The resulting wall structure—viscoelastic damping sandwiched between two drywall layers—forms a classic constrained-layer damping system. Constrained-layer damping is widely used in many industries and is a science unto itself. When the wall is flexed by the acoustic pressure of low-frequency sound inside the room, sheer forces in the viscoelastic layer dissipate that energy as a minute amount of heat in the viscoelastic material. Remember that the resilient channel allows the interior surfaces of the listening room to bend but not vibrate, which is a key element in a constrained-layer damping system. The walls and ceiling are turned into giant diaphragmatic bass absorbers. Moreover, the energy dissipated in the viscoelastic damping is energy that’s not putting the walls into structural resonance. The framed structure is rendered quiet and inert.
A small tweak is to specify different densities of drywall for the two layers. One layer of standard drywall and one layer of moisture-resistant drywall will do the trick. The idea is that the different wallboard densities will exhibit different resonant characteristics, and thus distribute the resonant energy that remains over a wider band but at a lower amplitude.
The Iso-Wall construction technique solves the three main problems with listening rooms: 1) it greatly reduces wall shudder; 2) prevents sound inside the listening room from getting out; and 3) absorbs excess bass energy by virtue of the flexible walls, turning them into giant membrane bass traps.
Between the stud dampers, the damping rectangles between the resilient channel and the studs, the damping material on the channel, and the damping between the layers of sheetrock, there are several hundred pounds of ASC WallDamp viscoelastic material built into the room. The goal is to make the room as inert as possible, reducing the amount of wall resonance and shortening the decay time of the resonances that remain. When you think of the extraordinary effort that some loudspeaker designers put into creating inert and well-damped speaker enclosures, it makes sense that one would want similar performance from the structure in which the loudspeaker is operating.
To realize the room’s full isolation (sound-proofing) potential, the door between the listening room and house must be carefully considered. At a minimum you should use a solid-core door with a door-seal kit such as ASC’s DoorKit. For greater isolation, consider installing a full-on studio isolation door. The specialized acoustic door I chose (the Studio 3D from Acoustical Surfaces) weighs 300 pounds, is 2 1/4" thick, has a robust three-point latch system, and cost about $4000. It has an STC rating of STC-56. As massive as this door is (it feels like opening and closing a bank vault), it’s still the weak spot in the room’s isolation. To achieve full soundproofing (such as would be required in a recording studio in Manhattan) a double-door structure is required. Nonetheless, in my application the door is highly effective and in practice works as intended.
Incidentally, because I have two doors going into the listening room (the 300-pound isolation door to the house and a standard solid-core door to the garage) I was able to compare the sound-proofing effectiveness of the two doors. The solid-core door performed much better than I thought it would, attenuating the sound quite well. But it was no match for the acoustic isolation door. When standing in the hallway in front of the door with music playing at a high level in the listening room, you can detect that music is playing on the other side but it’s not nearly loud enough to disturb anyone. Move a few feet away from the door and the sound is barely detectible. Mission accomplished.
The materials cost for my room was about $13,500—$8500 for the ASC Iso-Wall products (ASC supplies a kit with the appropriate amount of dRC-1 and dRC-2 resilient channel, WallDamp strips, squares, and StudPacks, perimeter gasket, and wall-bearing felt,), $4000 for the isolation door, and $1000 for plywood stud dampers, screws, acoustic caulk, and miscellaneous supplies. That’s on top of the cost of the additional layer of drywall. At the next level, Art Noxon will design a custom ASC Iso-Wall System for your room for a consulting fee. He will also specify all the surface acoustic treatments for the interior of the room once the Iso-Wall has been installed.
The finished and painted room looks just like any other room in the house—the acoustic infrastructure is completely invisible behind the drywall. The room is now ready for surface treatments. I treated the inside walls with the Acoustic Geometry Pro Room Pack 10 that I had been using successfully in my temporary rental house (see my review in Issue 291).
The AC power to a listening room has a greater effect on sound quality than one would expect. I followed the guidance of Shunyata Research founder Caelin Gabriel in specifying my room’s power. Although one can spend an enormous sum on a custom AC-power system, I employed a few simple and inexpensive techniques that Caelin Gabriel has found to be effective. I had a good head start on the AC power; my home’s power is supplied from its own transformer at the street. I have rooftop photovoltaic solar panels but have no idea whether this confers any sound-quality benefits.
AC power to the listening room is supplied by a separate sub-panel located right outside the listening room. The room’s five dedicated 20-amp AC outlets for the audio system are wired with identical lengths of 10AWG on the same phase. Thinner 12AWG wire is standard, but don’t let the electrician scoff at the need for 10AWG and talk you into the easier-to-manage 12AWG. The thinner wire will limit instantaneous current delivery to the power amplifiers. Identical lengths ensure that each component in the audio system sees the same ground potential. I specified five dedicated lines for equipment-placement flexibility. In practice, I need only three: one for each power amplifier and a third for the front-end components. During the build, cable manufacturer Audience sent me a small sample of its new in-wall AC cable, but it was too large in diameter to fit through the conduit that the electrician had already installed. I had used Audience’s earlier generation of in-wall AC cable in another house with excellent results.
The five dedicated lines terminate in floor plugs. The electrician wasn’t able to install the audio-optimized AC outlets (the same ones in Shunyata’s AC power conditioners) because the slightly oversized Shunyata outlets didn’t fit in the rings he had installed in the floor. Consequently, the electrician installed the stock AC outlets so that we could pass the electrical inspection. (The electrician and project foreman both thought the AC power requirements were bizarre and unnecessary—not to mention their opinion of the Iso-Wall construction.) Although I was disappointed at the time, the situation turned into a valuable learning experience. Two weeks after moving in I got the reference system up and running (see sidebar). I spent the next eight weeks listening to the system powered by the stock outlets. I then got another electrician to modify the existing floor-plug rings to accept the Shunyata AC outlets. I was thus able to listen to the effect of the custom AC outlet independently of any other variables. I was beyond shocked by what I heard. In my previous house I had compared dedicated AC lines and custom outlets to stock AC with stock outlets, but never to just the outlets themselves. After replacing the stock outlets with Shunyata outlets in the new room, the bass was fuller and deeper, textures were more densely rendered, dynamics wider, and the sense of individual objects existing in space was heightened. Replacing your AC outlets is a relatively inexpensive upgrade (see Neil Gader’s review of the Furutech GTX-DR NCF AC outlets in Issue 291).