In phonetics, sibilants are fricative consonants of higher amplitude and pitch, made by directing a stream of air with the tongue towards the teeth.[1] Examples of sibilants are the consonants at the beginning of the English words sip, zip, ship, and genre. The symbols in the International Phonetic Alphabet used to denote the sibilant sounds in these words are, respectively, [s] [z] [ʃ] [ʒ]. Sibilants have a characteristically intense sound, which accounts for their paralinguistic use in getting one's attention (e.g. calling someone using "psst!" or quieting someone using "shhhh!").

In the alveolar hissing sibilants [s] and [z], the back of the tongue forms a narrow channel (is grooved) to focus the stream of air more intensely, resulting in a high pitch. With the hushing sibilants (occasionally termed shibilants), such as English [ʃ], [tʃ], [ʒ], and [dʒ], the tongue is flatter, and the resulting pitch lower.[2][3]

A broader category is stridents, which include more fricatives than sibilants such as uvulars. Because all sibilants are also stridents, the terms are sometimes used interchangeably. However, the terms do not mean the same thing. The English stridents are /f, v, s, z, ʃ, ʒ, tʃ, dʒ/. Sibilants are a higher pitched subset of the stridents. The English sibilants are /s, z, ʃ, ʒ, tʃ, dʒ/. On the other hand, /f/ and /v/ are stridents, but not sibilants, because they are lower in pitch.[4][5][6]

"Stridency" refers to the perceptual intensity of the sound of a sibilant consonant, or obstacle fricatives or affricates, which refers to the critical role of the teeth in producing the sound as an obstacle to the airstream. Non-sibilant fricatives and affricates produce their characteristic sound directly with the tongue or lips etc. and the place of contact in the mouth, without secondary involvement of the teeth[citation needed].

The characteristic intensity of sibilants means that small variations in tongue shape and position are perceivable, with the result that there are many sibilant types that contrast in various languages.

Sibilants are louder than their non-sibilant counterparts, and most of their acoustic energy occurs at higher frequencies than non-sibilant fricatives—usually around 8,000 Hz.[7]

All sibilants are coronal consonants (made with the tip or front part of the tongue). However, there is a great deal of variety among sibilants as to tongue shape, point of contact on the tongue, and point of contact on the upper side of the mouth.

The following variables affect sibilant sound quality, and, along with their possible values, are ordered from sharpest (highest-pitched) to dullest (lowest-pitched):

Generally, the values of the different variables co-occur so as to produce an overall sharper or duller sound. For example, a laminal denti-alveolar grooved sibilant occurs in Polish, and a subapical palatal retroflex sibilant occurs in Toda.

The main distinction is the shape of the tongue. Most sibilants have a groove running down the centerline of the tongue that helps focus the airstream, but it's not known how widespread this is. In addition, the following tongue shapes are described, from sharpest and highest-pitched to dullest and lowest-pitched:

The latter three post-alveolar types of sounds are often known as "hushing" sounds because of their quality, as opposed to the "hissing" alveolar sounds. The alveolar sounds in fact occur in several varieties, in addition to the normal sound of English s:

Speaking non-technically, the retroflex consonant [ʂ] sounds somewhat like a mixture between the regular English [ʃ] of "ship" and a strong American "r"; while the alveolo-palatal consonant [ɕ] sounds somewhat like a mixture of English [ʃ] of "ship" and the [sj] in the middle of "miss you".

Sibilants can be made at any coronal articulation, i.e. the tongue can contact the upper side of the mouth anywhere from the upper teeth (dental) to the hard palate (palatal), with the in-between articulations being denti-alveolar, alveolar and postalveolar.

The tongue can contact the upper side of the mouth with the very tip of the tongue (an apical articulation, e.g. [ʃ̺]); with the surface just behind the tip, called the blade of the tongue (a laminal articulation, e.g. [ʃ̻]); or with the underside of the tip (a subapical articulation). Apical and subapical articulations are always tongue-up, with the tip of the tongue above the teeth, while laminal articulations can be either tongue-up or tongue-down, with the tip of the tongue behind the lower teeth. This distinction is particularly important for retroflex sibilants, because all three varieties can occur, with noticeably different sound qualities. For more information on these variants and their relation to sibilants, see the article on postalveolar consonants.

