An interdental or frontal lisp occurs when the tongue protrudes between the front teeth during production of sibilants like /s/ and /z/. It results in a “th”-like distortion that may be noticeable but still intelligible. While common in early speech development, it may require treatment if it persists. SLPs treat interdental lisps by training correct tongue placement and airflow.
Baum, Shari R., and McNutt, J. C. "An Acoustic Analysis of Frontal Misarticulation of /s/ in Children." Journal of Phonetics 18 (1990).
Lipetz, Heidi M., and Bernhardt, B. May. "A Multi-Modal Approach to Intervention for One Adolescent’s Frontal Lisp." Clinical Linguistics & Phonetics 27, no. 1 (2013).
