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

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "acoustics":
   Newtonian physics, acoustical engineer, acoustician, aerophysics,
   applied physics, astrophysics, basic conductor physics, biophysics,
   chemical physics, cryogenics, crystallography, cytophysics,
   electron physics, electronics, electrophysics, geophysics,
   macrophysics, mathematical physics, mechanics, medicophysics,
   microphysics, natural philosophy, natural science, nuclear physics,
   optics, philosophy, phonics, physic, physical chemistry,
   physical science, physicochemistry, physicomathematics, physics,
   psychophysics, radiation physics, radioacoustics, radionics,
   solar physics, solid-state physics, statics, stereophysics,
   theoretical physics, thermodynamics, zoophysics