Silencing air through duct work

I gear up a bathroom ventalation system using a 4" 300 cfm inline marine
blower and 4" rigid ductwork. The reason was to keep the noise of the fan
away from the bathroom. The blower is in the basement and vents the air
outside via a 4" dryer outlet. Everything works great except the noise and
it is not the blower. It is the air rushing through the ductwork. If I put
my hand partially over the intake grill in the bathroom it silences it a bit
but the volume of air is also reduced. Are there any tricks I could do to
quiet things down.
Reply to
mark
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I saw a demo of a ductwork system that uses fiberglass lining in the duct. Dramatic difference, basically a big glass-pack muffler. I wouldn't use it for a cold air return because I'm not sure the dusty unfiltered air wouldn't just saturate it with fuzz, but on a forced air "source" like a vent fan, it may be just the thing.
I can't dredge up a vendor name from my rusty memory at this time but maybe someone here knows it.
Dave Hinz
Reply to
Dave Hinz
typically 6" is used for >200 cfm, 4" for 50 -80 cfm, that's why it's noisy, you could try this
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Reply to
steve
ALL the duct work in the buildings I've worked in has this lining in it. And yes it's almost completely silent. Add the "white noise" speaker system they put in big office buildings, and you'll never notice any noise.
I wonder if you just replaced the last few feet of the duct at the end of each branching with this lined duct, if that would do it ?
Reply to
GatherNoMoss
(someone else wrote:)
Ken, I can't help but think - if the guy thinks he needs 300CFM of ventilation in the bathroom, I really, really don't need all the details.
Reply to
Dave Hinz
Thanks guys, I will check with a local duct company to see if they carry this type of ductwork. Only problem is that it is all behing drywall now but if it will help I will cut it out and replace it.
Reply to
mark
If as you say the noise you hear is not a component of the fan/motor assembly, then noise absorbent ducting together with several abrupt bends in the duct is not the solution. And the noise you hear is created by the speed of the air exiting your register or vent. As others have suggested you may want to reduce the amount of air delivered per minute or go to a more open register or even several. Basically the area of your register produces to high a velocity for the air to pass silently, or silently enough.
cheers T.Alan
Reply to
T.Alan Kraus
Something else you could try would be to splice in a "T" in the ceiling above, and put a butterfly damper plate in it, to adjust the amount of air drawn from the bathroom.... the "excess" air could be drawn in from the attic space. Ken.
Reply to
Ken Sterling
The metal duct is acting as a sound amplifier. My friend had a very noisy computer that could be heard all over the house, even when standing in the basement next to a running washer and dryer. It was painful to listen to, for any length of time. I totally silenced his computer by removing the metal cover and taping pieces of cardboard to the frame instead of the metal. This was the cardboard box that the computer came in. It was absolutely silent and everyone was much happier. Why do the engineers insist on building things out of steel and then go to great effort to try to silence it. Just replace the steel with something that has a more deadening sound like particleboard, MDF, etc. You will notice the difference immediately.
mark wrote:
Reply to
Buy_Sell
In the case of the computer it's to prevent interference with other electronics. At least that's what it says on the box. Karl
Buy_Sell wrote:
Reply to
kfvorwerk
You could just go to Home Depot and buy the quietest bath fan. I installed one 6 months ago in a friends house and it's amazingly quiet. Of course I realise that defeats the whole purpose of this group. Karl
mark wrote:
Reply to
kfvorwerk
You sure will. You just turned the faraday cage of the computer case into something RF transparent. So more interference in the house but it's quieter in the audio frequencies.
Computers are easy enough to make quiet - they have whisper fans with great airflow and features which make them quiet, especially on an idle system. Worth looking into.
Reply to
Dave Hinz
Or do as Junior did, and line 90% or the case with the sound deadening material used when installing high end auto stereo systems - basically a lead composite sheet coated with contact PSA. Gerry :-)} London, Canada
Reply to
Gerald Miller

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