Audio Speed Changer

Why change audio speed?

Adjusting audio speed is useful in many situations. Language learners slow down spoken dialogue to catch every word. Podcast listeners speed up episodes to consume more content in less time. Musicians slow down complex passages to learn them note by note. Transcriptionists reduce speed to type along more accurately. Audio producers adjust tempo to match a video timeline or synchronise with other tracks.

What does "preserve pitch" mean?

When you speed up audio normally, the pitch rises - voices sound like chipmunks. When you slow it down, everything sounds deep and distorted. Pitch preservation (also called time-stretching) changes the speed while keeping the original pitch intact. This tool uses ffmpeg's atempo filter, which applies a high-quality time-stretch algorithm to maintain natural-sounding audio at any speed between 0.25x and 4x.

How the atempo filter works

The ffmpeg atempo filter accepts values between 0.5 and 100.0, but for quality reasons each instance is limited to 0.5-2.0. To achieve more extreme speeds (e.g. 4x or 0.25x), this tool chains multiple atempo filters automatically. For example, 4x speed uses two passes of atempo=2.0, and 0.25x uses two passes of atempo=0.5. This maintains audio quality across the full speed range.

Live preview

The preview button uses the Web Audio API's playbackRate property to let you hear the speed change instantly without processing the file. The actual export uses ffmpeg.wasm for proper time-stretching with the atempo filter, which produces higher-quality results.

Privacy

Everything runs in your browser. No audio is uploaded to any server. Your files remain on your device throughout the entire process.