Understanding audio fades
A ramp, in and out.
Why every podcast starts and ends with a fade, the difference between linear and logarithmic ramps, and how short a fade needs to be to do its job.
The job of a fade.
A fade is a gain envelope: amplitude rises from 0 to 1 across the fade-in window (or drops from 1 to 0 across the fade-out). The ear hears clicks when audio jumps discontinuously from silence to full level (or vice versa). A 20-100 ms fade smooths the transition; nobody consciously notices it but the click is gone. For podcasts and music, the fade is the polite way to start and end.
Linear vs logarithmic.
Linear fade: gain grows in equal steps over time. Logarithmic (exponential) fade: gain follows a curve so the ear hears a more uniform ramp. Human hearing is logarithmic in amplitude — what we hear as "twice as loud" is roughly 10× the power. Linear fades sound abrupt-fast at the end; log fades feel natural. For fades of 200 ms or less the difference is inaudible; for fades of seconds (a slow fade-out at the end of a song), log is unambiguously better.
A worked fade.
A 60-minute podcast with a 100 ms fade-in and a 500 ms fade-out. FFmpeg: ffmpeg -i in.mp3 -af "afade=t=in:st=0:d=0.1, afade=t=out:st=3599.5:d=0.5" out.mp3. Two filter instances on the audio filtergraph; one fade-in at zero, one fade-out near the end. The clicks at start and end go away; everything else is untouched.
Fade-in + fade-out
afade=t=in:d=0.1, afade=t=out:d=0.5
Two filter passes on the same stream.
ramp 0→1 over 100ms ; ramp 1→0 over 500ms
= Clickless edges
Crossfading.
Joining two audio clips smoothly means fading the first out while fading the second in. The total combined loudness should stay roughly constant — that's why crossfades use logarithmic ramps that overlap at -6 dB (half power each). The "DJ crossfade" applied at the wrong curve gets you the audible dip in the middle. For podcast intros joining to dialogue, 200-500 ms crossfades sound natural.
When to skip the fade.
Hard-cut edits, intentional silences, music tracks with deliberate hard starts (think Beatles' opening chord) — all want no fade. The fade isn't a universal requirement; it's specifically a fix for silence-to-signal click. If the file starts or ends at zero amplitude anyway, no fade is needed.