Understanding audio reversal
Play it backwards.
The mechanical operation, the artistic history, and what backwards-cymbal swells have in common with classic record-spinning.
It's literally array-reverse.
Reversing audio is one of the simplest DSP operations: take the array of samples, read it back-to-front. For a stereo file, reverse each channel independently. No quality loss, perfectly bit-exact, fully reversible (reversing the reversed file gives the original back). FFmpeg's areverse filter does exactly this. The maths is nothing; the artistic effect is what matters.
What reversed audio sounds like.
Sounds that decay forward (cymbals, piano notes, plucked strings) become sounds that swell — the long quiet tail comes first, rising into the loud strike. Speech becomes unintelligible but recognisable as speech. Drum hits become whooshes. The reverse-cymbal swell is a Beatles invention (Strawberry Fields Forever, 1967) that every subsequent producer has used; the trick is that human auditory perception processes attack vs decay completely differently, and swapping them is jarring.
A worked reverse.
A 30-second cymbal crash sample. ffmpeg -i crash.wav -af "areverse" reverse-crash.wav — runs in milliseconds, produces a sweep that builds from silence to the original's loud peak, then ends abruptly. Layer the reverse-crash on top of a snare hit and you get the classic "build-up to the snare" sound used in every transition.
Reverse swell
afade=t=in attached after areverse for a clean start
Reverse the source, then add a short fade-in to avoid the hard edge.
ffmpeg -i in.wav -af areverse,afade=t=in:d=0.05 out.wav
= Clean swell into silence
The backmasking myth.
The mid-20th-century moral panic about "satanic messages" hidden in records played backwards relied on pareidolia — the human brain is so good at finding patterns in noise that reversed speech can sound like new meaningful phrases. Empirically, no major artist ever actually recorded reversed satanic instructions; the songs that were accused of it (Led Zeppelin's Stairway, the Eagles' Hotel California) just had reversed gibberish that listeners interpreted creatively. The effect is real but the conspiracy isn't.
Reverse + reverb = a different effect.
The interesting trick: reverse the signal, apply reverb, then reverse again. The reverb decays "backwards" relative to the source — instead of trailing each sound forward in time, it precedes it. The result is the "swelling reverb" sound that arrives before the dry signal. Common in shoegaze production, dream-pop, and any context that wants vocal lines to feel "haunted".