imperceptible vs unheard

imperceptible

adj
  • beyond the purview of man; too great and all-encompassing to be perceived. 

  • not perceptible, not detectable, too small in magnitude to be observed 

unheard

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

  • Not heard. 

  • Not listened to. 

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