(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.
adjective
Subtonic adj. (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.
... passes off into silence."—Kirkham's Elocution, p. 75. Thus the "unpractised student" is taught that b-y spells bwy; or, if pronounced "very deliberately, boo-i-ee!" Nay, this grammatist makes b, not a labial mute, as Walker, Webster, Cobb, and others, have called it, but a nasal subtonic, or semivowel. He delights in protracting its "guttural murmur;" perhaps, in assuming its name for its sound; and, having proved, that "consonants are capable of forming syllables," finds no difficulty in mouthing this little ... — The Grammar of English Grammars • Goold Brown