Why do we often associate “higher” sounding vowels (E, A, I) with sharp edges and “lower” sounding vowels (O, U) with round/dull/circular edges?

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A wiki on the topic https://en.m.wikipedia.org/wiki/Bouba/kiki_effect

In: Other

It may have something to do with the literal way that we make those sounds with our mouths, make an o or u sound and you will find your mouth contorting to that shape, we see other people make the o sound and so that’s the association that cements in our minds, since it’s the same mouth shape for that sound globally, it becomes a universal concept that o is round.

Does this really need an explanation? Just look at the shapes of the letters in your title lol.

To quote the researchers in the source of the link you included:

“The Kiki-Bouba effect corresponds to a sound-vision synaesthesia, probably between the tone of vowels and the vertical or horizontal aspect of figures.”

Essentially, in some humans, perceiving something with one sense will cause them to perceive something with another (e.g. some people who hear certain musical notes will perceive colours corresponding to those notes). It seems as though we are hardwired to make connections between different types of stimuli. In that experiment, it showed we associate rounder objects with longer, ‘softer’ noises (b, p, o, etc.) and sharper objects with noises that are more short and ‘hard’ (k, t, e, etc.).

I was taught when singing to round out my vowels so they don’t seem as screechy. Perhaps we relate skreechy noises with sharpness.

Make “eeee” with your mouth – you don’t have to do it out loud, it can be whispered/silent. Now, importantly, while maintaining that “eeee” position inside your mouth, *completely relax your lips.* Don’t pull them out into a smile, just completely relax them.

Notice how all the “eeee” sound requires is this one tongue position. And notice that the major characteristic of this position is that a large portion of your tongue is HIGH AND TALL, all the way at the top of your mouth, barely letting any air over it, constricting your airflow more than any other vowel. This is the skinniest and tightest your airway ever gets during the vowel parts of speaking. What’s more connotative of “skinny and tight and constricted” – sharpness and edges, or roundness and circles?

Now, also with totally relaxed lips, go “uhhh, ahhh” – your tongue is low in your mouth, and your vocal tract is as wide open and unrestricted as it can be. But you asked about O and U – good catch, as these also have some tongue height to them.

O and U do have your tongue rising somewhat to constrict the air flow. However, and you’ll notice this if you whisper “oh, oo, ee, oo, oh” to yourself, O and U are mainly the BACK of your tongue rising, leaving a bunch of space in front of your tongue for resonance (whereas “eee” has almost the whole tongue coming up and tightening away all the space in your mouth, comparatively). AND, importantly, O and U are given EXTRA space at the front of the mouth by lip rounding, which extends the vocal tract even further and provides even more space for resonance, another feature Eee doesn’t have. What’s more connotative of “space and resonance” – sharpness and edges, or roundness and circles?

Tldr: it’s about the amount of relaxed resonating space vs tight constriction left in your mouth by your tongue. Eee = very tight and constricted, which connotes sharpness and edges more. O/u = more space and resonance, which connotes roundness and circles more.

I hope that helps explain the association a little bit more 💜 source: degree in speech language pathology with undergrad study in linguistics