Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   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.

Source : WordNet®

acoustics
     n : the study of the physical properties of sound