Duct Liner - Where to Use

[DanCostello]

I think I know the answer to this, but I need an idiot check before I install something that winds up being hard to fix later:

I'm at the point where I'm about to install the ducts for the cold/stale air returns that run from from my vocal booth and control room back to the utility room. The ducts will be rigid galvanized steel and will be located entirely outside the inner leaves of these rooms. There will be no return ducting inside the rooms.

Should I install duct liner (i.e. the rigid stuff that goes inside the ducts) in these returns or do I only need the duct liner in the silencer boxes where they penetrate the inner leaves? The concern here is only for acoustic performance, not thermal insulation (which isn't really important for a return but will ultimately be taken care of via other means anyways).

My big concern, really, is the size of the ducts that I need - I've got some tight spaces to navigate and the smaller I can get the ducts, the better.

-Dan.

[DanDan]

https://www.thermaflex.net

[Gregwor]

DanCostello,

Should I install duct liner (i.e. the rigid stuff that goes inside the ducts) in these returns or do I only need the duct liner in the silencer boxes where they penetrate the inner leaves? The concern here is only for acoustic performance, not thermal insulation (which isn't really important for a return but will ultimately be taken care of via other means anyways).

This will only help to tame the sound of your air handler since your silencer boxes will have already provided the insertion loss necessary to attenuate the sound to/from your room. Ideally you want everything to have duct liner, however it does add insertion loss.

https://www.thermaflex.net

Dandan recommended this stuff and it is handy for snaking through some tight weird shaped spots. The problem with it is that it does introduce a ton of static pressure. For normal home HVAC, there are restrictions on how long these runs can be due to the static pressure concerns. Round steel ducts introduce the smallest amount of static pressure and should be used where possible.

My big concern, really, is the size of the ducts that I need - I've got some tight spaces to navigate and the smaller I can get the ducts, the better.

When I'm thinking about sizes for HVAC, I figure out the CFM and then try to feed the silencer boxes with duct sized based on a .1 friction rate.
Here is a chart to help with that:

Residential Duct Sizing Guide.png

Once you know that, the cross sectional area inside your silencer boxes should be at least twice that of your duct work.

Greg

[DanDan]

I suggested the Thermaflex because the insulated version would seem to be damped, avoiding the metallic resonances and transmission. Also a senior Acoustician appeared to promote it as a norm in a post which I can no longer find. Despite extreme differences of opinion on a translating listening curve, the guys engineering chops are significant.
So, not a personal experienced recommendation then. Just something I think worth considering based on a borrowed expert opinion.
Its been a while since the text books, and I really do not have the experience in quiet air.
Looking, well skimming, at the published data, Thermaflex seem at least comprehensive and presumably honest.
I see a bit of ribbing in the interior which presumably causes some turbulence which might slow air down.
Greg, given a straight lay..... ahem, is this enough to make a significant difference given equal diameter pipes?
Also, as steel is one of the best conductors of sound, any thoughts as to why plastic pipes are not used?

[Gregwor]

Greg, given a straight lay..... ahem

I love it hahaha

is this enough to make a significant difference given equal diameter pipes?

As far as I know, when super straight, there is no difference. I think the concern is raised when you're snaking it through odd places as causing lots of bends.

I've used this for flex duct in my calculator:

https://www.hartandcooley.com/tools/fri ... ible-ducts

Also, as steel is one of the best conductors of sound, any thoughts as to why plastic pipes are not used?

I believe that it is probably due to the following (I'm guessing here):
- steel duct work material can be shipped flat and then assembled later
- the wall thickness of steel can be much thinner than plastic would have to be to have equivalent strength

Greg