Room Acoustics: Isolation & Noise Control - page 3

Locating the Theater - Above Grade
We're now dealing with four (or more) potential walls plus a ceiling and a floor. If you have any "wiggle room" at all as to what space gets the nod as your new home theater it's helpful to have a basic understanding of how sound travels from inside the home theater and out to other parts of the house.

As can be seen in the slide, direct, through-the-air, sound paths are broken down into direct and "flanking" paths. Paths can be either airborne, structure-borne (low bass frequencies) and/or duct-borne. Leaks are also a problem and probably the most commonly overlooked mistake of do-it-yourselfer-type installations.

Effective use of buffer spaces around the theater can be quite beneficial. Buffer spaces include closets, storage rooms, hallways, lightly-used bathrooms, entry areas and living rooms in certain cases. Spaces to avoid locating adjacent to, above or below the theater would include; workshops, garages, bathrooms (heavy-use) bedrooms, exercise rooms, kitchens, dens/family rooms and children's play rooms.

Sound Isolation - Fundamentals
Once the location of the theater is fixed we can start figuring out which level of Noise Criteria (NC) or Room Criteria (RC), (a better number for low frequencies) we want to try to achieve. Increasing your system's dynamic range via internal sound isolation can be costly. A home theater, well isolated internally and also quiet externally, can easily add $25,000+ to the construction costs of a home. So it pays dividends up front to hire a professional who can apply a whole-room, systematic approach to designing each sub-system within and outside the theater.

Internal Sound Isolation - Walls
An isolated home theater can be thought of as four outside walls that are solidly affixed to the rest of the home structure with inside walls, floor and ceiling that are made to "float or hang" without ever being rigidly attached. Steve started out with some slides pointing out not-so-obvious fallacies that might get by a first-time home theater builder.

The left slide illustrates the old theory of rigidity versus mass. A standard 2 x 4 wall construction with double layers of sheet rock on each side still does not have the equivalent actual sound deadening of cement cinder blocks, especially at the low frequencies, even though they are both rated at STC 50.

In the CEDIA class, Steve played an actual recording of music as heard outside of both types of wall construction, and the differences, especially in the low frequencies, were amazing. Again, this is because the STC ratings are not defined below 125Hz. Keep this fact in mind then as we illustrate other types of wall construction and dispel some myths about just how effective other forms of "miracle isolation products" really are.

Single-spaced studs vs. staggered studs. (See center illustration above.). There will be almost no difference in sound transmission. Perhaps just a little less bass.

Standard drywall vs. engineered drywall. Or mass barrier vs. constrained layer damping. Only a very slight difference in low bass (below 125Hz) attenuation will be noticed. And putting mass loaded vinyl between 2 layers of drywall is only as good as adding a third layer of drywall.

Resilient channels (RC1 channels). Can allow too much movement of sheet rock setting up a severe resonance at a specific frequency.

Inner walls themselves are usually floated on engineered isolator pads and secured to the solidly affixed outer walls by means of "sway bracing". Left slide,below.

Next, The intersection of walls with floors and ceilings/joists are more areas of potential sound leakage. Center slide, below.

Penetrations of multiple objects such as beams, A/C ducts, wiring, etc. - can make sealing walls very difficult. And a common mistake, even for none-totally-isolated rooms is not sealing off areas of cut-out drywall where wires have been brought into the room for hook-up to the A/V gear.

Internal Sound Isolation - Ceilings/Roofs
Basic isolation for a non-full-bore isolated room would include batting insulation between joists and additional/varying board materials attached to the ceiling. Resilient channels can also be used on the ceiling but the same cautions as with walls apply. Going to the next level of isolation however gets significantly more involved. Here are two typical high performance ceiling/roof isolation solutions.

It should be obvious by now that one can't simply buy a couple of spring hangers, attach them to the solid ceiling above and hope that the spring rate will be correct for the weight of the ceiling installed. As I watched Steve present these various solutions I thought of all the poorly executed car suspensions we see on our highways versus the factory-computer-tuned suspension which is found on, say, a BMW. When you're looking toward a suspension that is to perform optimally there's no room for error in any of the floor, wall or ceiling calculations. And remember too that you're integrating lighting, A/C, and other wiring into the ceiling at the same time. Every subsystem needs to be perfectly routed, mounted and secured before the floating ceiling is buttoned up. Or be prepared for a heck of a lot of hassle if a rework is required.