microphone vs public address system

microphone

noun
  • A device (transducer) used to convert sound waves into a varying electric current; normally fed into an amplifier and either recorded or transmitted over radio. 

verb
  • To put one or more microphones on or in. 

public address system

noun
  • An electronic voice-amplification apparatus, usually containing a set of two loudspeakers and a central control unit, used for singing, giving a speech, play-by-play commentary at events, or broadcasting in public areas like in airports or stations. 

How often have the words microphone and public address system occurred in a corpus of books? (source: Google Ngram Viewer )