Strident vowels (also called sphincteric vowels) are strongly pharyngealized vowels accompanied by (ary)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 cords.
Strident vowels are fairly common in Khoisan languages, where they contrast with simple pharyngealized vowels. Stridency may be a type of phonation called harsh voice. A similar phonation, but without the trill, is called pressed voice or ventricular voice. The Bai language of southern China has a register system with allophonic strident and pressed vowels.
There is no official symbol for stridency in the IPA, though a superscript is often used. In some literature a subscript double tilde ( ) is sometimes used, as seen here on the letter a ():
This is found in the Charis and Doulos fonts (), and has been accepted into Unicode at code point U+1DFD.