar with no processing. On a drop where everything is already shouting, this rustle usually does not come out. Two people can listen to the same song and disagree: one caught the pause before the click, the other was in the middle of the chorus.

A recording you know is easier. You remember that it was clean before that click, and you hear the dirt at once. On an unfamiliar one it is easy to take the dirt for room noise.

If you want a number and not an impression, hide the filename and leave only the sound. The rounds are explained in the FLAC vs MP3 article. The tail after the hit, where the codec saves on the decay, is in the article on where you hear MP3. You can find your own threshold on the headphones you already use, on the setup page.