The Collaborative International Dictionary of English v.0.48:

Acoustical \A*cous"tic*al\, a.
   Of or pertaining to acoustics.
   [1913 Webster]

WordNet (r) 3.0 (2006):

acoustical
    adj 1: of or relating to the science of acoustics; "acoustic
           properties of a hall" [syn: acoustic, acoustical]

Moby Thesaurus II by Grady Ward, 1.0:

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