ethnomusic vs folk song

ethnomusic

noun
  • The traditional music associated with an ethnic group; the kinds of music studied in ethnomusicology. 

folk song

noun
  • folk music 

  • A song, in any folk style, originating among ordinary people, usually non-professional musicians, and handed down by oral tradition. 

  • A song in the style of folk music, a modern genre of popular music. 

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