acoustic vs acoustical

acoustic

adj
  • (of building materials) Used for soundproofing or modifying sound. 

  • Utilizing sound energy in its operation. 

  • (of an explosive mine or other weapon) Able to be set off by sound waves. 

  • Naturally producing or produced by an instrument without electrical amplification or the need thereof. 

noun
  • The properties or qualities of a room or building that determine how sound is transmitted in it. 

  • A medicine or other agent to assist hearing. 

acoustical

adj
  • Of or pertaining to hearing or acoustics. 

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