surd vs unvoiced

surd

adj
  • unvoiced; voiceless 

  • Involving surds, or irrational numbers; not capable of being expressed in rational numbers. 

noun
  • An irrational number, especially one expressed using the √ symbol. 

  • A voiceless consonant. 

unvoiced

adj
  • That does not contain voice. 

  • Spoken without vibration of the vocal chords. 

  • Of an IRC user: not granted voice privileges. 

  • Not spoken or expressed. 

How often have the words surd and unvoiced occurred in a corpus of books? (source: Google Ngram Viewer )