Sibilant, Fricative, Rhotacism, Allograph
Terms 10

Sibilant Having, containing, or producing the sound of or a sound resembling that of the s or the sh in sash. Non-sibilant is producing the sound of or a sound not resembling that of the s or the sh in sash, for example the th sound.

Fricative
In linguistics it is a sound made by forcing air out of your mouth through a narrow opening that is made using the lips, teeth, or tongue.

Rhotacism A defective pronunciation of r; especially : substitution of some other sound for that of r.

Allograph It is each of two or more alternative forms of a letter of an alphabet or other grapheme. The capital, lowercase, italic, and various handwritten forms of the letter A are allographs. In phonetics it is each of two or more letters or letter combinations representing a single phoneme in different words. Allographs of the phoneme f include the (f) of "fake" and the (ph) of "phase.