Trim Audio

Add a file and its waveform appears. Drag the left handle to where the part starts and the right one to where it ends. Drag across the waveform to choose a new part in one move, or inside the part to slide it along.

For exact marks, type a time under the waveform, such as 1:23.456 or 83.5, or focus a handle and press the arrow keys: 10 milliseconds a press, 100 with Shift, 1 with Alt. Zoom in with the buttons, or with a trackpad pinch, to see single beats. A button moves both marks to the quietest places nearby, so a cut falls in a pause instead of a word.

Play plays what you would save, a removal’s join and the fades included, so you hear the result before you make it. Nothing is cut until you press the button.

Keep saves the part between the handles: the ringtone, the best minute, a voice note without its first ten seconds. Remove does the opposite and saves everything else with the two ends joined: a cough, an advert, a false start in the middle of a take.

The status line says how long the result will be before you press anything. A removal that would leave nothing, or a part shorter than the file can be cut at, says so instead of running.

An MP3, an M4A, an Ogg or Opus file is a row of small packets of sound. Cutting it copies the packets you keep into a new file as they are, so the sound is not decoded and encoded again and nothing is lost. The cost is that a cut falls between two packets. Each cut lands at most half a packet from your mark, which is within 14 milliseconds in an MP3, 12 in an M4A or AAC file, 10 in an Opus file and 24 in an Ogg Vorbis file.

A WAV file has no packets, so it is cut to the sample, 0.02 milliseconds at CD quality. The page shows what your file allows before you press the button, and says “No quality lost” only when that is true.

A fade changes the sound itself, so a file that has one is encoded again. In a WAV the fade is done on the samples in place and nothing else changes. Any other file with a fade is saved as WAV, which holds the faded sound exactly, or as MP3 at 128, 192 or 320 kbps, which is a new lossy copy. The page tells you which it is before you choose.

A removal made without re-encoding joins two frames that were never next to each other, and the join can click. Saved as WAV or MP3, the 4 milliseconds either side of the join are faded down and up, which takes the click away.

It reads MP3, WAV, FLAC, M4A and AAC, Ogg (Vorbis or Opus) and the sound of MP4, MOV, MKV and WebM videos. A video comes out as a sound file: its picture is left behind, and a film with several sound tracks uses the first one that can be decoded. Dolby Digital and DTS sound can’t be read yet.

Each file is saved as its own kind where it can be: MP3 as MP3, M4A as M4A, Ogg as Ogg. FLAC is saved as WAV, which is just as lossless but bigger, or as MP3: ToolHero has no FLAC encoder yet, and a FLAC packet is too long to cut at.

The waveform of a 60-minute recording is drawn in about 9 seconds and fills in from the left while it is read. Cutting a part out of it without re-encoding takes about 1 second. A 4-minute song takes about 1 second to cut that way, and the same with a fade.

The file is read a piece at a time and the result is written to your browser’s private storage as it goes, so the browser’s memory grew by up to about 300 MB for the hour. We haven’t measured phones, so we promise no length there.

Title, artist, album and cover art are copied into the new MP3, M4A, Ogg or Opus file. A WAV can hold title, artist, album, comment, genre, year and track number, but no cover.