a cappella vs diapason

a cappella

noun
  • A vocal performance with no instrumental accompaniment. 

adv
  • In a manner of a choir with no instrumental accompaniment; literally, "in the style of the (Sistine) Chapel (in Rome)", such as a musical Mass done a cappella. 

adj
  • Singing solely or mainly without instrumental accompaniment. 

  • Related to a form of purely vocal music mostly associated with American college performance groups. 

diapason

noun
  • A harmonious outpouring of sound. 

  • The range or scope of something, especially of notes in a scale, or of a particular musical instrument. 

  • A tonal grouping of the flue pipes of a pipe organ. 

  • The musical octave. 

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