headphone vs mouthpiece

headphone

noun
  • A cushioned speaker placed on, over, or inside the ear, normally used in left/right pairs. 

mouthpiece

noun
  • The part of a telephone that is held close to the mouth. 

  • The part of a wind instrument that is held in or against the mouth. 

  • A lawyer for the defense. 

  • A spokesman; one who speaks on behalf of someone else. 

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