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 totally 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].
Japanese Wikipedia suggests this sound is 無声鼻腔摩擦音 ("voiceless nasal fricative"?).
But the article does not explain about this sound in detail.
- 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?