How to impose symmetric BCs for acoustic elements with Abaqus/CAE?

Dear all,

I am a beginner of Abaqus, and I am using Abaqus/CAE. I need to build an acoustic model for my M.S. thesis. The model is a cubic washing tank filled with water. the bottom of the tank is activated by harmonic sound. Since the geometric symmetry, only a quarter of the model need to be calculated.The question is, how should I impose the symmetric boundary conditions on the acoustic elements (water) with CAE?

I am tangling with this for weeks, and really get headache, can anybody help me out?

 Thanks very much!