pitter-patter vs synth pad

pitter-patter

noun
  • A soft, percussive sound, as of tiny feet, or of rain on a rooftop. 

synth pad

noun
  • A sustained chord or tone generated by a synthesizer, often employed for background harmony or atmosphere. 

How often have the words pitter-patter and synth pad occurred in a corpus of books? (source: Google Ngram Viewer )