a cappella vs psalm

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. 

psalm

noun
  • A sacred song; a poetical composition for use in the praise or worship of God. 

  • One of the hymns by David and others, collected into one book of the Old Testament, or a modern metrical version of such a hymn for public worship. 

verb
  • To extol in psalms; to make music; to sing 

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