Why Do This in Your Browser?
A fade is one of the most common audio edits, and one of the smallest: the samples are multiplied by a gain curve that runs from silence to full level (or back). Opening a full audio editor, importing the file, drawing the envelope and rendering is a lot of overhead for a two-second fade on a podcast intro.
This tool uses FFmpeg's afade filter, which offers many curve shapes; four are exposed here. We measured each one on a steady tone with FFmpeg 6.1 (the curve formulas are the same in the FFmpeg.wasm core). Values are the level reached, as a fraction of full level, at points through a fade-in:
Linear (tri): 0.25, 0.50, 0.75 at a quarter, half and three-quarters of the way. Quarter-sine (qsin, the default here): 0.38, 0.70, 0.92, so it is half-way up after about a third of the fade and eases into full level. Exponential sine (esin): 0.22, 0.29, 0.36, then a jump to full level at the end, so most of the change happens at the two edges. Logarithmic (log): already 0.80 after one tenth of the fade, so it sounds almost like a hard start.
A fade-out uses the same shape mirrored in time: a logarithmic fade-out holds close to full level and drops away in its last tenth, while a quarter-sine fade-out falls away gently at first and then more steeply.
How It Works
Drop an audio file, tick Fade In, Fade Out or both, and set each one's length (0.05 to 30 seconds) and curve. The fade-in starts at 0:00; the fade-out is placed so that it ends exactly at the end of the file, using the length your browser reads from the file. There is no waveform or preview step: press Apply Fades and listen to the result. A 4-second clip took 0.3 seconds in our test once the engine had loaded.
FFmpeg.wasm runs afade=t=in:st=0:d=N:curve=X,afade=t=out:st=T:d=N:curve=X and re-encodes the result in the same format it came in: MP3 at 192 kbps, OGG Vorbis at 160 kbps, AAC/M4A at 192 kbps, or lossless WAV (16-bit) and FLAC. Any other input, such as Opus, is written as MP3. There is no option to pick a different output format; use the Audio Converter for that.
Tip: For trimming the audio before adding fades, use the Audio Trimmer first — fades sound better on cleanly-cut edges. After fading, run a final loudness pass with the Audio Normalizer for distribution-ready output. For pitch correction on the faded clip (e.g., transposing a music intro to a different key), the Audio Pitch Shifter shifts up to ±12 semitones without changing duration.
Common Use Cases
Podcast Intro/Outro Music
Fade in the theme music at the start of an episode; fade it out at the close. The simplest professional polish.
Audiobook Chapter Transitions
Add a 1–2 second fade to the start and end of each chapter for smoother chapter-to-chapter playback.
Voice-Over Soft Starts
Quick 200ms fade-in eliminates the click that recording starts can produce.
Music Looping Setup
Fade out a song's tail to make it loop cleanly back to the start without a hard cut.
Sound Effect Tail Smoothing
Hard-cut sound effects can sound abrupt; a short fade-out smooths the tail.
Ringtone Preparation
Custom ringtones sound jarring without fades; a 200ms fade-in/out makes them sound polished.
How We Compare
Honest read on free, paid, and self-hosted options for this kind of job:
UDT Audio Fade (this tool):
Free, browser-based, FFmpeg.wasm afade filter. Four curve shapes and separate in/out controls. No trimming, waveform or preview. No upload, no watermark, no signup.
mp3cut.net (fade option):
Free web cutter from 123apps with fade-in and fade-out switches; it also trims, which this tool does not.
Audacity (desktop):
Free, full audio editor with fade effects and hand-drawn volume envelopes. The better choice for fades in the middle of a file.
GarageBand (Mac):
Free on Mac; full DAW.
Adobe Audition:
Paid, via an Adobe Creative Cloud subscription; full fade and envelope editing.
FFmpeg CLI:
Free, command-line; same engine as this tool. This is FFmpeg with a UI.
Frequently Asked Questions
Is this audio fade tool really free?+
Yes — completely free, no watermark, no time limits, no signup.
What's the difference between the four curve shapes?+
Linear rises at a constant rate. Quarter-sine is half-way up about a third of the way through and eases into full level. Exponential sine makes most of its change at the very start and very end, with a slow middle. Logarithmic reaches 80% of full level within the first tenth of the fade, so it sounds nearly instant. Fade-outs use the same shapes mirrored in time. Quarter-sine is the default.
Are my audio files uploaded anywhere?+
No. FFmpeg.wasm runs entirely in your browser. The 32MB engine downloads the first time and your browser usually caches it. Once it has loaded, processing needs no network connection while the page stays open.
Can I apply only a fade-in (or only a fade-out)?+
Yes. Untick the fade you don't want; each has its own checkbox. Durations can be set from 0.05 to 30 seconds, and a value outside that range is clamped to it.
What's a good fade duration?+
Depends on content. For music intros/outros, 1–3 seconds sounds natural. For voice content, 200–500ms is enough to smooth edges. For sound effects, 50–100ms eliminates clicks. Default is 1 second.
What's the maximum file size?+
Limited by browser memory. The whole file and the result are held in memory at once, so very large files, hundreds of MB and up, can fail, and phones run out sooner than laptops.
Can I add a fade somewhere in the middle of a file (a 'dip')?+
Not in the current version — the tool applies fades only at the start and end. For mid-file volume automation, an audio editor like Audacity is the right pick.
What's the underlying engine and license?+
FFmpeg.wasm v0.12.x with @ffmpeg/core v0.12.6 (afade filter built in). FFmpeg itself is LGPL by default, but the @ffmpeg/core build this tool loads includes x264, which makes that build GPL; the wasm wrapper is MIT.