SUBTONIC is a 8 letter word that starts with S.

1. \Sub*ton"ic\, a. (Phonetics)
   Applied to, or distinguishing, a speech element consisting of
   tone, or proper vocal sound, not pure as in the vowels, but
   dimmed and otherwise modified by some kind of obstruction in
   the oral or the nasal passage, and in some cases with a
   mixture of breath sound; -- a term introduced by Dr. James
   Rush in 1833. See Guide to Pronunciation, [sect][sect]155,
   199-202.
   
   

2. \Sub*ton"ic\, n.
   1. (Phonetics) A subtonic sound or element; a vocal
      consonant, as b, d, g, n, etc.; a subvocal.
   
   2. (Mus.) The seventh tone of the scale, or that immediately
      below the tonic; -- called also {subsemitone}.