This is just the same thing though?
It's not actually trivially obvious that different minds would agree on how "sharp" and "soft" apply to sounds and to shapes.
It is a bit mysterious why animals would all map e.g. sharp auditory waveforms to sharper / spiky lines (kiki figure), and then smoother sounds to smoother lineforms (bouba figure), but, reasonably, these senses evolved together and used far more shared underlying processing machinery than they do presently. After all, both light and sound reach us in waves and both carry energy.
Some primitive mush-blob perhaps just started processing all incoming energy waves together, differentiating e.g. smoother vs. more staccatto waves before making the distinction between e.g. sound vs. light. Or I dunno, but IMO it makes more sense when you consider evolution, and don't just look at processing now in isolation from the likely development of these senses.
If I stick to the more familiar territory of bad speech synthesis on 8-bit computers using envelopes, /k/ probably starts with a burst of white noise.
Lol, true.
But the place of articulation kinda flips, from rear consonant/front vowel (kiki), to front consonant/rear vowel (bouba).
If you flipped the voicing, it'd be 'gigi' vs 'poupa', which to my ears at least, is a lot less clear cut.
Yeah this I know, by "bouba" being opposite I meant the waveform.
Really the phonetic terms here are not actually relevant and have betrayed me. It's more that the 'k' sound is "more [ex]plosive" in that it seems to build up more pressure and turbulence (probably as you say, because the consonant is in the rear, so pressure builds and bounces around in the mouth more, whereas the "b" just mostly blasts out the front more quickly). And the "k" surely has a more peaky / spikey waveform in the audible range.
There's probably nicer programs that can do the same thing, aimed at people like musicians, but they won't have the phonetics-specific guides and help online.
There's also a large archive of speech recordings from a wide range of languages hosted by UCLA[1], which you can download and play around with. There might be newer archives now, but I remember this one, because Peter Ladefoged wrote the proverbial book on introductory phonetics.
Linguistics generally aligns quite pro-copyleft, so there will be a lot of freely available stuff online.
[0]: https://praat.org/ [1]: https://archive.phonetics.ucla.edu/
It would be more interesting if we had two sounds which didn’t have such apparent objective differences giving rise to specific and universal subjective differences, or if something that was not so cross domain elicited a different response, e.g. linking volume with sharpness as opposed to something like size.
It would be more surprising if the connection was indicative of genuine universality of brains and their representations, rather than ability to identify a cross-domain mathematical/physics-cal connection.
I’ll rush to posit something for others to poke holes in: the definition of a Kiki word is a word where the syllables have a sudden “attack” then decay. And that’s a function of sound, not language.
Same genetics produce the same outcome (mostly).