reverb vs undertone

reverb

noun
  • An electronic effect which simulates echoes or reverberations in the sound signal being processed. 

verb
  • To apply a reverb (electronic echo effect) to. 

undertone

noun
  • An auditory tone of low pitch or volume. 

  • A pale colour, or one seen underneath another colour. 

  • A low state of the physical faculties. 

  • An implicit message perceived subtly alongside, but not detracting noticeably from, the explicit message conveyed in or by a book, film, verbal dialogue or similar (contrast with overtone); an undercurrent. 

verb
  • To accompany as an undertone. 

  • To say or speak in an undertone. 

  • To present as less important, noticeable or prominent. 

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