Hi Acoustic Gurus!

I'm well underway with my 28'x15'x9'6" dedicated home theater construction. I promise I'll start a thread to document the process soon ;-) The space I'm utilizing is a former crawl space under the house. I've excavated out and built a retrofit basement by extending the original stem/foundation walls downward. The result is a partial concrete bunker with the original 2x6 mud sill and framing starting from a floor height of between 2' and 10' (on a hill hence the variation around the room). I will be building a room within a room with an emphasis on sound isolation from the rest of the house but still want to maximize the room width. Just how close to the concrete walls should I frame the room? I've read everything I can and yet cannot decide if a 1" air gap is sufficient in my case. I know that 4" or more is a rule of thumb but builds utilizing this rule don't seem to have the air space I will have from the upper walls venting to above the ceiling (the joists from the floor above are over 11' high!

So am I likely to suffer low frequency coupling with the following construction:
Double DW + GW ; 2x4 framing attached to floor only ; 1" airgap ; 8-14" concrete foundation with existing house framing build on top approximately midway up the wall?

Thanks!

What a digging project. Holy smokes. Some pics of that would be great.

The walls you described are considered decoupled. A double wall. Outer wall is foundation. Inner wall is wood frame. Simple R19 in the walls will be great. Make the walls as heavy as possible. Include as large an air gap as possible.

These factors will directly affect your wall / ceiling's ability to attenuate low frequencies.

Thanks Ted, yes this is an unusual starting space for a theater build but I was determined :-) The excavation is done and it is quite amazing what the removal of well over 100 yards of dirt from under the house looks like!

Back on the isolation question. From what I understand, the air gap will form a spring between the mass of the theater room wall and the concrete foundation -- the larger the gap the better the low frequency isolation. But, given that the air gap in the upper part of the wall is larger and it continues into the space between the theater rooms separate ceiling and the joists of the floor above, is a 1" separation sufficient or do I have to make it larger (4" seems to be the rule of thumb). Note that the whole purpose of going to the effort of building this retofit basement is isolation from the house so I want to make sure I'm getting this part of the design correct.

Cheers.

More air = bigger spring = lower resonant frequency. So you make the distance as big as you can deal with, and the walls as heavy as you can deal with.

Think of it this way. Even if you use 4 sheets of drywall and a 12" air cavity, you will still hear bass in very low frequencies. You just cannot build a facility in a house that will stop 20Hz soundwaves.

So make the walls as deep and heavy as is practical for you.

The design of my basement:

Concrete foundation to 6 feet tall on 3 of the 4 walls
2x6 16 inch OC studded walls on top of the wall to the joists above

I added a decoupled 2x4 16 OC inner wall that varies from 1.5 inches to 2 inches from the concrete/2x6 wall. The 2x4s are staggered differently from the 2x6s. The 2x4 wall is decoupled from the original wall and the joists above.

It adds some insulation expense, as well, as I have to fill the 2x6 cavity as well.

In the end, it will be worth it!

Sounds perfect. You could put R13 in those 2x6 walls and be OK, by the way.

Excellent info. John, your build sounds very similar to mine. So all I think I'm missing in my original design is a bit of R13/R19 insulation. Time to start that construction thread, but until then, here is a taster of the excavation...

http://farm4.static.flickr.com/3100/3254171614_9937073cca.jpg?v=0

Now that everyone has been focused on the walls, what are you going to do about the floor? :)

Well, I'll be starting with a 4" concrete pad that is being poured between the new foundation. I intend the frame the room anchored only to the floor. The layout (I'm preparing the pics for a construction thread now) will have a sand filled stage, staged seating for the second row that may/could extend all the way back to the back of the room. I was thinking of installing drycore on the concrete within the perimeter of the room and building on top of that.

I guess the room framing could be built on top of the drycore but it will still be anchored to the concrete floor.

Is there anything I should be doing differently?

Thanks.

- Paul.

Holy Monkeys, that's a LOT of excavating!

Sounds perfect. You could put R13 in those 2x6 walls and be OK, by the way.

Thanks!

I was hoping I could do that. The differences in insulation price are rather extreme.

Does anyone use closed cell sprayed foam insulation to help isolate sound? If not, why?

Holy Monkeys, that's a LOT of excavating!

Um, ah, yes! 105 yards / 37 F650 dump truck loads! Whilst I'll be doing the rest of the work on the theater myself, I realized I met my match with that kind of digging... although, how big was the tunnel in the Great Escape? :D

I may have missed it but what about the ceiling? Also, what about the utilities, lighting, hvac, etc.... they all need to be planned for and how you do them will have a large impact on your acoustic isolation or lack there of.

Closed cell foam is a transmitter, not absorber of sound (great for thermal applications). Foam insulation (open or closed cell) which contacts both sides of a barrier defeats mechanical isolation and provides a direct flanking path. Closed cell (and most open cell) foam insulation materials do little to slow down the movement of air molecules. If you look at the mechanics of fiberglass insulation, the air molecule strikes the fiber causing the fiber to move. The kinetic energy of the air molecule is reduced.

The concrete slab is a flanking path for LF and HF energy from the structure of the house (it's mass notwithstanding).

I may have missed it but what about the ceiling? Also, what about the utilities, lighting, hvac, etc.... they all need to be planned for and how you do them will have a large impact on your acoustic isolation or lack there of.

You haven't missed it. I'm going to start a construction thread today with all the details. The ceiling joists are not shared and I don't anticipate any problems there.

Construction thread started:
http://www.avsforum.com/avs-vb/showthread.php?t=1117148

Closed cell foam is a transmitter, not absorber of sound (great for thermal applications). Foam insulation (open or closed cell) which contacts both sides of a barrier defeats mechanical isolation and provides a direct flanking path. Closed cell (and most open cell) foam insulation materials do little to slow down the movement of air molecules. If you look at the mechanics of fiberglass insulation, the air molecule strikes the fiber causing the fiber to move. The kinetic energy of the air molecule is reduced.

The concrete slab is a flanking path for LF and HF energy from the structure of the house (it's mass notwithstanding).

Thank you.

..not to mention that the closed cell spray foam is like 3-4x the cost of regular insulation batts.