hard rock vs opera

hard rock

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

opera

noun
  • A theatrical work, combining drama, music, song and sometimes dance. 

  • A building designed for the performance of such works; an opera house. 

  • The score for such a work. 

  • Any showy, melodramatic or unrealistic production resembling an opera. 

  • A company dedicated to performing such works. 

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