a cappella vs burden

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. 

burden

noun
  • A phrase or theme that recurs at the end of each verse in a folk song or ballad. 

  • The proportion of ore and flux to fuel, in the charge of a blast furnace. 

  • The capacity of a vessel, or the weight of cargo that she will carry. 

  • A responsibility, onus. 

  • The tops or heads of stream-work which lie over the stream of tin. 

  • A fixed quantity of certain commodities. 

  • Theme, core idea. 

  • The drone of a bagpipe. 

  • A cause of worry; that which is grievous, wearisome, or oppressive. 

  • The total amount of toxins, parasites, cancer cells, plaque or similar present in an organism. 

  • A heavy load. 

verb
  • To impose, as a load or burden; to lay or place as a burden (something heavy or objectionable). 

  • To encumber with a literal or figurative burden. 

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