Practical question regarding ductwork

[ProperVenting]

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?

[SBKold]

Where does the air replacing all this exhausted air come in at?

Is this a belt drive centrifugal fan?

[ProperVenting]

The air comes from open windows and it is large inline fan, like Fantech FKD series which I believe is direct drive

[chrisevansac]

silencers only reduce sound of air velocity at terminals. not the sound of the fan if you were standing next to it or standing underneath it in your bedroom.

[ProperVenting]

Hmm..exactly opposite to what I was told by Fantech tech support

[chrisevansac]

doubt they have ever installed one themselves

Hmm..exactly opposite to what I was told by Fantech tech support

[ProperVenting]

So what do I do?

[jtrammel]

Live with the noise, or enter the 21st century and install an hvac system.

[timebuilder]

Large ducts, large diameter fan(s), slower blade speed.

[ProperVenting]

Live with the noise, or enter the 21st century and install an hvac system.

If u refer to A/C and Heating system, I am already in 21-st century and I am installing this one in addition. And my last observation, you should be less presumptuous in your opinions.

[chrisevansac]

well... 16" round flex will do what you want at a velocity of roughly 1100fpm for about 50-70 feet. they do make a blanket material you can use to reduce sound. or live with it. using flex alone will silence some of the sound of the fan at the terminals.

[ProperVenting]

Large ducts, large diameter fan(s), slower blade speed.

Tim, thanks for the suggestions. So forget silencers? I did mention that I am planning to oversize my ducts, what about using flex vs regular pipe? I really need to find a solution for the fan noise. If you guys are telling me that silencers will only help with aerodynamic noise and not fan noise, I am start questioning the whole installation idea.

[chrisevansac]

silencers reduce the sound at the registers only.