After evaluating the possible construction techniques for my room I chose one that simultaneously solves all three of these problems. This technique was developed about 25 years ago by Art Noxon and has been used successfully in many listening rooms and recording studios around the world. Called the ASC Iso-Wall System, it is a building method that achieves excellent isolation of the listening room (sound proofing), nearly eliminates wall shudder, and absorbs excess bass. It’s remarkable that Iso-Wall addresses these three major problems with a single construction technique.
Before describing ASC Iso-Wall, I’ll tell you about the listening room’s wall construction. The listening room adjoins the house on just two walls and only partially on one of those walls (the third wall adjoins the garage, the fourth the outside perimeter). The walls adjoining the hallway and garage are framed 2x6. The wall that partially adjoins the kitchen and dining room is a double 2x6 wall with an unbridged air cavity. This wall construction is more effective in reducing the transmission of sound, but is expensive and consumes some of the room’s real estate. I used it on this wall because it happened to work out with the rest of the floorplan. The ceiling height of the rooms surrounding the listening room is either 10' or 9', lower than the listening room’s 11' height. This is advantageous because the surrounding rooms’ ceiling framing helps to reinforce the listening room walls and give them greater structural rigidity. In addition, one of the room’s long walls is connected to a perpendicular outer wall, adding stiffness.
In designing an Iso-Wall installation for my room, Art Noxon specified an elaborate framing structure that would further reinforce the external side of the listening room walls. It involved screwing and gluing, rather than nailing, the wall structure together. The plan included other non-standard and labor-intensive framing techniques. Unfortunately, this special framing was outside my budget. (Builders and subcontractors don’t like to do things that are outside the ordinary, and quote very high prices to discourage custom work.) Still, if you have the budget, the glued and screwed walls, along with the upgraded framing structure, should strengthen the walls and transmit less bass, as well as making the walls less prone to vibration.
I did, however, adhere to Art Noxon’s specification that I damp the 2x6 studs by attaching strips of 5"-wide and 1/2"-thick plywood to the side of each stud, with two strips of ASC’s WallDamp, a self-adhesive viscoelastic damping material (Fig. 2), between the stud and the plywood over nearly the entire stud length. I went to my local Lowe’s and loaded up a lumber cart with sheets of plywood and wheeled it to the lumber-cutting area. The lumber-cutting guy wasn’t enthusiastic about the prospect of making 80 cuts in ten 4' by 8' plywood sheets. But it turned out he was interested in audio, and after discovering the purpose of my odd request, fed the sheets through the saw while I stacked the strips (I was prepared and brought hearing protection).
Back at the raw-framed listening room, I conducted an experiment to understand the effect of the stud dampers. I hit an undamped stud with a hammer and listened to the ring. Then I repeated the hammer strikes after installing the stud damper. The undamped stud rang for a long time; the damped stud produced a dull and short-lived thud. The difference was so dramatic that I recorded the experiment on my phone. Many weeks later, just before the drywall went up, I applied strips of ASC WallDamp viscoelastic material on the stud edges that face the house side of the wall for additional wall damping (Fig. 3).
After the insulation had been blown into the walls, I started the Iso-Wall installation in earnest. You can see an overview of the entire Iso-Wall structure in Figs. 4 and 5. The first step is to attach with construction adhesive and a few small nails a “perimeter gasket” to all the wall and ceiling edges—on the upper and lower stud plates along the ceiling and floor lines, on the stud edges where the walls meet, and around window and door frames. The perimeter gasket will support the entire edge of the drywall wall. Next, a strip of self-adhesive “wall-bearing felt” is attached to the floor where the floor meets the walls. You can see the black perimeter gasket and white wall-bearing felt in Fig. 6 (and in some of the subsequent photos).
Once the gasket and felt are installed you’re ready to attach ASC dRC-1 resilient channel to the studs and ceiling joists. Resilient channel is a “Z”-shaped strip of somewhat flexible metal that acts as a spring between the framing and the drywall. The resilient channel is attached to the studs, and the drywall to the resilient channel. Conventional resilient channel is commonly used between adjacent condos and apartments where STC-45 sound isolation is part of the building code. ASC has taken the resilient-channel concept to a much higher level, modifying a conventional resilient channel by facing it with ASC’s WallDamp viscoelastic material. The channel is laid out in horizontal strips across the studs and screwed to each stud it crosses. A small rectangle of WallDamp viscoelastic material (called a StudPad) is located between each stud and the dRC-1 channel to help damp vibration transfer (Fig. 7). The dRC-1 channel forms a spring bed for the drywall. The ceiling is treated the same way, but with a slightly different resilient channel (ASC’s dRC-2) and closer spacing between the channels.
The release paper on the WallDamp strips along the resilient channel is removed and panel adhesive applied to the face of the perimeter gasket. The first layer of drywall is laid up vertically and screwed to the channel’s wide face. The drywall is thus affixed to the channel rather than to the studs. The channel’s springiness “floats” the drywall on the channel, reducing the energy transmitted from drywall motion to the framed walls and ceiling. It is essential that every drywall screw goes into the channel and not into a stud—an errant screw that penetrates the stud will “short out” the structure and reduce its effectiveness. Chalk lines snapped on the drywall are essential to avoiding this problem. Fig. 8 shows my room with the channels installed. You can also see the perimeter gasket (black strips along edges and around the door) and the wall-bearing felt (the white strip along the floor at the wall).
The next step is to apply WallDamp Strips (1.5" by 48" by 1/16" thick) along the perimeter edges of each plane of drywall, around the wall perimeters, door and window openings, and over the vertical drywall seams. Then WallDamp Squares (4" by 4") are applied to the drywall face 12" on center (Fig. 9). Remember that the WallDamp Strips and WallDamp Squares are self-adhesive, requiring that you remove a release paper on both sides of the strips and squares. It’s a job in itself to clean up the backing paper from these strips, squares, and rectangles, but it has to be done or you’ll end up with a floor literally covered in it.
You’re now ready for the second and final layer of drywall, which is also installed vertically and with its seams offset from those of the first layer. Again, the drywall is screwed through the two drywall layers into the resilient channel, making sure that no screws penetrate the studs. The drywall is thus rigidly attached to the wood-framed wall only along the wall edges where it meets the perimeter gasket. The central area of each wall essentially “floats” on the flexible resilient channel rather than being rigidly affixed to the studs.
Finally, you seal all the drywall joints and corners, and around electrical boxes, light fixtures, door jambs—anywhere there’s not a tight seal. I went through 17 tubes of oversized (29-ounce) acoustical caulk. Don’t scrimp on the caulk; tiny gaps can allow a surprising amount of sound through, a phenomenon called “flanking.”
Although I installed the Iso-Wall system myself (except for hanging the drywall) it was a much bigger job than I anticipated. Attaching the resilient channel to the ceiling is particularly challenging for one person. If you hire out the work, I recommend that you be on-site at all times to make sure that the contractor is precisely following ASC’s installation procedures. A drywall contractor with experience installing resilient channel is recommended, but you still must monitor the process closely. Incidentally, you can download a PDF of the Iso-Wall installation guide from ASC’s website.