Acoustics Refresher: Boiling Down Room Treatment Essentials

August 6, 2012
If you’ve followed this column very long, you’ve probably noticed that I like to write about acoustical treatment. This is partly because I work in acoustics every day and think it’s really interesting. More significantly, though, I still continue to see high-end home cinemas that are poorly treated—or not at all. Hopefully, most of you are already up to speed on acoustical treatment and could just use a refresher. For those who aren’t—maybe this is your first time ever hearing about acoustical treatment—this won’t substitute for an acoustics textbook or a CEDIA lecture, but it will get you on your way.
You Gotta Have It
A small portion of the sound from speakers goes directly to the listener and the rest bounces all around the room. All those reflections cause frequency and phase errors that wreck clarity, spectral balance, and spatial imaging. This is especially true if the speakers don’t have good off-axis frequency response—which not a lot of speakers do. Even with flagship electronics and speakers, you have to manage reflected sound energy using acoustical treatment to get the right balance of direct versus reflected sound at the seats. It’s not optional, it’s not an upgrade, and it’s not something you can fix in DSP (I wish). Everyone from AES fellows to big record producers understand this, and many integrators in the residential sector have picked it up—some not quite as fast as I would like, but who’s counting?
Get Covered, but Not too Much
Sophisticated equations and computer modeling can predict exactly how much acoustical treatment a room needs. You can even tweak parameters, like what the room is made of and what is in it. Look at as many prediction results as I have, and you can pretty much boil it down to a simple rule of thumb: cover 15-20 percent of room surfaces with absorptive treatment and another 15-20 percent with diffusers that scatter sound reflections. Diffusers come in two general flavors, and you need both: 2D, which scatter sound out in a plane, and 3D, which scatter in a hemisphere. Bass absorption is important too, and it’s best confined to corners.
Are You Thick?
Thick is exactly what you want to be when it comes to acoustical treatments. By far the most common error in treating a room is to cover large portions of the surfaces with 1-inch absorptive material. This does nothing but suck out all the high frequency energy. It doesn’t address more serious acoustical concerns at mid and low frequencies. The room can actually sound worse than if you did nothing at all. I consider 40-inch deep absorption and 4-6-inch diffusion to be the starting point for a room targeting high-quality sound. You can get away with a small number of thinner treatments as part of a larger design, but you should be thinking thicker, not thinner. The treatments can all hide behind a fabric wall anyway, so depth is not an aesthetic issue.
Mix It Up
Historically, there have been differences of opinion on the best way to distribute absorption and diffusion around the room. Without going into great detail, a general balance of absorption and diffusion around the room is the objective. There are a few more practical guidelines to make the process easier.
Spread absorption modules evenly throughout the room. I like using 2x4-feet vertical strips of thick mineral wool absorption. Be a little absorber-heavy near the center of the back wall to catch the beefier energy from the front speakers. Favor 2D diffusion on the front portion of the sidewalls. Go with 3D diffusion toward the back of the sidewalls and on the outer portions of the back wall, flanking the absorption section there.
The front portion of a ceiling should have an absorption area between the speakers and the listeners. The back portion of the ceiling should have a scattering area to provide envelopment from the surround speakers and the wide dispersion energy of the front speakers. Remember that the front wall is going to see mostly back wave from the speakers, which is low frequency. Thinner treatments do nothing in that range, so mid-bass absorbers are a good call for the front wall.
Opposing hard surfaces—including the faces of some diffusive/reflective acoustical treatments—can generate undesirable slap echoes. If you have a diffuser on one side of the room, put an absorber on the other.
Assuming that your bass absorbers actually reduce deep bass energy, it is best to place them in the corners of the room where energy is highest. Making the riser into a large bass absorber by constructing it out of perforated wood and partially filled with absorption is a clever and efficient method for increasing bass absorption.
If you’re firing speakers through an acoustically transparent micro-perforated projection screen, absorb the front wall to catch slap-back off the screen. The same applies to a baffle wall.
Benefit and Profit
In addition to the technical benefits, acoustical treatment represents a great value among the products that go into a home cinema. The per-unit cost is low, the margin is high, and the product is easy to install and never requires upgrades or maintenance. Most acoustical treatment orders should represent about 10 percent of the total budget. That’s $3,000 for a $30,000 room or $30,000 for a $300,000 room, so we’re not talking about pocket change.
You could say that I’m just drumming up support for my business, but don’t let that keep you from the point. A great many otherwise-excellent rooms fall apart sonically due to poor or no acoustical treatment. It’s a problem that I identified long ago (way before I started my acoustical design firm) and figured out how to address. Hopefully, you will use this information as a starting point to learn more about how to include acoustical treatments into your business process. 
Chase Walton contributed to this column.
Anthony Grimani is president of PMI, Ltd., an award-winning home cinema engineering firm.

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