For tongue-down laminal articulations, an additional distinction can be made depending on where exactly behind the lower teeth the tongue tip is placed. A little ways back from the lower teeth is a hollow area (or pit) in the lower surface of the mouth. When the tongue tip rests in this hollow area, there is an empty space below the tongue (a sublingual cavity), which results in a relatively duller sound. When the tip of the tongue rests against the lower teeth, there is no sublingual cavity, resulting in a sharper sound. Usually, the position of the tip of the tongue correlates with the grooved vs. hushing tongue shape so as to maximize the differences. However, the palato-alveolar sibilants in the Northwest Caucasian languages such as Ubykh are an exception. These sounds have the tongue tip resting directly against the lower teeth, which gives the sounds a quality that Catford describes as "hissing-hushing". Ladefoged and Maddieson[1] term this a "closed laminal postalveolar" articulation, and transcribe them (following Catford) as [ŝ, ẑ], although this is not an IPA notation. See the article on postalveolar consonants for more information.

The following table shows the types of sibilant fricatives defined in the International Phonetic Alphabet:

Diacritics can be used for finer detail. For example, apical and laminal alveolars can be specified as [s̺] vs [s̻]; a dental (or more likely denti-alveolar) sibilant as [s̪]; a palatalized alveolar as [sʲ]; and a generic "retracted sibilant" as [s̠], a transcription frequently used for the sharper-quality types of retroflex consonants (e.g. the laminal "flat" type and the "apico-alveolar" type). There is no diacritic to denote the laminal "closed" articulation of palato-alveolars in the Northwest Caucasian languages, but they are sometimes provisionally transcribed as [ŝ ẑ].

The attested possibilities, with exemplar languages, are as follows. Note that the IPA diacritics are simplified; some articulations would require two diacritics to be fully specified, but only one is used in order to keep the results legible without the need for OpenType IPA fonts. Also, Ladefoged has resurrected an obsolete IPA symbol, the under dot, to indicate apical postalveolar (normally included in the category of retroflex consonants), and that notation is used here. (Note that the notation s̠, ṣ is sometimes reversed; either may also be called 'retroflex' and written ʂ.)

^1 ŝ ẑ is an ad-hoc transcription. The old IPA letters ʆ ʓ are also available.

^2 These sounds are usually just transcribed ʂ ʐ. Apical postalveolar and subapical palatal sibilants do not contrast in any language, but if necessary, apical postalveolars can be transcribed with an apical diacritic, as s̠̺ z̠̺ or ʂ̺ ʐ̺. Ladefoged resurrects the old retroflex sub-dot for apical retroflexes, ṣ ẓ Also seen in the literature on e.g. Hindi and Norwegian is ᶘ ᶚ – the domed articulation of [ʃ ʒ] precludes a subapical realization.

Whistled sibilants occur in speech pathology and may be caused by dental prostheses or orthodontics. However, they also occur phonemically in several southern Bantu languages, the best known being Shona.

The whistled sibilants of Shona have been variously described—as labialized but not velarized, as retroflex, etc., but none of these features are required for the sounds.[10] Using the Extended IPA, Shona sv and zv may be transcribed and . Other transcriptions seen include purely labialized and (Ladefoged and Maddieson 1996) and labially co-articulated sᶲ and zᵝ (or s͡ɸ and z͜β). In the otherwise IPA transcription of Shona in Doke (1967), the whistled sibilants are transcribed with the non-IPA letters ȿ ɀ and tȿ dɀ.

Besides Shona, whistled sibilants have been reported as phonemes in Kalanga, Tsonga, Changana, Tshwa—all of which are Southern African languages—and Tabasaran. The articulation of whistled sibilants may differ between languages. In Shona, the lips are compressed throughout, and the sibilant may be followed by normal labialization upon release. (That is, there is a contrast among s, sw, ȿ, ȿw.) In Tsonga, the whistling effect is weak; the lips are narrowed but also the tongue is retroflex. Tswa may be similar. In Changana, the lips are rounded (protruded), but so in /s/ in the sequence /usu/, so there is evidently some distinct phonetic phenomenon occurring here that has yet to be formally identified and described.[11]

Not including differences in manner of articulation or secondary articulation, some languages have as many as four different types of sibilants. For example, Northern Qiang and Southern Qiang have a four-way distinction among sibilant affricates /ts/ /tʂ/ /tʃ/ /tɕ/, with one for each of the four tongue shapes.[citation needed] Toda also has a four-way sibilant distinction, with one alveolar, one palato-alveolar, and two retroflex (apical postalveolar and subapical palatal).[citation needed]

