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|Manners of articulation|
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The term sibilant is often taken to be synonymous with the term strident, though historically this is incorrect, and there is variation in usage. The term sibilant tends to have an articulatory or aerodynamic definition involving the production of aperiodic noise at an obstacle. Strident refers to the perceptual quality of intensity as determined by amplitude and frequency characteristics of the resulting sound, i.e. an auditory, or possibly acoustic, definition.
Sibilants are louder than their non-sibilant counterparts, and most of their acoustic energy occurs at higher frequences than non-sibilant fricatives. [s] has the most acoustic strength at around 8,000 Hz, but can reach as high as 10,000 Hz. [ ʃ ] has the bulk of its acoustic energy at around 4,000 Hz, but can extend up to around 8,000 Hz.
Of the sibilants, the following have IPA symbols of their own:
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 postalveolar as [s̠], a transcription frequently used when none of the above apply (that is, for a laminal but non-palatalized, or "flat", postalveolar). Some of the Northwest Caucasian languages also have a closed laminal postalveolar, without IPA symbols but provisionally transcribed as [ŝ ẑ].
Only the alveolar and palato-alveolar sibilants are distinguished in English; the former are apical, while the latter are slightly labialized and generally called simply "postalveolar": [s̺ z̺] [ʃʷ̜ ʒʷ̜]. Polish and Russian have laminal denti-alveolars, palatalized denti-alveolars, flat postalveolars, and alveolo-palatals, [s̪ z̪] [s̪ʲ z̪ʲ] [s̠ z̠] [ɕ ʑ]; whereas Mandarin has apical alveolars, flat postalveolars, and alveolo-palatals, [s̺ z̺] [s̠ z̠] [ɕ ʑ].
Few languages distinguish more than three series of sibilants without secondary articulation, but Ubykh has four series of plain sibilants, [s z], [ŝ ẑ ŝʷ ẑʷ], [ɕ ʑ ɕʷ ʑʷ], [ʂ ʐ], and the Chinese dialect of Qinan, in Shandong province, is said to have five. Toda has a laminal alveolar, an apical postalveolar, laminal domed postalveolars, and sub-apical palatals. Since two of these could be called 'retroflex', Ladefoged & Maddieson 1996 have resurrected the old IPA diacritic for retroflex, the underdot, for apical retroflexes, and reserve the letters <ʂ, ʐ> for sub-apical retroflexes. Thus the Toda sibilants can be transcribed [s̪] [ṣ] [ʃ̻ ʒ̻] [ʂ ʐ], although the official IPA symbols [s̪] [s̠] [ʃ̻ ʒ̻] [ʂ ʐ] are also sufficient. (In some publications the underdot and underbar are interchanged.)
Other definitions of sibilant
Some authors, as for instance Chomsky & Halle (1964), group [ f ] and [ v ] as sibilants. However, they do not have the grooved articulation and high frequencies of other sibilants, and most phoneticians (for instance by Ladefoged & Maddieson 1996), continue to group them together with the bilabial fricatives [ ɸ, β ] as non-sibilant anterior fricatives. Some researchers judge [ f ] to be strident in one language, e.g. the African language Ewe, as determined by experimental measuerments of amplitude, but as non-strident in English.
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 supposedly non-sibilant [ θ ] voiceless alveolar non-sibilant fricative (Stone and Lundberg, 1996, Journal of the Acoustical Society of America, vol. 99: 3728-3737). More research on the phonetic bases of the terms sibilance and stridency, and their interrelationship, is required.
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