surd vs unheard

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. 

unheard

adj
  • Not heard. 

  • Not listened to. 

  • Not known to fame; not illustrious or celebrated; obscure. 

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