Denasalization
In phonetics, denasalization is the loss of nasal airflow in a nasal sound, such as a nasal consonant or a nasal vowel. That may be due to speech pathology but also occurs when the sinuses are blocked from a common cold, when it is called a nasal voice, which is not a linguistic term. The symbol in the Extended IPA is ⟨◌͊⟩.
When one speaks with a cold, the nasal passages still function as a resonant cavity so a denasalized nasal [m͊] does not sound like a voiced oral stop [b], and a denasalized vowel [a͊] does not sound like an oral vowel [a].
However, there are cases of historical or allophonic denasalization that have produced oral stops. In some languages with nasal vowels, such as Paicĩ, nasal consonants may occur only before nasal vowels; before oral vowels, prenasalized stops are found. That allophonic variation is likely to be from a historical process of partial denasalization.
Similarly, several languages around Puget Sound underwent a process of denasalization about 100 ago. Except in special speech registers, such as baby talk, the nasals [m, n] became the voiced stops [b, d]. It appears from historical records that there was an intermediate stage in which the stops were prenasalized stops [ᵐb, ⁿd] or poststopped nasals [mᵇ, nᵈ].
Something similar has occurred with word-initial nasals in Korean; in some contexts, /m/, /n/ are denasalized to [b, d]. The process is sometimes represented with the IPA [m͊] and [n͊], which simply places the IPA ◌͊ denasalization diacritic on [m] and [n] to show the underlying phoneme.