The other day I learnt the Japanese word for ‘reception’ (as in ‘cellphone reception’) - 電波 denpa/dempa. The two kanji that make up this word are 電 den ‘electricity’ and 波 ha ‘wave’. Naturally, its primary meaning is ‘electromagnetic waves’.

What struck me as odd about this word is that the second morpheme [ha] becomes [pa] in the compound. Had it become [ba], I wouldn’t have thought anything of it. It would simply a case of rendaku (sequential voicing), a topic I covered several weeks ago.

In this case, however, the /​p/ from Old Japanese pronunciation of 波 appears, but doesn’t undergo rendaku to become [b].

Checking the Japanese-English dictionary jisho.org, turned up the following compounds ending in 波¹:

• 電波 - dempa ‘electromagnetic wave; radio wave; reception; signal’
• 脳波 - nōha ‘brain waves’
• 寒波 - kampa ‘cold wave’
• 電磁波 - denjiha ‘electromagnetic waves’
• 衝撃波 - shōgekiha ‘shock wave’
• 余波 - yoha ‘waves that remain after the wind has subsided’
• 短波 - tampa ‘short wave’

Note how there’s an alternation between [h] and [p], but never [b]. Even more interestingly, [p] only ever occurs following a nasal. If you were to predict any sound change with these words, it would be h > b after a preceding vowel, but this never happens.　The kanji learning site WaniKani, incorrectly identifies the sound change here as rendaku. In reality, it shares none of the hallmarks of rendaku, i.e. there’s no sequential voicing and it never occurs after a vowel.

Curiously, Japanese has another set of words which behaves similarly: numbers suffixed with 分 fun, the counter for minutes. As in:

• 三分 - sampun ‘three minutes’
• 四分 - yompun ‘four minutes’
• 何分 - nampun ‘how many minutes’

But not in:

• 二分 - nifun ‘two minutes’
• 五分 - gofun ‘five minutes’

(Remember, ふ /​fu/ actually belongs to the set of h-sounds in Japanese (despite being realised as [ɸ]) so the sound change is analogous.)

To be honest, I have no idea what’s going on in these examples. I would guess that modern words like 電波 dempa ‘electromagnetic waves’ are applying the irregular rule of much older words like 寒波 kampa ‘cold waves’ by analogy. But why does [p] crop up after nasals? And why does rendaku never occur after vowels? Looks like it’s time for some reading…