Room Acoustics: Isolation & Noise Control - page 4

Projectors, Lighting Fixtures, Ducting and Doors
Ceiling mounted projectors must be attached to the solid structure above a floating ceiling, so the projector mount will penetrate through the floating ceiling. The penetrating part of the ceiling mount can be a simple pipe-type mount which requires only a bead of acoustical sealant around it or it can be a more elaborate scissors-type mechanism in which a whole panel carrying the projector drops down and into position for play. In either case, care must be taken to properly isolate the fixed structure above from the floating ceiling of the home theater to avoid short-circuiting the mechanical isolation between the two surfaces.

For lighting requirements; it is recommended that low voltage lighting be utilized and spaced a minimum of 6 feet apart. Running 117V AC through a ceiling and close to projector interconnects can cause problems so a high quality, low voltage lighting system is essential. It is usually recommended that a separate 117V outlet be mounted close to the projector and that it be properly grounded. It is helpful also to run a second, completely independent circuit for the low voltage lighting to avoid problems with transformer noise and the A/C so as to isolate turn-on spikes and system noise.

Ducting can be quieted by means of a product called duct lining. Use a minimum of 1" thick but preferably 2" thick duct lining to quiet A/C lines down to 500Hz. The ½" material sometimes used as duct lining is almost useless for home theater application so as long as you've got access to the A/C ducts use the thickest material that makes sense. Software programs for calculating the approximate dB attenuation you can expect from applying duct lining to your A/C can be found at the websites of major HVAC and Noise control manufacturers.

Finally, home theater doors should be considered an important subsystem themselves. The lowest level of a noise suppressing door would be one similar to the entry door in a home; solid wood with excellent gasketing around the sides, top and across the threshold is essential. Beyond the single solid door solution is a custom, STC-rated acoustical door with its own custom gasketing system or the more common approach for a well-isolated home theater which is the double door solution. One swings in, the other, out.

Setting a Realistic Isolation Goal
If you're considering an isolated home theater or even seeking some sort of benchmark with which to judge the current noise level of your listening room, you'll need the help of a qualified professional with the appropriate test gear. I like to think of these professionals as someone like an ISF calibrator for setting up an HDTV. The only difference is that you can buy a disc like Digital Video Essentials or Avia to try to do the ISF calibration yourself. But I know of no inexpensive test software which can accurately plot an NC curve. And, even if you manage to hire a professional to give you an evaluation, you'll have to do the work yourself, hope that you make no mistakes along the way, and rehire the professional to take the curves once again after you're done.

Earlier I gave two examples of very quiet environments. The THX certifiable room and the Harman Multichannel Listening Lab. Of the two, the room most of us would want to emulate would be the THX room with it's Noise Criteria rating of 22. Steve Haas suggests that most isolated home theater projects should set the NC range goal between 20 and 25. Study the slides below. Achieving an NC of 20-25 is a daunting task. An RC of 25 which takes into account the 16Hz and 31.3Hz bands is even more difficult!

Note that the Harman MLL with its NC 5 rating is a special-case design. The NC 5 figure is necessary because the speaker systems listened to double-blind in the Lab are only played at 75dB (B-weighted) at all times to avoid listener fatigue. This is not the situation with an isolated home theater. Also, at the "loud end" (115dB subs and 108dB satellites) of THX's home theater criteria you're talking about seriously powerful subwoofers, usually two or more 15"ers, and very high performance satellites with 6.5" or greater woofers which only need go down to 80Hz to satisfy THX.

In building an isolated home theater room from scratch, and keeping the total system under, say, $50K I would be comfortable recommending the following perimeters and dollar allocations;

• Given an outside neighborhood NC of 45, the interior NC and RC goal would be between 20 and 25, meeting both Noise Criteria and Room Criteria measurements. For a professionally managed, built and test-verified room allot $30K. (Here you're essentially buying 20dB more dynamic range than you would have with a non-isolated room with the windows open. Close the windows and drapes and the neighbors will still hear at least 10dB more noise than ambient within their neighborhood. It's likely the figure would be 15 dB more noise than ambient and you haven't even started to turn the system up to listening levels!)
• THX-certified home theaters aim to reproduce the maximum possible peak playback (115dB) SPLs of commercial theaters. Most people, once they experience these types of levels in small rooms, feel uncomfortable and sometimes nauseated if these SPL levels are sustained too long. Being in a smaller home theater environment exacerbates the situation, especially for less SPL-enamored significant others. Also, speaking of dollars, THX certification for home theaters is usually sought once the cost of the home theater exceeds $250,000.

I'd recommend a full 8dB lower bass level within the home theater at the listening position to keep the $30K cost-of-room figure in line. Extending the dynamic range on the high end by enabling the system to play louder will cost much more in sound attenuation than a $30,000 room can hope to contain. A maximum of 108 dB at the listening position for frequencies below 125Hz and 100dB for the satellites at the listening position seems a much saner goal. To put these levels in perspective, note that theater THX levels require 85 dB SPL two-thirds of the way back into the theater whereas when you set up your home receiver most manufacturers have you set your baseline level at 75dB. There is a far greater percentage of reflected sound within our small space home theaters when compared to commercial theaters. And this lower 75dB figure partly reflects that fact.

Now take another $5,000 out of the cost of the home theater room and system for "accessories" which we haven't even talked about like specialized seating, popcorn machines and wet bars whose plumbing-noise problems were not explored here and we're down to $15,000 left over for all the audio and video gear.

That $15,000 figure for audio and video gear without an isolated room is probably a pretty good average dollar figure for home theater enthusiasts who are contemplating the "next step" up that a professionally-built, isolated home theater represents. So that's where this story ends. To get past the performance plateau dictated by being a good neighbor or at the least, insuring domestic tranquility, an isolated home theater is about the only alternative, desert islanders not included.

If you are interested in learning more about room isolation, one of Steve Haas' most recommended books is Noise Control Manual for Residential Buildings by David A Harris , 1997 McGraw-Hill. Other titles can be found at www.inceusa.org.

To find a certified Home Theater building consultant in your area visit www.CEDIA.net. Or you can contact Steve Haas directly through www.shacoustics.com.