hard rock vs refrain

hard rock

noun
  • A rock music genre marked by a heavy regular beat, high amplification, and usually frenzied performances. 

refrain

noun
  • The chorus or burden of a song repeated at the end of each verse or stanza. 

  • A much repeated comment, complaint, or saying. 

verb
  • To stop oneself from some action or interference; to abstain. 

  • To abstain from (food or drink). 

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