baffled duct - PML 3D PROBLEM

Hello every one,
I’d like to excite a cylindrical waveguide (rigid in one face and flanged in the other one) with a monopole located in M(x,y,z) and compute the pressure inside the duct. Also, compute the far field radiation within comsol 3D.
I used Comsol to validate my analytical code in the case of a cylindrical duct with different boundary conditions. The results are perfect unless all the walls are hard except the outlet which is flanged (=I use the radiation impedance formulation of a baffled piston in terms of struve and bessel functions). To model the infinite baffle, I employed 2 PMLs . The first one has the same radius of the main duct. The second with a bigger radius (I obtained sound pressure level results as shown in the attached screen shot 1.5 a , 2a, 5a) .Please find herewith the model as well (with PLM 2 Radius = 1.5a) . I applied five elements in the swept direction in the PMLs ( the PML scaling factor is 1).

Could you please let me know how to adapt the 3D PMLs to the problem inorder to get better results ?

Best regards.

Aniss