Source : Webster's Revised Unabridged Dictionary (1913)

Visible speech \Vis"i*ble speech"\ (Phon.)
   A system of characters invented by Prof. Alexander Melville
   Bell to represent all sounds that may be uttered by the
   speech organs, and intended to be suggestive of the position
   of the organs of speech in uttering them.

Source : WordNet®

visible speech
     n 1: a phonetic alphabet invented by Melville Bell in the 19th
          century
     2: spectrogram of speech; speech displayed spectrographically