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Room Acoustics: Acoustic Treatments

by Patrick Hart October 24, 2004

Taught by Anthony Grimani of PMI Ltd.

Anthony Grimani's "Room Acoustics: Acoustic Treatments" class followed his course on "High Performance Home Theater Calibration" on the afternoon of my first day at CEDIA. I'm reviewing it here, as the third installment of our CEDIA Seminars because it flows well and builds upon John Dahl's "Acoustics 101" course. A CEDIA System Designer-in-training would normally take these two classes in this sequence so that is the order in which we are presenting them.

Critical Distance
Have you ever tried to grasp concepts in learning something new? Like room acoustics for instance? If you've read Parts 1 and 2 of The CEDIA Seminars you've heard it said that the speaker system and the electronics contribute 50% of the sound of the system and that the room is responsible for the other 50%. That was all well and good for me, though still pretty nebulous, until, that is, Tony introduced the concept of Critical Distance. Critical Distance is the distance to the listening position at which the sound from the speakers and the sound from the room are equal. And, he contends, that in a typical home theater, before acoustic treatment, that distance is usually only 6 feet from the speakers!

Wow. I found that stat' amazing. This seemed to possibly explain why for so many years I'd read of reviewers listening to very large stereo speaker pairs from what seemed like ridiculously close distances. Were they trying to get within the Critical Distance criteria? Had they been aware of the room reflections and realized that they were not, in fact, truly listening to the speakers when they sat back at more typical listening distances?

The slide above is one of the best you're likely to see on the subject of room reflections for mid and high frequencies. In addition to noting all the reflections vs. direct sound depicted on Tony's slide note also that the absolute worst position to sit if you're in a typical rectangular home theater would appear to be right in the middle. The middle spot gives you perfectly symmetrical 1st, 2nd, and 3rd order reflection paths so they're theoretically at equally balanced strength as they reach your ears! Not at all desirable.

But don't move away from that center position just yet all you old-time stereo diehards. Remember a couple of important points here.

  1. If you move off center figuring you can get away without acoustically treating the first reflections, forget it. You'll still get them. More reflections at slightly differing intensities but reflections none the less. So you've got to do something about them.
  2. Some might ask "What about the auto EQ functions built into many of today's more feature laden receivers?" The answer is, that these room EQ receivers, which come with a microphone intended to be placed at the listening position, are not capable of discerning a difference between a 1000Hz direct wave or the whole slew of 1000Hz waves that might arrive at the listening position only a couple milliseconds later. In other words, the receiver is trying to equalize for all the 1000Hz notes "heard" at the listening position! It follows then, that to allow the receiver's room EQ feature to work accurately, we must absorb strong, first reflection waves over a very broad bandwidth. And, Tony Grimani recommends not EQing frequencies above 1KHz reasoning that a quality speaker system, appropriate to the level of refinement of your home theater, should have no problem producing a tight tolerance, linear response in the first place.
  3. Virtually no room is perfectly rectangular. Most non-theater-specific rooms have doors, windows or perhaps archways leading into another living space. So it's highly unlikely that the "perfectly centered" scenario will ever be the most detrimental contributor to reflections. The speaker/room interface is itself the cause of the reflections and it's pretty darn hard to get rid of either. But it can be done. Stay with us.
  4. This is, after all, home theater, "surround sound". So unlike the old stereo mantra wherein the best sound was had by placing oneself exactly in the center of the audience, we've now got a center channel which carries 70% of all movie dialog and sound effects while also anchoring the center image for listeners at every position. This is true only when all other room set-up, tuning, level setting and equalization is done correctly and in a methodical way.


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