Synesthesia isn't about associating numbers, musical notes or other things with COLORS. It's associating any one collection of things of the the same type (say, days of the week) with pretty much any random set of objects, forms, colors, shapes or whatnot. I read the article and then all the comments here and it appears to me that most people are collectively blind to the fact that it isn't just about colors [1][2].
UPDATE. A thought I'd just had: it appears very intuitive that most people do have synesthesia, they're just not consciously aware of it, or, sometimes, don't even know what to call it.
> UPDATE. A thought I'd just had: it appears very intuitive that most people do have synesthesia, they're just not consciously aware of it, or, sometimes, don't even know what to call it.
The Wikipedia article is vague, but I've always thought of synesthesia as meaning that a stimulus in one "category" provokes a response in another "category"—Wikipedia and your post seem to say the same thing, but with different, and more technical, terminology in place of "category". More to the point, there seems to be some implication that this is a response that "shouldn't" happen; that there is an appropriate response to the stimulus, and that the actual response isn't the appropriate one.
But it seems that, if, as you suppose, most people have synesthesia, then the problem is one of definitions. If most people have an inappropriate response, then doesn't that mean that the definition of 'appropriate' is wrong—it's meaningless to say that everyone is abnormal!—so that mass-scale synesthesia must, if it exists, indicate nothing unusual per se about synesthetes, but rather a poor mapping on the part of diagnosticians?
I think almost everyone has associations between things of different "categories" (you can almost smell the grass if you picture yourself in a field). As a synesthete, I have both kinds of associations (synesthetic, and non-), and many lie in the spectrum between.
The strongly synesthetic associations are 1) always-on*, 2) automatic and immediate, even for novel stimuli**, and 3) consistent over time***
* You can turn your focus away from them, like focusing on a person's voice in a noisy room, but you're still aware of the noise.
** You won't experience the smell of grass looking at a photo of a field, if you've never been to a field before. But many synesthetic associations occur immediately even for new stimuli, especially if it's a simple input like sound. If it's a complex concept like someone's personality or an algorithm, they are initially in flux and solidify as understanding of the concept improves.
*** They can change very slowly over time (years)
The above is more of a personal definition from my experience, and not a clinically accepted one.
This is correct, I see visuals when I hear sounds. The textures and shapes correspond to the type of sound. For instance, a clean guitar has a different texture than a distorted guitar. Same for people's voices.
Came here to say that I experience similar, and my career is centred around audio. When people talk about "brighter" and "darker" sounds, that's very much a tangible concept to me and not some abstract thing. Different sounds have different colours, and my DAW templates are always based around this. Any gear that uses LEDs that can't be reassigned is a nightmare for me as it's visually overwhelming.
Funnily enough, I do a fair bit of scoring picture and games, and I find that the scene informs what kind of music I'll make. It's not painting-by-numbers though, but it's like sharing synaesthesia in a tangible form.
UPDATE. A thought I'd just had: it appears very intuitive that most people do have synesthesia, they're just not consciously aware of it, or, sometimes, don't even know what to call it.
[1] Wikipedia article: https://en.wikipedia.org/wiki/Synesthesia
[2] Cynically explained by Rustin Cohle: https://www.youtube.com/watch?v=FaYiq9x7odE