Trim & cut audio (MP3, M4A, WAV)

How to use it

1. Choose an audio file — MP3, M4A, WAV, FLAC and OGG all work.
2. Play it, then set the start and end: drag the handles, type the times, or snap either to the playhead.
3. Trim, and the clip downloads in the same format it went in.

An audio frame is about a fortieth of a second

Compressed audio is stored in frames too, but they are tiny, and there is no keyframe equivalent because every frame decodes independently. An MP3 frame carries 1,152 samples, roughly 26 milliseconds at 44.1 kHz; an AAC frame carries 1,024, closer to 21 milliseconds at 48 kHz. A copied cut snaps to the nearest boundary, which lands it within a fortieth of a second of where you asked.

Granularity varies a little by format. WAV is uncompressed PCM with no frame structure at all, so a cut can fall on any individual sample. Opus commonly uses 20-millisecond frames. FLAC blocks are the coarsest here, often 4,096 samples or about 93 milliseconds, still well below anything an ear registers as a late start.

Two quirks that survive a lossless MP3 cut

MP3 encoders use a bit reservoir: a passage needing more bits than its frame budget allows borrows space left unused by earlier frames. A frame is therefore not always as self-contained in practice as in principle, and the opening frame of a clip can be missing borrowed data it expected. If it is audible at all, it is a brief roughness in the first fraction of a second. Nothing after it is affected.

The second is encoder delay. A LAME-encoded MP3 opens with a short run of padding samples and records their length in an Info or Xing header. Trim the file and that header still describes the original, so some players report the old duration or place a sliver of silence at the front. Players that count actual frames get it right, and for a ringtone or a podcast pull it makes no audible difference.

Questions