Newbie question: HVAC vents

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Newbie question: HVAC vents

Postby neil.steiner » Thu Oct 24, 2013 3:45 am


I am completing my first pass through Rod Gervais' "Home Recording Studio : Build it Like the Pros," trying to design a drum room that won't disturb my neighbors, but I'm missing something basic.

I understand the concepts of MSM, flanking path, and mechanical decoupling. But then I am apparently supposed to slice through my nice, solid, decoupled two-leaf wall, and push vents through it. Yes, the ability to breath is compelling. But so is the ability to actually make noise in a room intended for that purpose.

Now instead of the acoustic energy having to pass through two double walls and an appropriately-sized air gap, it can take a much easier path through one layer of metal vent or through an open register, travel down the vent, and pass through one more layer of metal to exit the vent. Surely that renders most of my heavy decoupled wall moot, does it not?

I know the drums are going to be loud, though some of that probably depends on my playing. Getting consistent 60-70 dB of reduction outside the room will be hard enough without cutting a hole through the walls. It seems like I would need to build a two-leaf wall around my ducts, but that would still be of limited help unless the air handler and all the other ducts were similarly enclosed.

What do people do in cases like this?

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Re: Newbie question: HVAC vents

Postby bert stoltenborg » Thu Oct 24, 2013 2:04 pm

In these cases you use a muffler. :D
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Re: Newbie question: HVAC vents

Postby Bob » Thu Oct 24, 2013 5:09 pm

"drum room that won't disturb my neighbors"
Ah. Way to jump in with both feet there for your first attempt.

In general, whatever the reasons why, Rod's book's advice is what works. Follow it blind regardless of understanding, and it should be ok.

Variations on Bert's work too.
Such as a Paul Woodlock.

My hypothesis (mostly guesses, no facts, no studies, never seen anyone smart write anything like this) about air ducts is this:
1) the size of the duct relative to the size of the walls is small in terms of area, so only a % of the sound energy gets to the duct in the first place. Admittedly this is only a drop of a few db.
2) think about your house. If someone is talking in the basement, and you're up on the 3rd floor, you hear the low frequency noise of hammering transmitted through the framing of the walls, you hear the furnace fan and the wind of the duct, but you don't hear the voices. Why not? Someone may know, but it doesn't matter.
3) high frequency sound is fairly directional, and if there are a few bends in the hvac duct, particularly if the duct is round rather than square, then the sound is reflected around and back onto itself dramatically lowering the dBs.
4) low frequency sound is transmitted at least as much through the framing as through the ducts.
Bob Golds
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Re: Newbie question: HVAC vents

Postby Scott R. Foster » Fri Oct 25, 2013 6:08 pm

I induced isolation between my studio and mix suite by simply running oversized flexible duct [the soft shiny stuff] which because it was larger diameter than necessary I was able to make the runs very long - used the extra run length to allow a big slow "ess" curve. You gotta do this with both supply and return runs - tie the legs for each room together with a break out box all the way back at the air handler = example: sound coming into studio supply vents runs upwind in a complete circle of soft duct before it gets to the box over the AH and then downwind in another complete circle to get to the mix suite.

Waste of material but worked great.


*properly built mufflers I'm sure are cheaper - might even work as well [or better!].
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