Audio Speed Changer - Slow Down or Speed Up Music

This tool changes the pitch and the speed of an audio file as two completely independent controls. The primary control is the pitch shift, measured in semitones — the steps of a piano keyboard. Move it to +3 st and every note comes out three keys higher while the file keeps exactly its original length; move it to −12 st and everything drops a full octave, still at the original tempo. The secondary control is playback speed, from a quarter of the original speed to four times it, and it works the other way around: it changes tempo and duration while the pitch stays wherever you set it. Keeping the original speed is simply leaving the speed at its default 1×, which the readout confirms by reporting the duration as 100% of the original. Under the hood the processing runs on Signalsmith Stretch, an open-source, studio-grade time-stretch and pitch-shift engine compiled to WebAssembly and running entirely inside your browser. It analyzes the audio spectrally in overlapping blocks and rebuilds it at the new pitch and tempo, which keeps transients tight and voices natural far beyond what simple granular methods manage — vocals shifted a few semitones stay intelligible, and music slowed to half speed stays clean enough to practice against. If the WASM engine cannot load — an old browser, a strict content-security policy — the tool automatically falls back to a built-in WSOLA granular stretcher and tells you so in the result: the engine readout shows either Signalsmith (studio quality) or WSOLA (fallback), so you always know what produced the file you are downloading. The classic uses split neatly along the two controls. Pitch only: transpose a backing track into a singable key, fix a recording that is a semitone flat, deepen or raise a voice without the chipmunk speed-up. Speed only: slow a solo to 0.5× to learn it in the original key, listen to a lecture at 1.5× without the narrator turning cartoonish, gently slow fast speech to 0.75× for transcription. And because the controls are independent you can combine them freely — up two semitones and 10 percent faster in one pass, with no intermediate render. The math is shown live: output duration equals input duration divided by speed, and the pitch factor is 2 raised to the power of semitones divided by 12. Files longer than ten minutes are refused so the render stays responsive; trim first, then process. Everything — decoding, stretching, and encoding to WAV or 192 kbps MP3 — happens locally, so nothing is uploaded and nothing waits in a queue.