If you’re serious about your home theater, you’ve probably spent a lot of time agonizing over what gear to buy. But what about the room itself? Even with extremely high-end gear, you can’t achieve optimum audio performance without paying attention to the acoustics. Without room treatment, expensive speakers can sound awful, but even moderately priced speakers in a properly treated room can sound terrific. Some experts even say the speaker system and electronics contribute only 50 percent to your system’s overall sonics—with the room responsible for the other 50 percent. If you’re not factoring in acoustics, your system might sound only half as good as it could—and should.
To help you get your system sounding its best, I consulted with Anthony Grimani, founder of Performance Media Industries (PMI). Grimani specializes in the design and engineering of home cinemas and played a key role in developing many of the standards we take for granted in home theater technology during his tenures at Dolby Labs and Lucasfilm THX. He’s also the owner of Media Specialty Resources, Inc. (MSR), which distributes specialty audio/video tools such as acoustic treatment solutions, audio calibration kits, acoustic analyzers, and soundproofing materials. I’ll be citing some of MSR’s offerings as examples of what kinds of treatments to apply in different locations, but keep in mind that there are many suppliers of room treatments to select from as you’re planning your room.
The main reason to acoustically treat any room is to obtain a proper balance between direct and reflected sound at the listening position. When listening to a sound source, your ear hears both direct sound from the speakers and reflected sound bouncing off the surrounding hard surfaces. Moreover, only the direct sound provides your ears/brain with proper sound positioning.
In a home theater environment, especially a dual-purpose space such as a family room, you’ll have plenty of surfaces aside from the walls for things to bounce off: coffee tables, chairs, cabinets, fireplaces, floors, windows, etc. But even in a dedicated theater, you’ll still have the walls, floors, ceiling, and seating to contend with.
“The critical distance [to the ideal listening position] is that point where the direct sound energy is the same level as the sound energy from the reflected sound,” Grimani explains. “Any closer, and the direct sound dominates the mix. This provides better clarity and articulation, but it lacks envelopment. Move further back, and there’s only reflected sound. The soundfield appears larger; however, you begin to notice lower intelligibility, imprecise directional separation, and poor articulation. For optimum multichannel sound, it’s imperative to find this critical distance.”
Room treatments come in different types to perform different functions. Absorption materials absorb sound, and diffusive materials scatter sound; both are used to treat walls and ceilings. A polycylindrical diffusor, essentially a raised half circle or hump that protrudes from the wall or ceiling, is one type of scattering material. Other diffusors have irregular shapes and patterns.
Typically, 20 percent of the wall and ceiling surfaces are covered with absorptive materials that are at least 2 inches thick—preferably 4 to 6 inches—with a density of 3 pounds or more per cubic foot. Grimani’s opinion is that common foam materials are usually too light to control lower frequencies, and high-density fiberglass is often too hard to control high frequencies. He believes the ideal material is 4-to-6-pound Rockwool, a mineral fiber made from recycled volcanic rock (also known as slag). Another 20 to 30 percent of the room should be covered with scattering materials that are 4 to 12 inches deep. This level of thickness is required to diffuse audio waves down to at least 500 hertz. Having about 50 percent reflected sound generally strikes a good sonic balance.
You’ll need an absorber at the points of first reflection off the left, right, and back walls. To find the source of first reflections, slide a mirror along the room’s side wall. From your main listening position, you can determine the first reflection point when you see the speaker in the mirror. The process goes faster with a partner moving the mirror. Always intersperse the absorptive and scattering materials in even, vertical bands all around the room. If you place a diffusor on one side wall, there should be one in the corresponding location on the opposite wall.
The ceiling is a critical reflective surface, especially at the first reflection point between the front speakers and the seating position, and requires at least one or two absorption panels. The ceiling area behind the seating position may also require scattering materials.
Rumble Down Under
An untreated room radically alters a speaker system’s low-frequency performance. “A combination of resonances, like in any enclosed cavity, along with individual reflections from nearby boundaries, will completely mess up the performance built into the speaker by its designer,” explains Grimani. “There is simply no way around this when you wrap walls around a speaker system.”
Bass traps help overcome these low-frequency resonances and are generally placed in room corners. This is where resonances (also known as standing waves) from the three axes (two sets of parallel walls and parallel ceiling/floor) intersect and are usually the most powerful. Without bass traps, the effects of this resonance can create muddy, indistinguishable, and highly uneven low frequencies.
Despite the plethora of fiberglass and foam bass traps available, Grimani contends that most don’t have much effect on the lowest frequencies—particularly those coming from the subwoofer—and are best suited for treating midbass frequencies in the 100-Hz to 200-Hz region. Grimani’s favored treatment is the SpringTrap he offers through MSR, which absorbs standing bass waves from 30 Hz to 100 Hz. The patented trap combines a large pistonic diaphragm that actuates three Helmholtz cavities to dissipate low bass energy in the room.