Audio Speed Changer

Speed up or slow down audio files (0.25x to 4x) using FFmpeg.

Audio Speed Changer speeds up or slows down audio from 0.25x to 4x (slider step 0.05, default 1.00x) with quick buttons 0.25x through 4x. Pitch stays the same — FFmpeg’s atempo changes tempo only. A Local Processing alert confirms the work runs in the browser; middle step label is Process.

Result shows Speed changed to {n}x and downloads as speed-{n}x-{basename}.{ext} so the original is never overwritten. Duration scales inversely: 2x halves length (and typically file size for lossy sources).

How it works

buildAtempoFilter chains atempo filters because a single atempo is only reliable in 0.5–2.0: within band → atempo={speed}; >2 → repeated atempo=2.0 until remainder ≤2; <0.5 → repeated atempo=0.5 until ≥0.5. Full args: -i input -filter:a {chain} -c:v copy output (harmless for pure audio).

MIME falls back to the original file.type or audio/mpeg. Success toast: Speed changed to {speed}x!

How to use

Open Audio Speed Changer and read the Local Processing alert (middle step label is Process).
Drop an audio file (hint: MP3, WAV, OGG, AAC, FLAC — any audio format).
Preview the source; wait for FFmpeg load (button enables).
Drag the Speed slider (0.25–4, step 0.05) or click a quick button (0.25x, 0.5x, 0.75x, 1x, 1.25x, 1.5x, 2x, 3x, 4x).
Read the live readout (e.g. 1.50x) between 0.25x and 4x scale labels.
Click Apply Speed Change (Processing…) and watch progress % + log.
Play the result (Speed changed to 1.5x + size badge).
Download speed-1.5x-{file} or Start Over.

Tips

Pitch is preserved (tempo-only) — extremes can still audibly degrade quality.
Quick buttons cover common podcast/audiobook speeds (1.25x, 1.5x, 2x).
Outside 0.5–2.0 the code chains multiple atempo filters (4x → atempo=2.0,atempo=2.0).
1x is a valid re-encode pass (filter atempo=1) if you just want a new file.
Speed- prefix prevents overwriting the source.