Much of this is from: https://cramdvoicelessons.blog/encyclopedia/aperiodicity/ it's a very good overview. Archive link: https://web.archive.org/web/20201024140509/https://cramdvoicelessons.blog/encyclopedia/aperiodicity/
Aperiodicity, as far as I can tell, refers to the parts of sounds that do not repeat over and over. It's kind of like you get the underlying sound that the voice is producing, the tone of the actual thing being pronounced. Then you subtract that from the raw input voice and what's left is the noise/aperiodicity. I'm guessing that this is important to the
This term comes up a lot in [vocal-synthesis] literature.
The [world-vocoder] uses something called D4C for "band-aperiodicity" estimation: https://www.sciencedirect.com/science/article/pii/S0167639316300413
Two kinds of aperiodic sounds: transient noise like a popping baloon, or noise sound like air leaking from a tire [1].
[1] http://www.cas.usf.edu/~frisch/SPA3011_L06.html