Fricatives are consonants produced by forcing air through a narrow channel made by placing two articulators close together. These may be the lower lip against the upper teeth, in the case of [f]; the back of the tongue against the soft palate, in the case of German [x] (the final consonant of Bach); or the side of the tongue against the molars, in the case of Welsh [ɬ] (appearing twice in the name Llanelli). This turbulent airflow is called friction.
A particular subset of fricatives are the sibilants. When forming a sibilant, one still is forcing air through a narrow channel, but in addition, the tongue is curled lengthwise to direct the air over the edge of the teeth. English [s], [z], [ʃ], and [ʒ] are examples of sibilants.
The usage of two other terms is less standardized: "Spirant" is an older term for fricatives used by some American and European phoneticians and phonologists. "Strident" could mean just "sibilant", but some authors[who?] include also labiodental and uvular fricatives in the class.
All sibilants are coronal, but may be dental, alveolar, postalveolar, or palatal (retroflex) within that range. However, at the postalveolar place of articulation, the tongue may take several shapes: domed, laminal, or apical, and each of these is given a separate symbol and a separate name. Prototypical retroflexes are subapical and palatal, but they are usually written with the same symbol as the apical postalveolars. The alveolars and dentals may also be either apical or laminal, but this difference is indicated with diacritics rather than with separate symbols.
with allophonic trilling, but these might be better analyzed as pharyngeal trills. 
The lateral fricative occurs as the ll of Welsh, as in Lloyd, Llewelyn, and Machynlleth ([maˈxənɬɛθ], a town), as the unvoiced 'hl' and voiced 'dl' or 'dhl' in the several languages of Southern Africa (such as Xhosa and Zulu), and in Mongolian.
No language distinguishes voiced fricatives from approximants at these places, so the same symbol is used for both. For the pharyngeal, approximants are more numerous than fricatives. A fricative realization may be specified by adding the uptack to the letters, [ʁ̝, ʕ̝]. Likewise, the downtack may be added to specify an approximant realization, [ʁ̞, ʕ̞].
(The bilabial approximant and dental approximant do not have dedicated symbols either and are transcribed in a similar fashion: [β̞, ð̞]. However, the base letters are understood to specifically refer to the fricatives.)
In many languages, such as English, the glottal "fricatives" are unaccompanied phonation states of the glottis, without any accompanying manner, fricative or otherwise. However, in languages such as Arabic, they are true fricatives.[page needed]
In addition, [ʍ] is usually called a "voiceless labial-velar fricative", but it is actually an approximant. True doubly articulated fricatives may not occur in any language; but see voiceless palatal-velar fricative for a putative (and rather controversial) example.
Fricatives are very commonly voiced, though cross-linguistically voiced fricatives are not nearly as common as tenuis ("plain") fricatives. Other phonations are common in languages that have those phonations in their stop consonants. However, phonemically aspirated fricatives are rare. [sʰ] contrasts with [s] in Korean; aspirated fricatives are also found in a few Sino-Tibetan languages, in some Oto-Manguean languages, in the Siouan language Ofo (/sʰ/ and /fʰ/), and in the (central?) Chumash languages (/sʰ/ and /ʃʰ/). The record may be Cone Tibetan, which has four contrastive aspirated fricatives: /sʰ/ /ɕʰ/, /ʂʰ/, and /xʰ/.
Phonemically nasalized fricatives are rare. Some South Arabian languages have /z̃/, Umbundu has /ṽ/, and Kwangali and Souletin Basque have /h̃/. In Coatzospan Mixtec, [β̃, ð̃, s̃, ʃ̃] appear allophonically before a nasal vowel, and in Igbo nasality is a feature of the syllable; when /f v s z ʃ ʒ/ occur in nasal syllables they are themselves nasalized.
Until its extinction, Ubykh may have been the language with the most fricatives (29 not including /h/), some of which did not have dedicated symbols or diacritics in the IPA. This number actually outstrips the number of all consonants in English (which has 24 consonants). By contrast, approximately 8.7% of the world's languages have no phonemic fricatives at all. This is a typical feature of Australian Aboriginal languages, where the few fricatives that exist result from changes to plosives or approximants, but also occurs in some indigenous languages of New Guinea and South America that have especially small numbers of consonants. However, whereas [h] is entirely unknown in indigenous Australian languages, most of the other languages without true fricatives do have [h] in their consonant inventory.
Voicing contrasts in fricatives are largely confined to Europe, Africa, and Western Asia. Languages of South and East Asia, such as Mandarin Chinese, Korean, the Dravidian and Austronesian languages, typically do not have such voiced fricatives as [z] and [v], which are familiar to many European speakers. These voiced fricatives are also relatively rare in indigenous languages of the Americas. Overall, voicing contrasts in fricatives are much rarer than in plosives, being found only in about a third of the world's languages as compared to 60 percent for plosive voicing contrasts.
About 15 percent of the world's languages, however, have unpaired voiced fricatives, i.e. a voiced fricative without a voiceless counterpart. Two-thirds of these, or 10 percent of all languages, have unpaired voiced fricatives but no voicing contrast between any fricative pair.
This phenomenon occurs because voiced fricatives have developed from lenition of plosives or fortition of approximants. This phenomenon of unpaired voiced fricatives is scattered throughout the world, but is confined to nonsibilant fricatives with the exception of a couple of languages that have [ʒ] but lack [ʃ]. (Relatedly, several languages have the voiced affricate [dʒ] but lack [tʃ], and vice versa.) The fricatives that occur most often without a voiceless counterpart are – in order of ratio of unpaired occurrences to total occurrences – [ʝ], [β], [ð], [ʁ] and [ɣ].
Fricatives appear in waveforms as random noise caused by the turbulent airflow, upon which a periodic pattern is overlaid if voiced. Fricatives produced in the front of the mouth tend to have energy concentration at higher frequencies than ones produced in the back. The centre of gravity, the average frequency in a spectrum weighted by the amplitude, may be used to determine the place of articulation of a fricative relative to that of another.