Slow down a song without changing its pitch

How to slow down a song and loop a part

1. Press Open a song or video and pick a file from your device: MP3, M4A, WAV, FLAC, OGG, or a video's sound track.
2. Move Speed down, to 70% for example. With Keep pitch on, the song stays in the same key.
3. Drag across the waveform over the passage you want to learn, or press Set A and Set B as it plays, then switch Loop on. Nudge either end by 50 ms if it clips a note.
4. For practice, switch on the Speed trainer (60% rising 5% a pass to 100%) and a Count-in at the song's tempo.
5. To change the key, move Transpose: −1 semitone to play along on a guitar tuned down a half step, or up or down to a key that suits your voice.
6. Press Download the loop or Download the whole file for a WAV with the same speed and pitch.

What it cannot do

• Below about 50% speed, drums and spoken consonants smear: a snare hit turns into a short flam and an echo, because a time-stretch repeats small pieces of the sound. Sustained notes, sung vowels and chords hold up much better, though at 25% they start to waver.
• It saves WAV only, not MP3: the free MP3 encoders are under licences this site does not ship. A 4-minute stereo WAV at 44.1 kHz is about 42 MB (twice that at 50% speed).
• Transposed playback is a rendered copy: each change of pitch re-renders the file before you hear it (a second or two per minute of music on a laptop, longer on a phone). Speed changes are instant.
• Live playback uses this browser's own pitch-keeping; the download uses this page's time-stretch, so the two can sound slightly different on drums.
• It cannot open a link to a streaming or video site. The file must be on your device.
• DRM-protected files (older protected M4P purchases, songs saved inside streaming apps) cannot be decoded, nor can formats this browser does not decode, listed under the open button.
• The whole file is decoded into memory: an hour-long video or a file over about 1 GB may be too big for a phone.

How the slowing and transposing work

Playing. Browsers can play a file slower while keeping its pitch (the preservesPitch setting of an audio element), so practice speed changes are instant. Switching Keep pitch off is tape mode: at 50% the song plays an octave lower, as a slowed tape or record would.

Saving. For the download the page stretches the sound itself with WSOLA (waveform-similarity overlap-add). It cuts the sound into 40 ms slices that overlap by half. To play at 50%, it lays slices down every 20 ms but moves through the original only 10 ms at a time, so each moment is heard twice as long. A plain repeat would click, so each next slice is taken from within 12 ms of its place at the point where its waveform best lines up with how the previous slice would have continued. A 440 Hz tone stretched to 50% stays at 440 Hz and lasts exactly twice as long; this page's test checks both.

Transposing. To raise a song by 12 semitones (an octave, twice the frequency) at the same speed, the page first stretches it to twice its length with WSOLA, then plays that back twice as fast by resampling, which halves the length again and doubles every frequency. One semitone is a factor of 2^1/12 = 1.0595; a cent is a hundredth of that. When speed and transpose are both set, the two steps are combined into one stretch and one resample.

Privacy

The file is opened and decoded by this browser, and every stretch and export runs in this page (and a background worker loaded from this same folder). No request carries the file, its name or any sound. The page counts an anonymous page view and, the first time you open a file or download, the fixed words slowdown-open or slowdown-export, never a file name, speed or length.

To pick up where you left off, the speed, pitch, loop points, markers and trainer settings for each file are kept in this browser's local storage, under a key made of the file's size and length (not its name or its sound). The last 40 files are kept.

Clear saved settings

Questions

How do I slow down a song without changing the pitch?

Open the song, leave Keep pitch switched on and move the speed slider down, for example to 70%. The browser plays it slower in the same key. To save that version, press Download the whole file: the page renders it with its own time-stretch and gives you a WAV.

Can I slow down audio on an iPhone?

Yes, in Safari or any iPhone browser. Tap Open a song or video and pick a file from Files (a song bought in the Music app is DRM-protected and cannot be opened, but files you own as MP3, M4A or WAV can). Speed, loop and transpose work by touch; the downloaded WAV lands in Files under Downloads.

How do I slow down an audio file and save it?

Set the speed, and a transpose if you want one, then press Download the whole file, or Download the loop for just the A–B part. You get a 16-bit WAV at the file's own sample rate, rendered on your device. A 4-minute song at 50% takes a few seconds on a laptop.

Is there a free online audio speed changer that does not upload the file?

This one. It is free with no account, and the file is opened and processed by your browser: the page sends no request that carries the file, its name or any sound. You can check it in the browser's network panel while you use it.

Can I loop part of a song and speed it up gradually?

Yes. Drag across the waveform, or press Set A and Set B while it plays, to set the loop. Turn on the Speed trainer, for example from 60% rising 5% per pass to 100%, and every time the loop comes round it plays a little faster.