The Collaborative International Dictionary of English v.0.48:

Acoustic \A*cous"tic\, n.
   A medicine or agent to assist hearing.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Acoustic \A*cous"tic\ (#; 277), a. [F. acoustique, Gr. ?
   relating to hearing, fr. ? to hear.]
   Pertaining to the sense of hearing, the organs of hearing, or
   the science of sounds; auditory.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

acoustic
    adj 1: of or relating to the science of acoustics; "acoustic
           properties of a hall" [syn: acoustic, acoustical]
    n 1: a remedy for hearing loss or deafness

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "acoustic":
   audible, audile, audio, auditory, aural, auricular, hearing,
   hypersonic, otic, otological, otopathic, otoscopic, phonic, sonic,
   subsonic, supersonic, transonic, transsonic, ultrasonic