Why is there no rendaku occurring in the word ふかふか?


Why isn't the second `ふか` changed to `ぶか` in `ふかふか`; thus, becoming `ふかぶか`? I
would expect to see `ふかぶか` instead of `ふかふか` for the same reason that `ひとびと`
is `ひとびと`, not `ひとひと`. There is no voicing; thus, Lyman's Law cannot be
suppressing it.




It is impossible to explain exactly when
[[連濁]{れんだく}](https://en.wikipedia.org/wiki/Rendaku) happens and when it
doesn't. The linked Wikipedia article also explains:

> In modern Japanese, rendaku is common but at times unpredictable, with
> certain words unaffected by it.

The way I understand it is it happens when two kanji words are combined. For
example from the above link:

> [人]{ひと} + [人]{ひと} → [人々]{ひとびと}
>
> hito + hito → hitobito ("person" + "person" → "people")
>
> [時]{とき} + [時]{とき} → [時時]{ときどき}
>
> toki + toki → tokidoki ("time" + "time" → "sometimes")

However, words such as 'ふかふか' from symbolism ([象徴音]{しょうちょうおん}) don't have
corresponding kanjis and [連濁]{れんだく} doesn't occur in such a case.

The link further explains:

> Rendaku thus remains partially unpredictable, sometimes presenting a problem
> even to native speakers, particularly in [Japanese
> names](https://en.wikipedia.org/wiki/Japanese_name), where rendaku occurs or
> fails to occur often without obvious cause. In many cases, an identically
> written name may either have or not have rendaku, depending on the person.
> For example, 中田 may be read in a number of ways, including both Nakata and
> Nakada.




As the other answers point out, _rendaku_ doesn't occur in mimetic or
onomatopoeic words. The basis is always a particular sound (usually two
morae), e.g.

> ふわ

with a particular quality. From this sound the most common
mimetic/onomatopoeic words are those obtained by doubling the sound, e.g.

> ふわふわ

but in many cases there is also a common corresponding _to_ -adverb, e.g.

> ふわっと

**If _rendaku_ were to occur, it would change the quality of the sound.**

In fact, for ふか → ふかふか "fluffy", there is also ぶか → ぶかぶか with a completely
different meaning of "too big" or "baggy" [clothes].

