did you start the fan with no ducts on it
70 db is no louder than the common TRANE condensing unit. Lol
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Use 26" pipe off of the fan and at least 2 90s to reduce mechanical noise.
1. First of all, are you talking about flex pipe? Does it even exist in this size? Frankly speaking, I thought that going from nominal 16“ to 20” was already overkill so you put completely different prospective on my project and I really appreciate that.
2. Secondly, I assume that 2 90’s will slow down air velocity even more and therefore will further reduce the aerodynamic noise.
3. What size and kind of pipes you suggest to use for each bedroom / bathroom (they are all different sizes). Originally, I thought that for the simplicity sake, I would bring 8” pipe and install one 8” round intake grill regardless of the room size. Am I right about that? Somehow, after I read your opinion, I have a nagging feeling that I am totally wrong.
4. And finally what about the idea of using special sound insulating jackets on a motor?
I was talking metal pipe. Flex over 20" is not standard in most areas. I'd run flex or sound attenuated duct(last 6 foot) to the individual grilles. Grilles have to be sized for the room they'll be serving and so must the duct be sized for the room.
You could also run duct board as the main trunk line. Duct board has sound absorbing properties. 2 90s in it would still help reduce noise even more.
Small duct work makes for loud air movement noise.
I know that prices are not permitted here but can you at least give me an idea on the difference between all these materials, so I know where to spend most of my time.
Having have done some recording studio's, if you have the $ one can create sound isolation by breaking the sine wave of sound and cancelling up to 80% using two methods without losing more than 10% CFM and FPM. at each branch run make a canvas break from round into equivalent duct board with internal 33 degree angled baffling that is up to 1/2 height x 3 into output canvas break again and back into final 5 ft hard pipe or flex at that point. The factoring is outside the box thinking and if sound waves cannot stay in sync with the air at any velocity then "Energy Itself Must Transfer" theory. The duct board and canvas become a heat sink in theory from the fibers of both canvas break and duct board vibrating removing the sound energy.
Take a metal pipe suspended like a wind chime tube and tap it with a metal screwdriver, it rings loudly. now just hold it with your hand and tap it, now it barely has a thud to it. (Do not even ask about related harmonic coupling). Apply that process to your design.