In phonetics, a trill is a consonantal sound produced by vibrations between the active articulator and passive articulator. Standard Spanish <rr> as in perro, for example is an alveolar trill.
Trills are very different from flaps. Whereas with a flap (or tap), a specific gesture is used to strike the active articulator against the passive one, in the case of a trill the articulator is held in place, where the airstream causes it to vibrate. Usually a trill vibrates for 2–3 periods, but may be up to 5, or even more if geminate. However, trills may also be produced with only a single period. Although this might seem like a flap, the articulation is different; trills will vary in the number of periods, but flaps do not.
Trill consonants included in the International Phonetic Alphabet:[r] - alveolar trill
[ʙ] - bilabial trill
[ʀ] - uvular trill
[ʜ] - voiceless epiglottal trill
[ʢ] - voiced epiglottal trill
In addition,[ʩ] - velopharyngeal fricative found in disordered speech sometimes involves trilling of the velopharyngeal port, producing a 'snort'.
The bilabial trill is uncommon. The coronal trill is most frequently alveolar [r͇], but dental and postalveolar articulations [r̪] and [r̠] also occur. An alleged retroflex trill found in Toda has been transcribed [ɽ] (that is, the same as the retroflex flap), but might be less ambiguously written [ɽ͡r], as only the onset is retroflex, with the actual trill being alveolar. The epiglottal trills are identified by the IPA as fricatives, with the trilling assumed to be allophonic. However, analyzing the sounds as trills may be more economical. There are also so-called strident vowels which are accompanied by epiglottal trill.
The cells in the IPA chart for the velar, (upper) pharyngeal, and glottal places of articulation are shaded as impossible. (The glottis quite readily vibrates, but this occurs as the phonation of vowels and consonants, not as a consonant of its own.) According to Esling (2010), palatal trills are also implausible. The upper pharyngeal tract cannot reliably produce a trill, but the epiglottis does, and epiglottal trills are pharyngeal in the broad sense. A partially devoiced pre-uvular (i.e. between velar and uvular) fricative trill [ʀ̝̊˖] has been reported to occur as coda allophone of /ʀ/ in Limburgish dialects of Maastricht and Weert. It is in free variation with partially devoiced uvular fricative trill [ʀ̝̊].
Voiceless trills occur phonemically in e.g. Welsh and Icelandic. (See also voiceless alveolar trill, voiceless retroflex trill, voiceless uvular trill.) Mangbetu and Ninde have phonemically voiceless bilabial trills.
The Czech language has two contrastive alveolar trills, one a fricative trill (written ř in the orthography). In the fricative trill the tongue is raised, so that there is audible frication during the trill, sounding a little like a simultaneous [r] and [ʐ] (or [r̥] and [ʂ] when devoiced). A symbol for this sound, [ɼ], has been dropped from the IPA, and it is now generally transcribed as a raised r, [r̝].
Liangshan Yi ("Cool Mountain" Yi) has two "buzzed" or fricative vowels /i̝/, /u̝/ (written ṳ, i̤) which may also be trilled, [ʙ̝], [r̝].
A number of languages have trilled affricates such as [mbʙ] and [dʳ]. The Chapakuran language Wari’ and the Muran language Pirahã have a very unusual trilled phoneme, a voiceless bilabially post-trilled dental stop, [t̪͡ʙ̥].
A nasal trill [r̃] 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.
A linguolabial trill [r̼] is not known to be used phonemically, but occurs when blowing a raspberry.
Snoring typically consists of vibration of the uvula and the soft palate (velum). Although the former part is simply a uvular trill, there is no standard linguistic term for the latter. It does not constitute a velar trill, because the velum is here the active articulator, not the passive; the tongue is not involved at all. (Speculative Grammarian has famously proposed a jocular symbol for this sound (also the sound used to imitate a pig's snort), a double-wide ʘ with double dot, suggesting a pig's snout. The Extensions to the IPA identifies a fricative pronounced with this same configuration as velopharyngeal.)
Lateral trills are also possible. They may be pronounced by initiating [ɬ] or [ɮ] with an especially forceful airflow. There is no symbol for them in the IPA. Lateral coronal trills are sometimes used to imitate bird calls, and are a component of Donald Duck talk. A labiodental trill, [ʙ̪], is most likely to be lateral, but laterality is not distinctive among labial sounds.
Ejective trills are not known from any language, despite being easy to produce. They may occur as mimesis of a cat's purr.