Trim an audio file

Frames, and the couple of hundredths you cannot see

Compressed audio is not a continuous stream of samples but a sequence of frames, each holding a fixed block of sound that must be decoded as a unit. An MP3 frame carries 1,152 samples, which at the usual 44.1 kHz works out at just over 26 milliseconds. A cut that copies the audio rather than re-encoding it can therefore only land on a frame boundary — so the start may fall up to about a fortieth of a second either side of the point you picked, and the page tells you the time it actually used.

That default is the right one nearly always, because copying means the sound is bit for bit what it was: no second generation of compression, and a long recording cuts in a moment. Twenty-six milliseconds is below what anyone perceives as a timing error in speech or music. Switching on the exact cut re-encodes so the boundary lands precisely where you asked, at the cost of a generation of quality — worth it when the cut has to sit between two syllables or on a beat.

A click at a cut is a real phenomenon and not a fault in the file. Sound is a waveform moving above and below a centre line, and cutting at a moment when it is far from that line leaves an instantaneous jump in level, which a speaker reproduces as a click. It is most audible in loud, dense material and inaudible in a quiet passage. The usual fix is a very short fade across the join; fades are not available here yet, so choosing a cut point during a pause is the practical workaround.

Taking a piece out of the middle joins what remains, and the same boundary rules apply at both ends of the removed section. Nothing is committed until you download — every split and deletion can be undone — and the file comes back in the format it arrived in.

When you want something else

ffmpeg does the same copy cut with no re-encoding: ffmpeg -ss 00:01:30 -to 00:04:00 -i in.mp3 -c copy out.mp3. For MP3 specifically, mp3splt is better still — it understands the format's frame structure and its gapless headers, so it can split at frame boundaries without disturbing anything, and it will even split an album into tracks from a cue sheet.

For anything involving fades, crossfades, noise reduction or level matching, Audacity is free, runs everywhere, and shows you the waveform so you can place a cut at a zero crossing and never hear a click. Anything beyond taking a piece out of one file is already an editing job.