earphone vs headphone

earphone

noun
  • A transducer that converts electric signals into sound and is held near the ear, especially as part of a telephone; an earpiece or headphone. 

headphone

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

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