Abaqus - Coupled structural-acoustic analysis

I need some advices because I'm pretty new on acoustic analysis.

I'm doing a exterior problem :

a solid structure is placed in an air bubble, coupled by surface-based Tie constraint. Overall, the model has about 1/2 million elements. 

I did consulted the Manuel ("acoustic, shock, and coupled acoustic-structural  analysis), but I am still confused about some details:

  • It is recommended to use direct-solution steady-state dynamic analysis for this type of problem but it take an enormous CPUs time, so i decided to use modal superposition with AMS eigensolver with projection of the structural-acoustic coupling specified. Will it induced critical inaccuracy?
  • I use acoustic infinite element for the terminating surface of the region of acoustic medium. But what is the diameter of the acoustic part minimum required to have accurate results?

Abaqus