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.

  • 1
    Maybe it wouldn't sound fluffy anymore. There's also ふわふわ
    – Darcinon
    Aug 18 '16 at 16:16
  • 6
    Isn't ふかふか sound symbolism? Most sound symbolism doesn't undergo rendaku.
    – Angelos
    Aug 18 '16 at 16:33
  • 1
    Hmm, got any examples of sound symbolisms that DO undergo rendaku?
    – kuchitsu
    Aug 18 '16 at 16:52

It is impossible to explain exactly when [連濁]{れんだく} 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, 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.


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


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


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].


ふかふか is more like an onomatopoeia (the formation of a word form a sound associated with what is named) instead of connecting two words. More examples are:

  • きらきら (shining) [e.g. このダイヤはキラキラしている]
  • はらはら (being nervous) [e.g. ハラハラした]
  • からから (drying) [e.g. のどがカラカラ]
  • かりかり (irritating) [e.g. カリカリしないで]

ひとびと is 人々 so it's connecting two words. if you say ふかぶか then it changes the meaning to 深々 (lit: deeply) (e.g. 深々とおじぎする)

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