True nasal fricatives, formerly known as nareal fricatives, are fricatives whose airflow is only through the nasal cavity. In extIPA, they are notated by [ ͋], e.g. [m͋].
That would mean the mouth is closed, making a nasal stop. If both voicing and strong airflow are present, that means the phonation is breathy. Does this mean [m͋]≡[m̤]?
I'm curious about this because, my native language is Korean, and I tend to mash [ㅁㅎ] ([m.ɦ]) together to one consonant. I would describe this consonant as above, but I'm unsure how this should be transcribed to (ext)IPA. This also happens to [ㄴㅎ] and [ㅇㅎ]. Examples include 엄호, 난해, 몽환.