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Articulatory properties of alveolar approximant
‘Voiced alveolar approximant’ is the standard description of /ɹ / in American English. It is articulated with the tongue tip or blade placed closely to the region behind the teeth called the alveolar ridge. Some people claim that in their pronunciation of /ɹ / the tip or blade of the tongue is placed behind the alveolar ridge and could, therefore, be characterized as post-alveolar.

Magnetic Resonance images of vocal tract configurations during /ɹ / production show that speakers of American English employ a wide range of articulatory strategies and shape their tongue differently to produce /ɹ /. This may suggest that a great mastery of tongue muscles is required in order to reach the correct target for /ɹ /.

Auditory properties of alveolar approximant
The American English /ɹ / displays a fairly stable pattern of the low third formant (F3) and the close proximity of the second and third formants (F2 and F3). The acoustic analysis of word-initial /ɹ / done by closely inspecting spectrograms of words pronounced by children and adults has led many researchers to believe that the acoustic interaction between F2 and F3 is a primary cue in distinguishing /ɹ / from other approximants /w/, /l/, and /j/, as well as labiodental /ʋ/ in speech perception.

As an allophone of other rhotic sounds, /ɹ / occurs in Edo, Fula, Murinh-patha, and Palauan.

Typical Developmental Trajectory of Feature
Research in phonological development references articulatory and acoustic descriptions. Children’s acquisition of consonants differs both in where and how the vocal tract is closed. Children lack control of their articulatory muscles due to the immaturity of their vocal tract structure. This could explain why the alveolar approximant is one of the last sounds mastered by children, approximately at the age of four. Acoustically, children’s pronunciations of /ɹ / revealed higher frequencies for F2 and F3 formants, and a larger distance between them than those of adults.

Gliding
As children age, they develop a systematic way in which to adjust the sounds of their target language in order to fit within the range of sounds they can produce. These systematic transformations are called phonological processes. One phonological process typically found in child phonological acquisition is gliding – a segment substitution process involving the alveolar approximant. The gliding change is one in which sounds resembling /l/ and /ɹ / which are traditionally classified as liquids are replaced by the sounds /j/ and /w/ which are traditionally classified as glides. For example, the alveolar approximant /ɹ / is replaced with the liquid consonant /w/ so that ‘rabbit’ is pronounced as /wabit/ and  /j/ replaces /l/ in ‘lamp’, and /w/ replaces /ɹ / in ‘red’.

Difficulties in Acquisition
The alveolar /ɹ / is among the last of the phonemes to develop normally, and is also one of the most commonly misarticulated sounds due to its difficult pronunciation and similarities to other sounds. The nature of the sound’s production requires the speaker to manipulate different parts of the tongue, lips, and pharyngeal wall in relation to the palate making it more complex than most English sounds. In addition, the subtle contrast between /ɹ / and /w/ may be difficult for children to differentiate in adult speech. As the English alveolar approximant sound has various contributing articulations that are not often audible or obvious, articulatory-delayed children and children with hearing loss often have difficulty acquiring it. In English, misarticulation is generally characterized by a high third formant and courses of treatment typically aim to lower the formant to normal levels. When a child misarticulates the alveolar approximant, they frequently substitute it for the /w/ sound, or distort it to a sound that falls somewhere between /ɹ / and /w /. If difficulties articulating this approximant persist, an oral mechanism examination can be done to ensure the muscles of the mouth are working properly, and if they are and the errors are not caused by physiological limitations, the child might undergo articulation treatment. In the circumstance that misarticulation is caused by physiological abnormalities such as the Pierre Robin Sequence or banded lingual frenulum, treatment may rarely involve surgical intervention, but further therapeutic treatment is still necessary. In either case, a typical course of treatment involves having the child produce affected words while being instructed on the correct articulation of alveolar /ɹ / for a speech language pathologist. Tasks designed to elicit use of the phoneme may involve the patient reading from a selected passage, verbally identifying objects in pictures, or producing spontaneous speech. Some errors that can be treated in therapeutic speech may remain present in spontaneous or informal speech, so the use of both reading and spontaneous tasks in therapy is imperative.