aphonous vs surd

aphonous

adj
  • Without a voice; voiceless; aphonic. 

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. 

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