a cappella vs expression

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. 

expression

noun
  • The tone of voice or sound in music. 

  • A specific blend of whisky. 

  • A colloquialism or idiom. 

  • The action of expressing thoughts, ideas, feelings, etc. 

  • An arrangement of symbols denoting values, operations performed on them, and grouping symbols. 

  • The process of translating a gene into a protein. 

  • A facial appearance usually associated with an emotion. 

  • (manufacturing) The act of pressing or squeezing out. 

  • A particular way of phrasing an idea. 

  • A piece of code in a high-level language that returns a value. 

  • emotional involvement or engagement in a text read aloud rendered by the voice of the reciter or the reader 

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