Nasalization

In phonetics, nasalization (or nasalisation) is the production of a sound while the velum is lowered, so that some air escapes through the nose during the production of the sound by the mouth. Examples of archetypal nasal sounds include and.

In the International Phonetic Alphabet, nasalization is indicated by printing a tilde diacritic above the symbol for the sound to be nasalized:  is the nasalized equivalent of, and  is the nasalized equivalent of. A subscript diacritic, called an ogonek or nosinė, is sometimes seen, especially when the vowel bears tone marks that would interfere with the superscript tilde. For example, are more legible in most fonts than.

Nasal vowels
Many languages have nasal vowels to different degrees, but only a minority of world languages around the world have nasal vowels as contrasting phonemes. That is the case, among others, of French, Portuguese, Hindustani, Nepali, Breton, Gheg Albanian, Hmong, Hokkien, Yoruba, and Cherokee. Those nasal vowels contrast with their corresponding oral vowels. Nasality is usually seen as a binary feature, although surface variation in different degrees of nasality caused by neighboring nasal consonants has been observed.

Degree of nasality
There are languages, such as in Palantla Chinantec, where vowels seem to exhibit three contrastive degrees of nasality: oral e.g. vs lightly nasalized  vs heavily nasalized,  although Ladefoged and Maddieson believe that the lightly nasalized vowels are best described as oro-nasal diphthongs. Note that Ladefoged and Maddieson's transcription of heavy nasalization with a double tilde might be confused with the extIPA adoption of that diacritic for velopharyngeal frication.

Nasal consonants
By far the most common nasal sounds are nasal consonants such as, or. Most nasal consonants are occlusives, and airflow through the mouth is blocked and redirected through the nose. Their oral counterparts are the stops.

Nasalized consonants
Nasalized versions of other consonant sounds also exist but are much rarer than either nasal occlusives or nasal vowels. The Middle Chinese consonant 日 ( in modern Standard Chinese) has an odd history; for example, it has evolved into and  (or  and  respectively, depending on accents) in Standard Chinese; / and  in Hokkien;  / and / while borrowed into Japan. It seems likely that it was once a nasalized fricative, perhaps a palatal.

In Coatzospan Mixtec, fricatives and affricates are nasalized before nasal vowels even when they are voiceless. In the Hupa, the velar nasal often has the tongue not make full contact, resulting in a nasalized approximant,. That is cognate with a nasalized palatal approximant in other Athabaskan languages.

In Umbundu, phonemic contrasts with the (allophonically) nasalized approximant  and so is likely to be a true fricative rather than an approximant. In Old and Middle Irish, the lenited $⟨m⟩$ was a nasalized bilabial fricative.

Ganza has a phonemic nasalized glottal stop while Sundanese has it allophonically; nasalized stops can occur only with pharyngeal articulation or lower, or they would be simple nasals. Nasal flaps are common allophonically. Many West African languages have a nasal flap (or ) as an allophone of  before a nasal vowel; voiced retroflex nasal flaps are common intervocalic allophones of  in South Asian languages.

A nasal trill has been described from some dialects of Romanian, and is posited as an intermediate historical step in rhotacism. However, the phonetic variation of the sound is considerable, and it is not clear how frequently it is actually trilled. Some languages contrast like Toro-tegu Dogon and Inor. A nasal lateral has been reported for some languages, Nzema language contrasts.

Other languages, such as the Khoisan languages of Khoekhoe and Gǀui, as well as several of the !Kung languages, include nasal click consonants. Nasal clicks are typically with a nasal or superscript nasal preceding the consonant (for example, velar-dental ⟨ŋ͡ǀ⟩ or ⟨ᵑǀ⟩ and uvular-dental ⟨ɴ͡ǀ⟩ or ⟨ᶰǀ⟩). Nasalized laterals such as (a nasalized lateral alveolar click) are easy to produce but rare or nonexistent as phonemes; nasalized lateral clicks are common in Southern African languages such as Zulu. Often when is nasalized, it becomes.

True nasal fricatives
Besides nasalized oral fricatives, there are true nasal fricatives, or anterior nasal fricatives, previously called nareal fricatives. They are sometimes produced by people with disordered speech. The turbulence in the airflow characteristic of fricatives is produced not in the mouth but at the anterior nasal port, the narrowest part of the nasal cavity. (Turbulence can also be produced at the posterior nasal port, or velopharyngeal port, when that port is narrowed – see velopharyngeal fricative. With anterior nasal fricatives, the velopharyngeal port is open.) A superimposed homothetic sign that resembles a colon divided by a tilde is used for this in the extensions to the IPA: is a voiced alveolar nasal fricative, with no airflow out of the mouth, and  is the voiceless equivalent;  is an oral fricative with simultaneous nasal frication. No known language makes use of nasal fricatives in non-disordered speech.

Denasalization
Nasalization may be lost over time. There are also denasal sounds, which sound like nasals spoken with a head cold. They may be found in non-pathological speech as a language loses nasal consonants, as in Korean.

Contextual nasalization
Vowels assimilate to surrounding nasal consonants in many languages, such as Thai, creating nasal vowel allophones. Some languages exhibit a nasalization of segments adjacent to phonemic or allophonic nasal vowels, such as Apurinã.

Contextual nasalization can lead to the addition of nasal vowel phonemes to a language. That happened in French, most of whose final consonants disappeared, but its final nasals made the preceding vowels become nasal, which introduced a new distinction into the language. An example is vin blanc white wine, ultimately from Latin vinum and blancum.