John Sayers' Recording Studio Design Forum Archive

My calculations show 12" duct requirement for the CR.

That sounds rather large for a typical home studio. How did you arrive at that size?

After designing an HVAC silencer for the CR ceiling (using two 6" ducts, each with a silencer),

Two 6" ducts is not the same as one 12" duct. Two 6" ducts has only half the capacity of one 12" duct (half the cross sectional area). You would need four 6" ducts to get the same capacity as one 12" duct.

realized it might be better to slightly oversize the duct and go with two 8" ducts since my calculations are right at 12".

Two 8" ducts is still not the same as one 12" duct. Two 8" ducts is only 88% of the capacity of one 12" duct. Two 8" ducts plus one 6" duct is about right to replace a single 12" duct.

QUESTION: Does anyone have any advice or experience building an 8" round duct silencer to fit in such a space? Thank you.

If you have an 8" duct, then the box cannot be slimmer than about 12", and possibly more, depending on how much isolation you need (in addition to the insertion loss), which defines how much mass you need, and therefore the thickness / type of wood. So for 8" duct, there's no way to make it less than about 12" for each of the minor dimensions. Therefore you'd need at least 14" of space to fit that box in, which means it won't fit in a standard stud bay or joist bay (Or rather it can be made to just fit, but not with good spacing).

But getting back to the original issue here: I suspect that you may have made a mistake in your calculations, or your layout. I vary rarely need individual ducts 8" or larger when designing typical studios. I normally use plenums for the main runs, which can handle greater flow, then use small round ducts only for the runs to the individual silencers / registers.

What flow rate and flow velocity are you using for your calculations? And what static pressure are you using? Something doesn't seem right.

- Stuart -

Stuart

It has been a while since I did the calculations. I will go back and review my notes and post the data I used for the calculations. Maybe I missed something or mis-understood a concept.

JK

OK. I feel really stupid after reading your response Stuart. It's simple math for the area. (2 - 6inch circles = 56.5 sq inches. 1 - 12" circle = 113 sq inches). Thanks for pointing that out.

I may have figured out where my previous calculation went wrong. My room requires 258cfm. I used the calculator at https://www.cdicurbs.com/ductcal

If you look at the two pictures (screen shots of the calculator), one of them suggest a flexible duct of 8", the other 10.8" (I still think it was even higher the last time. I must have done something different). Anyway, the difference in the results is due to the number of 180 turns. I entered (3) originally because I thought that is what the Silencer was really doing. Maybe that was a mistake. Please see the screens shots to see the calculations.

Do you still think 8 inches will be enough based on my CFM requirements?

FYI: The house Furnace is rated at 1600 cfm and the Air Conditioner is 3.5 tons

Thank you

JK

I'm just wondering if I'm close here, or if there is more information I need.

On a side note, it appears to me that folks are using the same size return ducts as the sends. The residential HVAC guides I have seen suggest the return is supposed to be larger. Having said that, I think I read somewhere on the forum that it was suggested that there be some "back pressure" developed in the room, which maybe is the reason I have seen the Sends and Returns the same size. This is all very confusing to me. The more I read about HVAC, the more I read. Not a lot is sinking in. Just the basics.

So my outstanding questions are:
1) Am I looking at 10.8" duct requirement or 8" based on previous data given?
2) Should the send and return Silencers be identical in size and design?

Thank you
JK

As I mentioned in a previous post, there just is not enough room above the ceiling for an HVAC silencer. Especially for one to accept a 10" flexible duct. I have decided to take some length off the room and bring the duct in from the rear. It really is my only other option. I have attached two pictures. One shows the silencer on the rear wall. The other shows an extension off the silencer into the room. I would install an HVAC diffuser at the end of the extension.


JK

That looks like it should work, but the silencer itself doesn't seem to be very big, for that amount of air flow.

You might also want to consider running that protruding part horizontally above the couch, side to side, feed from the middle by the silencer box, and with a vent on each end.

- Stuart -

I've made a little headway lately. I started framing the CR and ISO room. I need to get these up and running to begin work on a project that starts early next year. The Live room is probably a late 2016 project. Anyway, I thought I would share a few pictures of the current construction going on.

JK

In the second photo 'Entry to ISO' it looks like you have wooden brackets connecting the stud wall to to the concrete wall. There also appears to be a couple of small crossbeams joining both stud walls between the ISO and CR.

Aren't you creating flanking paths between your two leaves?
Whats the purpose of these components, perhaps just temporary bracing during construction to be removed later?

GraGra

Yes you are correct. They are just temporary bracing to insure everything is plumb and square until I finish all of the framing.

JK