surd vs toneless

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. 

toneless

adj
  • Lacking tone or expression. 

  • Lacking vitality; listless. 

  • Lacking tone or pitch, have the neutral tone. 

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