How to Slow Down and Loop a Song — Measured Speed and Pitch Accuracy
Set points A and B around the passage in the practice player, turn on looping and lower the speed. From 0.25× to 2× speed, pitch stayed within 0.1 cent, and key shifts from −12 to +12 semitones landed within 0.1 cent of the setting (measured 2026-10-01).
By Chaez · Published · Last updated · Data as of
What steps should you follow?
- Choose an audio file or open the demo in the practice player. The file is played and processed on your device only.
- Press A at the start of the passage and B at the end.
- Turn on looping: whenever playback passes B, it restarts from A. The position is checked on every screen refresh.
- Lower the speed slider to a value you can play comfortably (for example 0.5×) and raise it gradually as you improve.
- If the recording is in a different key from your instrument, shift the pitch in semitone steps.
Does slowing down change the pitch?
No. Processing a 440 Hz pure tone at 0.25× to 2× speed changed its pitch by no more than 0.1 cent, and playback length stayed within 0.7% of the set ratio (measured 2026-10-01). A semitone is 100 cents, so 0.1 cent is not audible.
| Speed | Playback length (set) | Pitch change | Processing noise (vs tone) |
|---|---|---|---|
| 0.25× | 4.013× (4×) | −0.1 cent | −55.8 dB |
| 0.5× | 2.005× (2×) | −0.1 cent | −55.9 dB |
| 0.75× | 1.338× (1.333×) | 0.0 cent | −60.5 dB |
| 1× (unchanged) | 1.003× (1×) | 0.0 cent | −103.2 dB |
| 1.25× | 0.805× (0.8×) | −0.1 cent | −54.3 dB |
| 1.5× | 0.669× (0.667×) | −0.1 cent | −55.7 dB |
| 2× | 0.500× (0.5×) | +0.1 cent | −53.2 dB |
Does a key change move exactly by semitones?
Yes. Shifting the key from −12 to +12 semitones produced pitch changes within 0.1 cent of the setting, and playback length stayed within 0.6% (measured 2026-10-01).
| Key shift | Measured pitch change | Playback length | Processing noise (vs tone) |
|---|---|---|---|
| −12 semitones | −1200.0 cents | 1.003× | −60.5 dB |
| −7 semitones | −699.9 cents | 1.003× | −61.1 dB |
| −5 semitones | −500.0 cents | 1.004× | −60.6 dB |
| −1 semitone | −99.9 cents | 1.003× | −62.2 dB |
| +1 semitone | +100.0 cents | 1.002× | −56.3 dB |
| +5 semitones | +500.0 cents | 0.994× | −56.7 dB |
| +7 semitones | +699.9 cents | 0.999× | −50.5 dB |
| +12 semitones | +1200.0 cents | 0.999× | −50.6 dB |
Changing speed and key together kept both accurate. With 0.5× speed and −2 semitones combined, playback length was 2.005× and the pitch change was −199.9 cents.
Does changing speed or key alter the sound?
Slightly. Played unchanged, the pure tone had content outside its frequency at −103 dB; changing speed or key added processing noise at −50 to −62 dB (measured 2026-10-01). The practice player uses the SoundTouch library, which cuts audio into short segments and splices them back together, adding a little content that was not there.
These figures come from a single pure tone. Real music with several instruments, especially at extreme settings such as 0.25× or ±12 semitones, may sound noticeably different in texture or ambience. This measurement is not a listening test.
How we measured
- Processing path: the practice player plays audio through the PitchShifter in soundtouchjs 0.3.0, which sets SoundTouch’s tempo and pitchSemitones. We ran the same package’s SoundTouch and SimpleFilter offline in Node.js.
- Test signal: a 44.1 kHz, 440 Hz pure tone synthesized by us. Any content outside that frequency can be attributed to processing.
- Pitch: the middle of each output was analyzed with an FFT, and the strongest frequency was compared with 440 Hz in cents.
- Playback length: to cancel the ~0.2 s held back at the end of processing, the ratio was taken from the output length difference between a 30 s and a 60 s input. Resolution is about 0.3%.
- Processing noise: energy outside ±25 Hz of the detected frequency compared with the energy inside, in dB.
Frequently asked questions
- Does slowing a song to 0.5× lower its pitch?
- No. In our 2026-10-01 measurement, 0.5× speed changed pitch by −0.1 cent. The practice player changes speed while keeping pitch.
- Does lowering the key by one semitone move it exactly one semitone?
- Yes. A −1 semitone setting measured −99.9 cents, and every setting from −12 to +12 semitones was within 0.1 cent.
- Can I change speed and key at the same time?
- Yes. With 0.5× speed and −2 semitones combined, playback length was 2.005× and the pitch change −199.9 cents, both matching the settings.