What Is an Audio Speed Changer?
An audio speed changer is a tool that increases or decreases the tempo of an audio file — making it play faster or slower — either with or without changing its pitch. This separates two properties that are physically linked in analog audio: when a vinyl record spins faster, it plays faster and higher in pitch. Digital time-stretching algorithms, like the atempo filter used in this tool, break that link so you can independently control speed and pitch.
Speed changing is used daily by millions of people: podcast listeners speed up shows to 1.5× or 2× to save time, language learners slow audio down to 0.75× to catch every syllable, musicians slow recordings to practice along with them, and content creators speed up reference audio to fit time limits.
Speed vs. Pitch — What's the Difference?
Pitch-Preserved Speed Change
Time-stretchThe tempo changes, but every note and voice stays at its original frequency. A speaker at 1.5× sounds exactly like them — just talking faster. This is what podcast apps, YouTube, and this tool's 'Preserve Pitch' mode do. The algorithm is called time-domain pitch synchronous overlap-and-add (PSOLA) or phase vocoder, implemented in FFmpeg as the atempo filter.
Natural Speed Change
ResampleSpeed and pitch change together, exactly like playing a tape or record faster. Doubling the speed raises the pitch by one octave. This is the basis of 'nightcore' (sped-up + higher pitch), 'slowed + reverb' (slowed-down + lower pitch), chipmunk voices, and classic DJ scratch effects. Implemented via FFmpeg's asetrate filter.
Pitch Shifting in Semitones
A semitone is the smallest interval in Western music — the distance between adjacent keys on a piano. There are 12 semitones in an octave. Shifting pitch by +12 semitones raises it exactly one octave; −12 semitones lowers it one octave. Common musical uses: shifting a song from the key of C to D is +2 semitones. Transposing from E to A is +5 semitones. The tool supports ±12 semitones (a full octave in either direction) with 1-semitone precision.
| Semitones | Effect | Music Use |
|---|---|---|
| +12 | One octave higher | Harmonize above, create ultra-bright vocal effect |
| +5 | Perfect fourth up | Transpose C → F, G → C |
| +2 | Whole tone up | Transpose C → D, common key change |
| +1 | Semitone up | C → C♯/D♭ |
| 0 | No change | Original pitch |
| −1 | Semitone down | C → B |
| −2 | Whole tone down | Transpose D → C |
| −5 | Perfect fourth down | Transpose F → C |
| −12 | One octave lower | Deep bass effect, lo-fi or horror audio |
Common Speed Settings & Use Cases
Slow-motion effect, transcription, music practice
Calm listening, 'slowed + reverb' aesthetic (with natural mode)
Original speed — no change
Light speed-up for lectures and dense content
The most popular podcast speed — saves 33% of listening time
Fast content consumption for familiar topics
Double speed — half the time, high comprehension for practiced listeners
Extreme review mode, scanning recordings, audio skimming
Frequently Asked Questions
Technical note: Pitch-preserved speed change uses FFmpeg's atempo filter (0.5×–2.0× per stage; stages are chained automatically for extreme values). Natural speed change uses asetrate to change the sample rate, making FFmpeg decode audio at a different speed. Pitch-only shift combines asetrate with a compensating atempo to shift pitch while keeping tempo. All processing runs via FFmpeg compiled to WebAssembly, entirely in your browser.