This second post in my quest for audio nirvana presents my home theater room construction techniques, and the acoustic absorption materials selected for the room. I thought that it may be more interesting to see how the room was built before reviewing my thought process for its design.
The new room was to be situated at the far end of my house adjacent to the breakfast and family rooms and well away from the bedrooms. I like loud music and lots of bass…don’t we all! So the room’s construction had to be based upon an isolated ‘room within a room’ detached from the main houses structure, it also needed to include its own low velocity HVAC system. This would hopefully ensure that my late night viewing and music listening did not disturb those sleeping or in adjacent rooms.
After reviewing the measurement results of many drywall wall construction techniques – Catalog of STC and IIC Ratings for Wall and Floor/Ceiling Assemblies, I opted to go with a double isolated wall with staggered studs spaced at least 2” apart using two layers of damped 5/8” drywall either side. This would provide, as will be seen, the required sound reduction index (SRI) of at least 60dB. I now feel that this construction technique could have been further improved using “green glue” damping. I will comment upon this issue in a later post.
In order to allow the internal room wall a greater ability to vibrate and absorb sound energy the studs were placed on 24” centers. All external wall studs were on 16” centers in order to stop both walls having similar resonance frequencies, thereby reducing the ‘coincidence effect’. If both walls had the same resonance frequencies (critical frequency) then sound would be more easily transmitted at that frequency to the outside areas. Making the walls have different stiffness’s ensures (hopefully) that at least one of them would be actively reducing sound transmission at all low frequencies. This document from Bruel & Kjaer provides a very readable introduction, with little mathematics, into many acoustic parameters and measurements.
I also intended to seal off this rooms attic space from the rest of the house by taking the external breakfast and family room walls all the way up to the new roof and extending each rooms two layers of 5/8” sheetrock all the way to the roof.
The following drawing shows the AV room floor plan and wall cross-sections.
Sound Absorption Material
Sound absorption material would be required for treating both the rooms internal acoustics and damping for all walls and the ceiling. This may be accomplished with either Fiberglass or Rockwool. However, if you compare the acoustic performance of the two for sound absorption and damping for a wall, Roxul RW40 (56Kg/m3) wins hands down for a given thickness, in this case 3”. Note. Rockwool is not designed as a thermal insulation material like fiberglass is. This is important to understand as it meant that the two external walls, floor and ceiling required an additional layer of fiber glass in order to meet heat loss and thermal insulation code requirements.
When considering the use of either of these materials for sound absorption inside the room, things becomes more complex. There is a specification called ‘flow resistance’ that defines how easily sound enters the material for absorption as heat. Generally speaking fiberglass has a lower flow resistance than Rockwool. As with everything these things are more complicated than just considering just this one parameter and the design and type of absorber becomes important. For those mathematicians amongst us here is a detailed introduction to Flow Resistance from NASA!
Many people use the Owens Corning Fiberglass 700 series for acoustic absorption inside rooms, and I have too, but in this case, and to cut a long analysis short, I also ended up using Roxul RW40 for all internal room treatments. It also has the benefit of being easier to install and easier on your hands and lungs.
Foundations and Floor
So, starting with the foundations that were to support both the house extension and the new music room. I went with 8” concrete block with the three walls that supported the music room being filled with concrete to remove all voids and increase mass. This would reduce sound transmission to adjacent parts of the house from energy that passed into the rooms basement. These walls had a double row of J bolts to hold the two separated plates one row for the external and internal house walls and one for the music room plates. Building a second foundation wall just to support the A/V room was not an option due to budget, nor would it have provided any significant improvement in my isolation. The mass of the void-less wall was more than sufficient to reduce any sound transmission between the isolated plates.
The spacing between the plates was typically 1”-2” around the new perimeter, while I positioned the room to provide at least a 4” spacing between the rear music room wall and our family room where we watch TV and play a grand piano. The rear wall of the room was isolated from the main house by sitting that wall on a pair of joists that spanned the new outside wall to the new internal wall isolated plate.
The plate spacing isolated my A/V room floor from the house main structure and provided sufficient additional space to create a 2” space between the three remaining room internal and external walls.
Impact and vibration tests on the A/V floor confirmed that the transmitted sound to both the family and breakfast room floors was inaudible.
What you cannot see from the above pictures is that all flooring to joist, joist to plate and rim joist etc. were all caulked both sides of the breakfast/music room foundation wall sealing any open-air gaps. This was really tedious and time consuming. It doesn’t take much of a penetration or void to significantly impact an NRI rating, and as I was striving for at least 60dB nothing could be ignored. Cutting holes in the rooms walls or the adjacent breakfast room and family room walls to install light switches and power outlets can seriously reduce the room’s isolation. So this was prohibited wherever possible. This meant that all power outlets along the adjacent breakfast wall were to be floor mounted, just as in the A/V room. The only light switch that was allowed in the external wall was the one next to the house back door due to a framing issue, and all back boxes for lights, wall speaker outlets and power were installed in specially constructed acoustically sealed boxes that were part of the wall construction.
The back box was mounted inside an airtight ¾” MDF box that was later injected with hard polyurethane foam. This provided an acoustic performance similar to what would be achieved from the 2 layers of drywall. This technique was also used for all internal wall boxes that fed speakers, projector, utility socket and switches.
The Wall Framing
The outside wall framing was relatively conventional 16” on center (OC). The isolated adjacent family and breakfast room walls were framed right up to the roof. This would allow me to acoustically seal off the music room volume from the rest of the house.
Once these external walls were framed they were covered with two layers of 5/8” drywall; all joints were taped and staggered. This was again all caulked to seal any air holes between the construction/drywall and house framing. These walls would then be filled from outside with 4” Roxsul to dampen them.
I now had a sealed area and a fully isolated floor that I could frame the A/V room on. In order to reduce the chance of having the same resonance and coincidence wall frequencies and therefore significantly reducing the isolation at those frequencies the internal room studs were placed on 24” centers; external walls were on 16” centers.
Wall Damping and Absorption
Once this “free standing” frame was constructed it was in-filled with Roxsul. The wall framing (inside and out) had wire supports added to ensure that the Roxsul would not fall into the wall void between the two walls and that it would press up against the drywall, thereby applying maximum damping. I realize that in the pictures it looks like the wall Roxsul is a little recessed but it wasn’t by the time the drywall was applied. This technique was not necessary for the outside walls above the ceiling or in the family room as the Roxsul could be packed between the 16” studs from the outside.
Finally two layers of taped and staggered 5/8” drywall wall added to the rooms studs. Please note that ALL drywall associated with this rooms construction was put up using screws, nails were never going to be an option as they loosen with time. I expected significant SPL’s in the music room so the wall vibration would be an issue with nails. It is the drywall that is the ONLY thing that holds the internal shell square so its movement was not an option.
I am only posting a small sample of the photos that I have just to give an idea as to what was being built. If you have any specific construction questions after reading this I will try to answer them and include any relevant photos.
Errors, Omissions and Updates
It was determined after the room was built that the ceiling joists, while meeting appropriate building codes WERE NOT sufficient to stop a troublesome VLF resonance. I should have installed engineered I beams of the same type and size as those used in the floor. The issue that arose was that at about 7Hz and 15Hz the ceiling would flex and due to its mass this resonance could not be easily damped. This issue and its solution will be discussed in a later post. But be warned, as I could not retrofit a whole new ceiling once this issue was discovered.
The next post will examine the doors, HVAC and electrical systems.