What does "acoustic" mean?

Acoustic: Pertaining to the sense of hearing, the organs of hearing, or the science of sounds; auditory. Acoustic duct, the auditory duct, or external passage of the ear. -- Acoustic telegraph, a telegraph making audible signals; a telephone. -- Acoustic vessels, brazen tubes or vessels, shaped like a bell, used in ancient theaters to propel the voices of the actors, so as to render them audible to a great distance.