hard rock vs instrumental

hard rock

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

instrumental

noun
  • A composition written or performed without lyrics, sometimes using a lead instrument to replace vocals. 

  • The instrumental case. 

adj
  • Pertaining to, made by, or prepared for an instrument, especially a musical instrument (rather than the human voice). 

  • Applied to a case expressing means or agency, generally indicated in English by by or with with the objective. 

  • Serving as an instrument, medium, means, or agency. 

  • essential or central; of great importance or relevance. 

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