American English Phonetics

11.1.2 Grooved and Slit Fricatives

A major distinction within the category of GA fricatives is that between the sibilants or grooved fricatives /s,z,ʃ,ʒ/ and the non-sibilants or slit fricatives /f,v,θ,ð/.

For the former, the air is channeled through a groove in the surface of the tongue, while for the latter, the air escapes over a larger area of the tongue or lower lip, producing less intense and lower-pitched friction. In fact, the friction for /f,v/ and especially /θ,ð/ may be so weak as to be almost inaudible.