Building the Hi-Fi House Page 3
Next, I needed to consider acoustic isolation---keeping noise from the rest of the house out of the listening room, and keeping music in it. Perfect isolation within an integral, one-piece structure is nearly impossible, of course, at least not without resorting to drastic measures. But there are a few almost sensible possibilities.
One of the listening room's long walls is shared with the garage. That, plus the outside walls which occupy perhaps one-quarter of the room's perimeter, take care of more than half the problem. For the two walls adjoining other living spaces, I used an insulated, double-stud construction consisting of two 2" by 4" stud walls, each with its own separate footing, separated by a small air space to keep them from touching each other (footnote 3).
When wallboarded on both room sides and insulated, the result is a break in the structural continuity between the listening room's walls and the walls of adjoining rooms which dramatically reduces sound transmission. And while, contrary to popular belief, insulation within the wall space does very little to minimize transmission of sound, it does limit wall-cavity resonances, which can reduce the effective isolation at those resonant frequencies. Such insulation should be loosely packed, as dense packing would have the effect of coupling the two sides of the wall---the very thing we're trying to avoid.
Unfortunately, construction practices required that my isolating double walls be attached at the top. I did request that the builder extend the double walls all the way to the roof line (with typical Santa Fe flat-roof construction, that's about 18" above the ceiling) to prevent the sound from bypassing the walls through the attic space. Unfortunately, he failed to understand or follow this specification.
To minimize sound transmission through the attic, I asked that a 1"-thick barrier consisting of two thicknesses of Sheetrock be glued and screwed into the trusses in the attic space which separated the listening room from the adjoining rooms. The builder, who by now was sure I was crazy, complied---at least for one of the walls. On the other, the attic structure was complex enough that I wanted to do it myself to ensure that it was done right. I spent many a chilly March afternoon cutting Sheetrock into small enough spaces to fit the gaps, gluing it into place in a double layer, then using silicone caulk to fill in any gaps. I also used silicone caulking to fill problematic-looking gaps in the pre-Sheetrocked walls, and used expandable aerosol foam (nasty stuff) to fill larger gaps.
The house was to have in-floor heating, so noise isolation here wasn't a problem. However, a roof-mounted swamp cooler feeding the rooms via air ducts threatened to be. The ducts couldn't be lined---the cooler would cause moisture absorption in any insulated lining. I ran a completely separate duct from the listening room to the cooler---a distance of about 25'---so that any sound traveling from another room would have to go all the way back to the cooler through its own duct, then another 25' to get to the listening room.
A room's door often defeats attempts at sound isolation. Fortunately, the layout of my house allowed for two doors between the listening room and the other common areas of the house---one in the room itself, another in an adjoining hallway. For the hallway, I chose a double-glass door intended for outdoor use, with full, weather-stripped frame and threshold. The door to the listening room is a normal interior door with a gap at the bottom. So far, it's been more than adequate; but it could be changed to a heavy, solid-core door with a seal at the bottom if further isolation is ever needed. The large windows in the room are of double-paned, insulated glass. Because of the relatively isolated nature of the house, I thought more elaborate window treatment would be unnecessary; so far, this judgment has proven correct.
Footnote 3: Another technique, simpler, less expensive, and nearly as effective, is to use staggered 2x4 studs on a 2x6 or larger footer, so that no one stud makes contact with both wall surfaces---though the footers do. I opted for the more elaborate construction; had I chosen the other and been disappointed, there'd have been no going back.