a cappella vs madrigal

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. 

madrigal

noun
  • A polyphonic song for about six voices, from 16th century Italy. 

  • A song for a small number of unaccompanied voices; from 13th century Italy. 

  • A short poem, often pastoral, and suitable to be set to music. 

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