pronunciation - IPA for ふーん/んふふ pronounced with your mouth closed

I know that ふ is normally pronounced as [ɸɯ]. But ふ is sometimes used to represent a clearly different sound that occasionally appears in spoken Japanese, and I'm having trouble identifying its IPA representation.

The following is what I know about this sound:

• It does sound like the h consonant to me, but it can be produced with your mouth completely closed.



• No normal Japanese word contains this sound, but a few interjections and onomatopoeias like ふーん, ふふんっ, んふふ, ふっふっふ, ふがふが are often pronounced with this "sound".


• It's the very first sound in this video. It's written as ふーん but it's obviously not [ɸɯ].


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  Japanese Wikipedia suggests this sound is 無声鼻腔摩擦音 ("voiceless nasal fricative"?).

  
  

  
  

  無声鼻腔摩擦音を「ふ」で表すことがある（例：「ふん」と鼻で笑う）。

  
  

  
  

  But the article gives no IPA representation, and this term appears nowhere else in Wikipedia.

  
  


• I think English "hmm" and "humph" have a similar sound, and some sources including this suggest this sound is [m̥] (the voiceless version of [m]). But other sources including this and this seem to suggest [m̥] is a rare consonant.

So what is this sound in IPA? How do you transcribe ふーん and んふふ said with your mouth closed?

Answer

When we say //ha// or //ho//, the actual phonetic realization of the //h// sound is the same as the following vowel, but voiceless. In other words, we could potentially choose to transcribe [[hɑ]] as [[ɑ̥ɑ]], with a ring diacritic under the first vowel to show that it's been devoiced.

Likewise, English hmm //hm// can be transcribed phonetically as [[m̥m̩]]. The English interjection uh-huh //mhm// is typically pronounced [[ʔm̩m̥m̩]] when your mouth is closed, with a short voiceless period separating the two syllabic nasals. If you pronounce this word slowly, you'll notice that you're making the same sound, just stopping and starting the vibration of your vocal folds partway through. Aside from the opening and closing of the vocal folds, nothing else is changing, so it's hard to say that [[m̥]] is an incorrect transcription.

As long as we're talking specifically about interjections, [[m̥]] is not really such a rare sound, but then, interjections are special in many ways. In Japanese, the interjection うん //ɴ// is typically pronounced [[ʔɴ̩ː]] or [[ʔm̩ː]], but outside of interjections Japanese words are usually considered not to begin with //ɴ// due to phonotactic constraints. This is another way that interjections are special.

I see no reason not to transcribe the sound in question as [[m̥]] if your lips are closed, as in ふーん [[m̥m̩ːː]] and んふふ [[ʔm̩ːm̥m̩m̥m̩]].