a cappella vs trill

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. 

trill

noun
  • A tremulous high-pitched vocal sound produced by cats. 

  • A rapid alternation between an indicated note and the one above it, in musical notation usually indicated with the letters tr written above the staff. 

  • A type of consonantal sound that is produced by vibrations of the tongue against the place of articulation: for example, Spanish ⟨rr⟩, /r/. 

adj
  • true, respected 

verb
  • To impart the quality of a trill to; to utter as, or with, a trill. 

  • To create a trill sound; to utter trills or a trill; to play or sing in tremulous vibrations of sound; to have a trembling sound; to quaver. 

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