Don’t Spread It Too Thin

Making The Case Using Thicker Acoustical Wall Treatments This graph outlines the absorption coefficients for Auralex ELiTE ProPanels, which are fiberglass, fabric-wrapped panels. The graph shows data
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Making The Case Using Thicker Acoustical Wall Treatments

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This graph outlines the absorption coefficients for Auralex ELiTE ProPanels, which are fiberglass, fabric-wrapped panels. The graph shows data for 1-, 2-, 3-, and 4-inch thicknesses

Home theater acoustical treatments need to be at least two inches thick, and preferably four inches thick for decent audio quality. Why? Because one-inch materials only treat sounds down to 1kHz. Everything down below that is freely bouncing around the walls of the room.

For proper audio imaging and articulation, you need to control sound reflections down to at least 500Hz, and preferably down to 250Hz. Remember that the Middle A on a piano keyboard is 440Hz, and you want to go down to that, at least.

Just as bad, however, is creating an acoustically “dead” room.

It is convenient to go in with one of the franchised stretched fabric wall systems to cover up all of the walls in a theater. They are simple, quick, clean, cost-predictable, but very wrong.

They are so wrong, in fact, that I have had several customers call us after such an install to ask for help in making the room more “comfortable.” Yes, a room that is “anechoic” above 1kHz not only results in bad sound, but is also uncomfortable to be in. The brain is confused as to why there are, in fact, no sound reflections even though the eyes see that there are wall boundaries.

As for sound quality, these “one-inch” rooms usually end up with a “thick” character. That’s because sound waves above 1kHz are immediately sucked up by the wall surfaces, while sounds below 1kHz freely hang in the room for up to one second for the lower frequencies. The overall tonal balance is heavy on the bottom end, because those frequencies are sticking around a lot longer in the room.

Ideally, a well-engineered and well-balanced acoustical treatment system for a home theater would have about 25 percent coverage of at least four-inch-thick absorption, with another 25 percent of the walls covered by scattering devices. This results in a good tonal balance and good spatial integration of all speakers in the room.

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Anthony Grimani (www.pmiltd. com) is president of Performance Media Industries, a Californiabased acoustical engineering firm specializing in home theater design and calibration.

I usually find that arraying absorbing and scattering devices around in even, two-foot-wide strips works well. Of course, it’s best to actually engineer all of this with predictive calculations, ray tracing, and better yet, auralization programs. This sophisticated software allows you to build up a 3D CAD room, place the speakers and treatments in the room, and then listen to the results on your computer. It’s much easier to do trial-and-error acoustical adjustments by clicking and dragging, than by doing it in the room with power drills.

Implementation and decoration of rooms with four- or six-inchdeep treatments is admittedly more complex than traditional one-inch stretched fabric schemes. You first need to build out a lightweight framing structure that juts into the room the right amount then apply the fabric on top of that.

You can now hide the treatments in the deeper gap behind the fabric. The added bonus is that you can now hide flush-mounted on-wall surround speakers and subwoofers behind the fabric. Also, there is no need to cut large sound-leaking and rattling holes into the sheetrock, and you can surface-mount all AV wiring to the wall.

Initially, the stretched fabric vendor you use may resist this new paradigm, but ultimately the work is not so different than what they have been doing all along. And, after hearing the superior sonic results, they will realize that it was worth the extra effort.