How are custom onomatopoeia made?

Question

I am currently reading a recently released remake of 月姫, and I encountered the following sentence:

指で触ってみたら、つぷり、と指先が沈みこんだ

The context is that the character can see lines that no one else can, and he is describing here what happens if he tries touching them.

I assume its supposed to be the sound that is made, but I can't find anything about it, so I assume this is one is made by the author.

What kind of rules exist when creating these and understanding what they are supposed to indicate?

Also, as a minor thing, for what reason would the author put a comma between the sound and と? It looks odd to me.

Answer 1

As for creating onomatopoeia in general, I don't think there are anything clear enough that can be called rules. On rare occasions when they are made, they are most probably based on existing ones + super fuzzy associations between sounds and actions or objects, in the following sense. They may well be based on the sounds that the onomatopoeia describes, but how they sound (and how they are transcribed) are likely to be affected by existing onomatopoeia.

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The rest is about つぷり in particular.

For me, つぷり sounds like you are putting something into water which is kind of thick and has honey-like tension on the surface. You don't feel any real push back, but do feel you touch something, then push the finger inside. The following at least partially explains why I understand it that way.

Apparently there is research on what I called association above. The following table is from this article.

つぷ has t as C1 and p as C2, so following this table, it describes something with weak tension (弱い張力) is breaking/bursting （破裂）.

Of course, it is not that we think in terms of these individual sounds, but the above 'bursting of something with weak tension' is not too far from the impression I get to see つぷり.

つぷり may not be completely new, as naruto says, but is not very common either. Still we see how it sounds because of the kind of associations given above, and existing onomatopoeia: ちゃぷ（putting slowly something into water), ぽちゃ（dropping something in water), どぼん（dropping with splash). Note all have t,d and b,p sounds like many others related to water.

Another thing regarding why つ (instead of other タ行 character): An onomatopoeia used for moving the tip of your finger along some surface (like swiping on iPad) is つー(っ).

Answer 2

つぷ (or つぷり, つぷっ) is not a new or rare onomatopoeia at all. You can google it (results may contain materials that are NSFW). It's basically a lighter variant of ずぶ (or ずぶり, ずぶずぶ, ずぶっ).

Only a very few creative writers try to make totally new onomatopoeias in their works, and there is no "rule" for such creative activities. Most can be understood as variations of existing ones (e.g., わきゃわきゃ). Mangaka may tend to play on onomatopoeias more often because they can explain the situation using a picture. This page introduces some unique onomatopoeias and how they were made.

Rules around commas are generally fairly loose in Japanese, and it's not uncommon to insert a comma before と. Here, that comma may help the onomatopoeia stand out from its surroundings. They could have used brackets (触れてみたら「つぷり」と) as well, but commas are less distracting.