hammered dulcimer vs psaltery

hammered dulcimer

noun
  • A type of dulcimer whose metal strings are stretched over a trapezoidal sounding board that has two bridges, one bass and one treble, set on a stand and played with small mallet-like hammers. 

psaltery

noun
  • A zither-like musical instrument consisting of a soundboard with multiple strings, played by plucking the strings with the fingers or a plectrum. 

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