Acoustics 101 Page 2
You should place bass traps in corners that are the most energized with standing waves. You can find them by listening to bass-heavy program material from all eight corners (including both the floor and ceiling) of the room. An SPL (sound pressure level) meter can help measure the sound energy.
Generally, one or two traps in the corners that require treatment will increase clarity and provide more even bass response. In a 1,100-cubic-foot room (say, 11.5 by 11 feet with 9-foot ceilings), this can minimize standing waves quite dramatically.
Lay It Down
Theater flooring provides either an absorptive or diffusive effect. Wall-to-wall carpet can introduce a lot of high-frequency absorption because carpets and pads are relatively thin. It’s arguably more important to control the mid frequencies and let the higher frequencies “bloom the room,” to provide a sense of spaciousness and air.
When possible, Grimani prefers combining carpet and hard surface flooring. Reduce the carpeted area to the theater’s central section, then leave a band of hard surface around the perimeter to prevent overabsorption. Use thick carpet with natural fibers and extremely thick padding with fibrous material, not foam. In more advanced home cinemas, he creates a 4-to-6-inch-deep absorptive pit at the first reflection point from the front speakers. The pit is covered with a thick perforated metal grate, then padding and carpet is added, providing excellent control of vertical reflections off the floor. This can significantly improve dialogue clarity and stereophonic imaging under the right conditions.
After the acoustical treatments are installed, walk around the room and clap your hands. You shouldn’t hear strong echoes; just a nice smooth sound decay that takes about a half second to die out. Also, avoid dead areas, which indicate too much absorption. A dead (or overdamped) room sounds unnatural. Every environment has some level of natural reverberation, so take care to preserve this balance.
It’s easier to acoustically treat a dedicated theater, as you have more control of the room design. However, acoustical materials aren’t always welcome in a multipurpose space.
So, in a multipurpose media room, you can strategically place furnishings to accomplish the needed absorption and diffusion. Use heavy carpet and drapes for absorption, and remember to leave at least 3 inches behind the drapes for airspace. If you have a bookcase, create some scatter-like diffusion by staggering the books so they don’t line up. Decorative acoustical panels (such as MSR’s Salon Acoustics line) use fabric to cover the actual acoustical material and can be customized to integrate into any décor. Again, shoot for 15 to 20 percent coverage of absorption, which includes the floors, and always address the ceiling’s first refection points. Optimize subwoofer placement and seating to deal with bass resonance issues.
The bulky audio analyzers and other measuring tools that theater designers use easily run into the thousands. If you own an iPhone, iPad, or iPod touch, there’s a more cost-effective solution—the AudioTools app from Studio Six Digital ($20). The basic modules include an SPL meter, a real time analyzer, a delay finder, wave generator, line level tester, and stereo oscilloscope. For accurate measuring with a high-quality microphone, you’ll want the companion piece of hardware called the iAudioInterface ($400). This is the setup Grimani uses for quick measurements. He also advises, “Always use your ears to confirm measurements. Mistakes can happen in the analyzer setup, and microphones don’t listen the way our ears do.”
The old axiom holds true: A system is only as good as its weakest link, so it’s more cost effective to spend money on room analysis and treatment than upgrading your speakers and amps. Plan to allocate 10 percent of the total installed equipment budget to acoustical tuning. For example, a $25,000 A/V system will benefit from a $2,500 acoustical budget, which is a relatively small investment to ensure optimum performance.
To learn more on the subject, you can read the Master Handbook of Acoustics by F. Alton Everest. There’s a lot of information on the Internet, too—just be aware that there are differing opinions on the subject. Grimani believes, “We are still in the infancy or adolescence of understanding the acoustical properties of small room sound reflections and their interactions with loudspeakers. This is common during the early phases of a science or industry.”
Discussion forums can also be helpful for interacting with other DIY enthusiasts. You should also consult with the manufacturer of the acoustical materials you plan to purchase, ensuring that you get the best options for your particular theater.
In the end, just be prepared to experiment, and don’t be in a hurry to complete this part of your installation. It may take some time, but getting it done right will make a world of difference in the sound of your system.