Author Topic: We "soundproofed" our AC ductwork and fan-unit  (Read 3529 times)

Spinnenacht

  • Guest
We "soundproofed" our AC ductwork and fan-unit
« on: May 26, 2008, 12:07:22 AM »
This was by accident, actually, and I thought I'd share and maybe you could help me figure out just what we did and why it seemed to make such a big difference.

Last year our AC was on the fritz - wild animals and so on had torn almost all of the insulation away and the metal was so old it had rusted through and was just falling apart. (In our crawlspace, btw)
So this year we decided to do some gross DIY and replace all the old solid-metal ductwork with more modern flexi-duct that comes already insulated in hopes to improve the air quality, flow and so on.
The unit was so loud you could hear it in the entire house and actually worked as a wonderful "background" noise generator, in fact, and we used it frequently for that purpose alone.

So we ripped all of the old ductwork out but decided to leave the air intake in place (The section of solid ductwork that runs from our return-vent in the hallway to the unit outside. We didn't wrap it with insullation or anything - it's as it's always been, a 10' length of 14" diameter ductpipe running under the house and out the side to the AC unit.

Once we replaced all the old ductwork we turned on the unit and it was amazingly quiet.
So quiet - in fact - that you really can't hear it run at all.

So - how did replacing the ductwork with flexi-duct actually dampen the sound? Was it the fact that the ductwork is now flexi-duct (a long metal-coil wrapped with plastic to form the air-tube and then sleeved with insulation?) Or because (regardless of the material the pipe is wrapped with) the dampening came from wrapping it?

johnbergstromslc

  • Guest
Re: We "soundproofed" our AC ductwork and fan-unit
« Reply #1 on: May 28, 2008, 12:11:45 AM »
So - how did replacing the ductwork with flexi-duct actually dampen the sound? Was it the fact that the ductwork is now flexi-duct (a long metal-coil wrapped with plastic to form the air-tube and then sleeved with insulation?) Or because (regardless of the material the pipe is wrapped with) the dampening came from wrapping it?

Well, the noise reduction comes from the flexi-duct itself; conceptually, it's a loosely hanging spring as opposed to a rigidly-connected cylinder of metal.  It transmits almost zero noise through mechanical vibration.  The majority of your noise reduction is due to this. 

The insulated sheath helps kill airborne noise propagating in duct.

It sounds like you've solved your noise problem, but if you need more reduction, add a few gentle curves to the duct so that it's not a straight-shot anymore.  This will slow the airflow for less noise and 'confuse' the sound waves - get them bouncing around at odd angles so more absorption can take place inside the duct.   

 

anything