So, I decided to install powerful inline fan in my attic capable of moving close to 3000 CFM and do air exchange every 4-6 minutes. This will be my whole house fan solution for the 2-nd floor only.

I will install main trunk and then branch off to each individual bedroom/bathroom on a second floor. I have to use silencers because fan is definitely loud (seems like close to 70 db) and in addition to fan noise I would have to address aerodynamic noise coming from moving air @ pretty decent velocity through the ductwork.

The immediate problem is that the largest available silencer off the shelf is only 12" and my fan requires 16" so I can't install it without further restricting air flow through both inlet and exhaust side of it.
Secondly, I am torn apart what material/pipe to use as 16" galvanized pipe in 26 gauge is definitely not cheap and in addition I would have to spend extra time and money to further insulate it to reduce aerodynamic noise from air movement.

Therefore I was thinking about insulated flexible pipe (very cheap and easy to install) but this one has its own issues as there are a lot of spirals/ridges inside it and in order to avoid potential problems I would have to increase its diameter from 16" to 18" or even 20" to reduce air velocity.

So, what do I do? Any suggestions from Pro's?