a cappella vs poem

a cappella

noun
  • A vocal performance with no instrumental accompaniment. 

adj
  • Singing solely or mainly without instrumental accompaniment. 

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

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. 

poem

noun
  • A piece of writing in the tradition of poetry, an instance of poetry. 

  • A piece of poetic writing, that is with an intensity or depth of expression or inspiration greater than is usual in prose. 

  • A literary piece written in verse. 

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