Strident vowel
Strident vowels (also called sphincteric vowels) are strongly pharyngealized vowels accompanied by epiglottal trill, where the larynx is raised and the pharynx constricted, so that either the epiglottis or the arytenoid cartilages vibrate instead of the vocal chords. They are fairly common in Khoisan languages, where they contrast with simple pharyngealized vowels. There is no official symbol for stridency in the IPA, but in Khoisanist literature, a subscript double tilde (≈) is sometimes seen.