Axiom Home Page Forums General Discussion Home Theater HT in-progress

TBD ... A separate 8" supply duct (some of which is flex) was run off the main supply far from the HT. The overall length is something like 40 feet to the vent in the HT. Right outside the HT the 8" duct expands into a 6 foot S shaped 16" duct surrounded by safe and sound after which goes back to an 8" duct again. The 16" flex is designed with plenty of length (like an accordion) to move and absorb energy. My thinking is that the sound pressure would be reduced 4x by expanding into the larger flex and that a lot of energy will be dissipated by the large flex wall area. Some will make it though to the other side but how much I don't know. Since I put in a access panel I can experiment.

Ok, the 2nd part to the screw holding power and rattle

When I examined the resilient I notice that a lot of the screw holes were bent outward making them larger so it wasn't engaging the screw well. This was probably why when I shook one part of the wall another part would rattle, The screw was rubbing the resilient and not holding the board. Cause ... the installer was over torquing the screws. It would not be good if that explosive movie brought the ceiling down on me ... that's a little bit more in the action then I'm going for ...

I did some experiments and this was the case. If I over torqued the screw the resilient could easily be pulled away from the board easily. If I torqued it snug the resilient was on tight.


I'm not sure this is a good idea. I'd want a solid connection where the screw was firmly attached to the channel. If you allowed the screw to move around it might expand its hole or work it's way out.

I am planning OSB as the first layer. My main reason was so I could attach things to the walls like molding/speakers if needed. This is especially true of the ceiling as many speakers will be hung on it.

The room is being built the way it is to be as sound proof as I can make it. My understanding was that the extra board would make the sound proofing better. It was also my understanding that there is little low end sound adoration with two wall boards.

Now you have me questioning that. It would be great if anyone could give me more feedback on this ...


My current plan for the low-end is to drive it with 4 subs to even things out.

Oh sh*t ... I'll have to get one in there ...
Wish I thought about this before ... it's not going to be easy now ...

Make sure your installer uses fine thread drywall screws -not normal coarse. They wont pop. I put over 2000 in my room with an impact driver. None popped.

Worst place for rattles was near corners. The silicone will smoosh completely flat. Just eliminates any flapping in the channel. With your layers not a concern. Your wall wont flex at all.

Here is a link for common wall assemblies

https://www.stcratings.com/assemblies.html#05

I think you would hit 60-65 or so with your plan.

I was thinking about what you said here and I may have a solution.
But first a few thoughts ...

There are two different goals which maybe fighting one another:

- sound isolation (keeping the wife happy & minimizing ambient noise)
- taming modal reflections in the low frequencies

STC does not cover frequencies below 125Hz ... so a better STC does not necessarily translate to better low frequency isolation. My understanding was that three 5/8 sheets were better than two because the heavier large area on clips would work to better dissipate the low-end. But now you have me wondering if the OSB is too stiff to let the double wall green board sandwich do its job (which possibly make STC worse ... )

Wish I had data on all this but ... An interesting data point on ply/green-glue isolation is that green glue is often sandwiched between plywood sheets in ceilings and floors. Also, I've seen many write-ups where a wall board green glue sandwich was attached to the ply under the floor to do the same.

The 2nd issue you pointed out is that there are benefits to not stiffening the wall too much as the double wall green glue sandwich absorbs modal energy. I wish I knew more about this as I was considering the resilient/walls to only be for isolation w/o benefit to sound inside the room.

I have no practical experience on this topic ... but it makes sense to me that for the green glue to do it's job the wall board needs to flex. So, without any data to back this up, I'm thinking of putting 1/4" wood strips between the OSB and the double drywall. It would allow the double wall board to vibrate and do it's job while still having a backer board to screw into. It also provides the same mass wall on the clips which might help dissipate the lower frequency energy (yet another "I think it might" unknown: does it help, hinder or do nothing). It's my hope that it will will help (especially in the ceiling) with the low end and reduce flanking.

Anyway, I'd like to get any feedback on the above and especially on implementing the 1/4" spacing idea.

This is all very impressive.
The idea is to (aside from getting good sound within the room) to keep any sound from emanating out? Almost sound proof?

I had looked into various sound control design methods and came across a manual of sorts put out by the NRC or some similarly educated group of engineers. It had great info on STC ratings for various materials, insulation, etc. It had notes about offset studding and dual walls, etc. Space was a limitation to some extent so inches upon inches of extra walls and materials wasn't realistic nor worth the cost, but the STC ratings were a huge help.

I decided that simply building a room that could reduce sound outflow by even 20dB would be sufficient to control anything but heavy bass to a very acceptable level within the rest of the house so i used the STC ratings in the manual to add up various materials to attain a STC somewhere between 45 and 60 (again this is so long ago i don't remember the exact number). Double dry walled ceiling (with a denser board type, cannot quite recall but "concrete" board or something like that?) with 12" of stuffed Roxul and same for the room walls except only 6" depth on the Roxul was reasonably inexpensive and has been very effective. Closing the doors to the room (which have an inside latch for a tight seal and to remove any door rattle) cuts the sound outside the doors by 30dB. On the main floor immediately above, you can only hear bass and it isn't defined enough to really make anything out.
The remainder of the basement rooms were all finished with interior Roxul insulation as well for two reasons. Further reduction of sound on that floor, up the hall from the media room and second, to help with basement room heat/cooling insulation.
I have to say, for the slight extra cost, we didn't have to go crazy and it controls sound outflow very well.

Chess, I started off reasonable ... double wall board on staggered studs but then the builder told me he knew how to do resilient channel ... and once I started down that rabbit hole I had to keep going ...

The only issue I see with leaving a gap is the triple leaf effect. Between layers any air gap non damped with material is going to create a space for sound to resonate and actually lower the stc.

Line the osb with a soundmat then drywall. Bunker. Yet flex in drywall. This ties the floor assembly logic you mention into a wall. You should be able to find reliable data on the idea or similar

Very important to understand your home vapour flow before sealing a room with non permeable membranes. Ensure your home is dry and know moisture always dries to the outside in cold climates. Below grade, be sure you have conditioned air movement and low humidity. Any moisture between the mlv and osb will dry slowly. Ensure this space never gets cold/moist to reduce chances of condensation.

https://www.usg.com/content/usgcom/...nt-prep/sound-absorbing-mats-boards.html

Page 10 info. Wood wall assemblies page 20.
http://www.usg.com/content/dam/USG_...ssets/acoustical-assemblies-en-SA200.pdf

The basement stays pretty dry. The only time I’ve seen any wetness was when they were building the house before grading and gutters … and then it took a constant down pour with all the water from the the roof running down the sides of the foundation. Haven’t see anything since and we’ve had some pretty good storms. Bottom line is I’m pretty safe … but that said it’s not 100% certain.

I think I’ll beef up the drainage system a bit anyway …. especially on that side of the house.

Are foundation slabs and walls not protected with an external non-permeable membrane nowadays?