The now-extinct Ubykh language was particularly complex, with a total of 27 sibilant consonants. Not only all four tongue shapes were represented (with the palato-alveolar appearing in the laminal "closed" variation) but also both the palato-alveolars and alveolo-palatals could additionally appear labialized. Besides, there was a five-way manner distinction among voiceless and voiced fricatives, voiceless and voiced affricates, and ejective affricates. (The three labialized palato-alveolar affricates were missing, which is why the total was 27, not 30.)[citation needed] The Bzyp dialect of the related Abkhaz language also has a similar inventory.[citation needed]

Some languages have four types when palatalization is considered. Polish is one example, with both palatalized and non-palatalized laminal denti-alveolars, laminal postalveolar (or "flat retroflex"), and alveolo-palatal ([s̪ z̪] [s̪ʲ z̪ʲ] [s̠ z̠] [ɕ ʑ]).[citation needed] Russian has the same surface contrasts, but the alveolo-palatals are arguably not phonemic. They occur only geminate, and the retroflex consonants never occur geminate, which suggests that both are allophones of the same phoneme.[citation needed]

Somewhat more common are languages with three sibilant types, including one hissing and two hushing. As with Polish and Russian, the two hushing types are usually postalveolar and alveolo-palatal since these are the two most distinct from each other. Mandarin Chinese is an example of such a language.[citation needed] However, other possibilities exist. Serbo-Croatian has alveolar, flat postalveolar and alveolo-palatal affricates whereas Basque has palato-alveolar and laminal and apical alveolar (apico-alveolar) fricatives and affricates (late Medieval peninsular Spanish and Portuguese had the same distinctions among fricatives).

Many languages, such as English, have two sibilant types, one hissing and one hushing. A wide variety of languages across the world have this pattern. Perhaps most common is the pattern, as in English, with alveolar and palato-alveolar sibilants. Modern northern peninsular Spanish has a single apico-alveolar sibilant fricative [s̠], as well as a single palato-alveolar sibilant affricate [tʃ]. However, there are also languages with alveolar and apical retroflex sibilants (such as Standard Vietnamese) and with alveolar and alveolo-palatal postalveolars (e.g. alveolar and laminal palatalized [ʃ ʒ tʃ dʒ] i.e. [ʃʲ ʒʲ tʃʲ dʒʲ] in Catalan and Brazilian Portuguese, the latter probably through Amerindian influence,[12] and alveolar and dorsal i.e. [ɕ ʑ cɕ ɟʑ] proper in Japanese).[13]

Only a few languages with sibilants lack the hissing type. Middle Vietnamese is normally reconstructed with two sibilant fricatives, both hushing (one retroflex, one alveolo-palatal). Some languages have only a single hushing sibilant and no hissing sibilant. That occurs in southern Peninsular Spanish dialects of the "ceceo" type, which have replaced the former hissing fricative with [θ], leaving only [tʃ].

Languages with no sibilants are fairly rare. Most have no fricatives at all or only the fricative /h/. Examples include most Australian languages, and Rotokas, and what is generally reconstructed for Proto-Bantu. Languages with fricatives but no sibilants, however, do occur, such as Ukue in Nigeria, which has only the fricatives /f, v, h/. Also, almost all Eastern Polynesian languages have no sibilants but do have the fricatives /v/ and/or /f/: Māori, Hawaiian, Tahitian, Rapa Nui, most Cook Islands Māori dialects, Marquesan, and Tuamotuan.

Tamil only has the sibilant /ʂ/ and fricative /f/ in loanwords, and they are frequently replaced by native sounds. The sibilants [s, ɕ] exist as allophones of /t͡ɕ/ and the fricative [h] as an allophone of /k/.

Authors including Chomsky and Halle group [f] and [v] as sibilants. However, they do not have the grooved articulation and high frequencies of other sibilants, and most phoneticians[1] continue to group them together with bilabial [ɸ], [β] and (inter)dental [θ], [ð] as non-sibilant anterior fricatives. For a grouping of sibilants and [f, v], the term strident is more common. Some researchers judge [f] to be non-strident in English, based on measurements of its comparative amplitude, but to be strident in other languages (for example, in the African language Ewe, where it contrasts with non-strident [ɸ]).

The nature of sibilants as so-called 'obstacle fricatives' is complicated – there is a continuum of possibilities relating to the angle at which the jet of air may strike an obstacle. The grooving often considered necessary for classification as a sibilant has been observed in ultrasound studies of the tongue for the supposedly non-sibilant voiceless alveolar fricative [θ] of English.[14]