The Collaborative International Dictionary of English v.0.48:

Acoustics \A*cous"tics\ (#; 277), n. [Names of sciences in -ics,
   as, acoustics, mathematics, etc., are usually treated as
   singular. See -ics.] (Physics.)
   The science of sounds, teaching their nature, phenomena, and
   laws.
   [1913 Webster]

         Acoustics, then, or the science of sound, is a very
         considerable branch of physics.          --Sir J.
                                                  Herschel.
   [1913 Webster]

   Note: The science is, by some writers, divided, into
         diacoustics, which explains the properties of sounds
         coming directly from the ear; and catacoustica, which
         treats of reflected sounds or echoes.
         [1913 Webster]

WordNet (r) 3.0 (2006):

acoustics
    n 1: the study of the physical properties of sound