Sibilants are louder than their non-sibilant counterparts, and most of their acoustic energy occurs at higher frequencies 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 .
Only the alveolar and palato-alveolar sibilants are distinguished in English; the former may be either apical or laminal, while the latter are usually apical, slightly labialized and generally called simply "postalveolar": or and ), as in sin [s̻ɪn] and shin [ʃʷ̜ɪn]. Although laminal and apical sibilants are not distinguished in English, Basque does distinguish these two phonemically, as well as having true postalveolars (). Polish and Russian have laminal denti-alveolars, palatalized denti-alveolars, flat postalveolars, and alveolo-palatals whereas Mandarin has apical alveolars, flat postalveolars, and alveolo-palatals ().
Few languages distinguish more than three series of sibilants without secondary articulation, but Ubykh has four series of plain sibilants, , as does the Bzyp dialect of the related Abkhaz, 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 , although the official IPA symbols are also sufficient. (In some publications the underdot and underbar are interchanged.)
Some authors, as for instance Chomsky & Halle (1964), group and 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 , β] as non-sibilant anterior fricatives. For a grouping of sibilants and , the term strident is more common. Some researchers judge [f] to be strident in one language, e.g. the African language Ewe, as determined by experimental measurements 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.