Fricatives are consonants with the characteristic that
air escapes through a narrow passage and makes a hissing
sound. The dental fricatives are sometimes described as if the
tongue were placed between the front teeth, and it is common
for teachers to make their students do this when they are
trying to teach them the sound. The thing is, however, that the
tongue is normally placed behind the teeth; the air escapes
through the gaps between the tongue and the teeth. There is
a distiction between fortis (unvoiced) fricatives, as in the word
“thin”, and lenis (voiced) fricatives, as in “thus”. (Roach 2003)
(Mark Roach,. English Phonetics and Phonology.
Cambridge: CUP, 2003. Adaptado)