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A fricative is a consonant sound produced by friction, or increased air pressure, between two oral structures. These may include the teeth, tongue, lips and palate. The proper production of fricative sounds requires that each of these structures come in close enough contact to change the sound without completely stopping it. Since precise placement of the articulators involved is required to produce a clear, easily understood sound, children tend to develop the ability to make fricatives later than other sounds.
Fricatives are sibilant sounds produced by partially constricting the flow of air between two articulators. They are classified based on the place where the constriction occurs, such as at the lips or further back in the mouth, the structures used to create the friction, and the presence or absence of voicing. They also require continuous air flow, as opposed to sounds such as p which stop the air with production. The inability to produce the necessary constriction or the needed air flow, results in perceptibly distorted sounds and reduced intelligibility.
Place of production is a key fricative distinguishing feature. Each combination of articulators used to produce a fricative is labeled along with the sounds they produce. In English these are the labiodental sounds f and v produced with the lips and teeth; the interdental sounds, voiced and voiceless th produced with the tongue and teeth; the alveolar s and z produced with the tongue and alveolar ridge; and the alveopalatal sh and final dz sound in "judge" produced with the tongue and palate.
Each place of fricative production in English features two distinct sounds based on voicing. Voicing is the vibration of the vocal folds in speech production. In many languages, including English, voicing is a key distinguishing feature between speech sounds. For example, the interdental sounds f and v are produced in exactly the same place with the exception of voicing.
Speech production depends on the coordination of breath control with very fine motor movements in the mouth and throat. Fricatives are especially difficult because they require the very careful placement of articulators to produce the perfect amount of friction in the air flow. As a result, fricatives are often poorly articulated in young children. For example, the interdental sounds th plus and minus voicing are often very late developing and are not considered delayed until seven years of age.