subfloor vs underneath

subfloor

noun
  • The floor structure supporting and underlying the visible flooring or other finishing surface such as a carpet 

underneath

noun
  • The lower surface or part of something. 

  • A background radio sound track played during a specific announcement or program. 

prep
  • Under the control or power of. 

  • Under, below, beneath. 

adv
  • On the underside or lower face. 

  • Below; in a place beneath. 

adj
  • Under, lower. 

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