Acoustically, nasal stops are sonorants, meaning they do not restrict the escape of air and cross-linguistically are nearly always voiced. A notable exception is Icelandic which has four unvoiced nasal sounds. (Compare oral plosives, which block off the air completely, and fricatives, which obstruct the air with a narrow channel. Both stops and fricatives are more commonly voiceless than voiced, and are known as obstruents.)
However, nasals are also stops in their articulaton because the flow of air through the mouth is blocked completely. This duality, a sonorant airflow through the nose along with an obstruction in the mouth, means that nasal stops behave both like sonorants and like obstruents. For the purposes of acoustic description they are generally considered sonorants, but in many languages they may develop from or into plosives.
Acoustically, nasal stops have bands of energy at around 200 and 2,000 Hz.
|[m]||voiced bilabial nasal|
|[ɱ]||voiced labiodental nasal||[F]|
|[n]||alveolar or dental nasal: see alveolar nasal|
|[ɳ]||voiced retroflex nasal, common in Indic languages||[n`]|
|[ɲ]||voiced palatal nasal, a common sound in European languages as in: Spanish ñ; or French and Italian gn; or Catalan, Hungarian and Luganda ny; or Occitan and Portuguese nh.||[J]|
|[ŋ]||voiced velar nasal, commonly written ng.||[N]|
|[ɴ]||voiced uvular nasal||[N]|
Examples of languages containing nasal consonants:
Catalan, Occitan, Spanish, and Italian have [m], [n], [ɲ] as phonemes, and [ɱ] and [ŋ] as allophones. (In several American dialects of Spanish, there is no palatal nasal, but only a palatalized nasal, [nʲ], as in English canyon.)
The term 'nasal stop' will often be abbreviated to just "nasal". However, there are also nasal fricatives, nasal flaps, and nasal vowels, as in French, Portuguese, Catalan (dialectal feature), Yoruba, Gbe, Polish, and Ljubljana Slovene. In the IPA, nasal vowels are indicated by placing a tilde (~) over the vowel in question: French sang [sɑ̃].
However, several of the Chimakuan, Salish, and Wakashan languages surrounding Puget Sound, such as Quileute, Lushootseed, and Makah, are truly without any nasalization at all, in consonants or vowels, except in special speech registers such as baby-talk or the archaic speech of mythological figures (and perhaps not even that in the case of Quileute). This is an areal feature, only a few hundred years old, where nasal stops became voiced plosives ([m] became [b], etc). The only other places in the world where this occurs is in a dialect of the Rotokas language of Papua New Guinea, where nasal stops are only used when imitating foreign accents (a second dialect does have nasal stops), and in some of the Lakes Plain languages of West Papua.