Helmholtz PDE for Acoustic scattering

I´m traying to use the General PDE- Helmholtz equation to solve an acoustic scattering problem, I know I can use the Acoustic Module, but i want to do it with the general PDE as this give me a little more flexibility an setting the properties. I´m having a hard time to get similar results than the ones on the accoustics module, The problem is very simple, its a sound hard rigid circle (r=1 centered in (0,0)), te propagation medium is water, the incident wave is planar, the frequency es 200hz. The area I´m modeling is a square of side 10.

Any suggestion or tip on how can I solve this problem?

Regards