Room Acoustics: Acoustic Treatments - page 4

How Much Acoustical Treatment?

Tony had a "Reverberation Guidelines" slide which gives a recommended delay time between 0.2 and 0.4 seconds within a typical room. There is even a graph showing the tolerance limits for reverberation time vs. frequency.

If this sounds like it's getting really complicated, it is. To do the math requires that you know the absorption coefficients of the materials in your room (carpet, sofa, drapes, etc.), figure out the square footage area of each and apply the right amount of mid-high absorption plus the right amount of bass absorption (Helmholtz or diaphragm type). Or, just use Tony's handy rule of thumb for achieving that goal: strive for ~25% of absorptive wall area.

That's a lot of absorptive surface for most non-room-specific home theaters (living rooms or great rooms). So, if using less than the recommended 25% of absorption per sidewall, the critical placement and overall effectiveness of qualified materials can make a major difference. Done correctly, it is heard in the system's expansive front soundstage and imaging depth along with how that front soundstage effectively blends with the rear surround channels which provide a feeling of "immersion" or "envelopment".

Standing Waves

John Dahl had touched upon standing waves in Part 2 of the CEDIA Seminar series so let's look a bit more at some possible solutions. Realize though that if your room is capable of sustaining standing waves (aka room modes) around your listening position odds are you'll hear (and feel) waves of similar or greater intensity in many places within the room.

Room modes can never be entirely eliminated. That is not the goal. From the listening position, the goal is to smooth-to-an-average-overall-level, the strength and specific frequency or frequencies which tend to peak in the low frequency region below approximately 150Hz.

Note here that we've now gone in frequency below the area in which reflections were dealt with earlier. Standing waves consist of substantially longer wave lengths which produce much larger errors in frequency response by virtue of the fact that the bass wave lengths and the room dimensions can act aggressively by either reinforcing or canceling each other at many places within the room.

Not surprisingly, given the strength of these room modes, the methods used to deal with them can sound quite drastic:

• Change one or more room dimensions. Professionals like Tony Grimani often use a false, specially configured wall at the front of their project theaters to accomplish this as well as other design goals.
• Move seating location. Here we're usually talking about inches, not feet. One caveat for your prime listening position(s) is to try to always stay away from the rear and side walls.
• Move subwoofer location or add more subwoofers which, when calibrated properly, can ameliorate to a great extent the standing wave problem at the listening position(s).
• Use bass absorption. Constructing a diaphragmatic absorber is discussed in "The Master Handbook of Acoustics" by F. Alton Everest. Tony warned, however, that this is a very tough project to build accurately if you are trying to absorb a single, problem, low frequency peak. There are professionally built diaphragmatic bass absorbers which are effective across a wide bandwidth of bass frequencies.
• Equalize. Use a 1/12 octave minimum and preferably a parametric equalizer for the low bass region. You need to be able to have an EQ which is capable of centering within 2 or 3 Hz of a bass peak to effectively attenuate it. Dips in frequency response at these frequencies are always left alone. Attempting to boost a dip in bass response is an exercise in futility and serves only to suck the power reserves out of the bass amplifier.

This last slide shows how the "Every seat a good seat" is accomplished. Microphones representing each of the seating positions are portrayed above. The information is fed to a central computer equipped with software which can average the frequency response reading for all positions and apply a global equalization scheme. If the theater owner wants his two or three most common seating positions to have a better, more tightly defined frequency response it is a simple matter to tell the software program to "weight" certain microphone positions more heavily than others.

Example Treatment Layouts

Tony saved the best for last and so have we. Five of the last six slides show an ideally treated, dedicated home theater room. This level of treatment would normally only exist in professionally designed and executed home theaters costing in excess of $50,000. This theater is the reason, gentle reader, I decided to place photos of my much less ambitious living room/home theater at the beginning of this article!

Again, many thanks to Tony Grimani of Performance Media Industries Ltd., www.pmiltd.com , and CEDIA, www.cedia.org , for their generous assistance with this article.

- Patrick Hart