March:
Each month we look at a different acoustical design or calibration Element. Taken from the HAA Acoustic Design and Calibration Review checklists, these elements detail the many attributes that define high end home theater sound. An HAA Calibrator can perform an analysis of your system by reviewing each of the over 70 elements. This month we look at ADR Element #35.
Acoustical Design Element 35: Listening positions should not be pre-disposed to extreme bass response anomalies.
One of the problems associated with small rooms is the effect of standing waves or room modes on low frequency response smoothness. In rooms which are generally rectangular, assumptions can be made on the expected response at any given location. For less symmetrical rooms, computer modeling must be employed to understand the response. Other acoustical issues such as boundary gain and boundary interference also point to avoiding certain listening positions in the room. In most cases, the least desirable locations are near any wall and the center of the room.
The concept that there are good and bad places to sit in a theater is not earth breaking news to most cinema aficionados. Whether it’s in the neck-breaking front row or eye squinting back row, most folks know exactly where not to sit at the big show. Transferring this insight to the home cinema is not straightforward. For one thing, most home cinemas (excuse my honesty) are not exactly havens for sonic ecstasy. The bass is usually turned up too loud anyway and someone is always complaining that they didn’t catch the last line of dialogue. My apologies to those amongst our fair readership that have excellent sound; I’m merely pointing out that many folks are too involved in suppressing the fatigue of poor sound to begin exploring the solace of the best seat in the house. Besides, there is usually only a couch or maybe a couple of Lazy Boys to choose from and the video display is not (usually) hidden behind the bobbing head of the guy in front of you. In the end, most live in ignorant bliss assuming the boomy bass is correct and even applauding its excess.
For many audiophiles the best seat is a well defined spot diametrically placed between the left and right speaker. On the occasions when they are “forced” to use the center speaker for a movie, the dialogue blurts out like its hanging from the car window at a drive-in movie theater. (That’s not too obscure a reference for everyone is it? My town actually still has a drive-in and the good old metal speakers have been largely replaced by FM transmissions to the car radio.) The center channel has been relegated to the collective mind set of the populace as the “dialogue speaker” and it is best to be turned several dB louder than the other speakers… right? Well, this Pearl is not about the true value of a properly tuned and matched center channel changing the proverbial sweet spot into a sweet area, but rather about smooth bass. I will leave to a future discussion why a center channel is the most important speaker in the house. It is mentioned in this article because of our desire to make many listening positions good sounding and its pivotal role toward that aim. Let’s examine why one would not usually want to sit in the center of the room.
This heretical concept of not listening from the center of the room is usually greeted with boo’s and hisses from my audiophile friends. The truth is I have spent more that a few years listening from the room’s central sweet spot drinking in the luscious soundstage so painstakingly crafted from the careful repositioning of the left and right speakers. Then I got a subwoofer, and became both enthralled by the majesty of those elusive bottom two octaves and repelled by the insidious boominess and loss of clarity. The truth is the center of the room is where the rooms’ resonant frequencies, referred to as modes, are the most schizophrenic. Certain modes cause narrow frequency bands to swell up to boomy heights, meanwhile other equally narrow frequency bands are suppressed to virtual oblivion. The end result is a roller coaster ride response curve providing little satisfaction to the enlightened listener. So how do I keep my soundstage and have my smooth bass? You guessed it; scoot your Lazy Boy and your center channel (right and left speaker in tow) out of the rooms’ center. The front soundstage will still be intact if you symmetrically move everything to the right or left a foot or so and similarly the listener forward or backward. The soundstage remains symmetrical and the bass is not predisposed to poor quality. Where else can we expect poor bass quality?
The typical placement of listening seats near walls in our living rooms and home theaters is a direct result of the human mania for proper space management. In some cases, we are trying to place 10 lbs of “doo doo” in a 5 lb bag or alternatively we need that open “space” feeling in the room. In other cases, the room is laid out such that the walk ways enter near the center of the room; it is just darn hard to put a sofa anywhere but against a back or side wall. Poor quality bass is often overshadowed by the impression that more is better. If you want more, sitting near a room boundary or two will give it to you (for bass that is). While in the center of the room, modes sound either too loud or too quiet. Against the wall though, they are all on full blast! If you have ever listened to your system from one of your rooms' corners, you’ve experienced the finest manifestation of this phenomenon.
Most folks, allow me to be more direct, most males do not verbally observe excessive poor quality bass even if they sense it subconsciously. On the other hand, most women tend to be brutally honest about it. I have heard about more than one wife or girlfriend refusing to watch a movie until it is “turned down”. Sorry for my sexist observation, but I believe most folks will agree with my assertion and it is a great complement to these frank ladies that they won’t stand for bad sound. Men seem to gut their way through the loud boomy passages yet over time succumb to the bass induced listener fatigue and either turn the sub down or listen less. The proper calibration of a subwoofer is an exercise in subtlety. A properly calibrated subwoofer is sublime; a football linebacker that can ballet to repeat a metaphor. Of the five basic elements of subwoofer calibration, the first is listening position. ADR Element 35 embodies this concept. Good bass is not wimpy, on the contrary, it is deep and can be quite loud; it is above all smooth. Most folks find a properly calibrated sound system to be quite tolerable and even inspiring at high sound levels.
I’ll finish by pointing out a bonus to the common household rectangular room. By maneuvering away from the rooms’ center and the boundaries we find four areas of modal moderation. These moderate zones, as I have taken to call them, provide not only the smoothest bass, but also as a bonus, display similar acoustical qualities. This means that if we make changes to the subwoofer position or equalization to smooth the response, we’ll be generally improving all of these areas similarly. The true challenge to designing a great theater is not discovering where to set a listening chair but rather how to arrange seating to place most of our guests in these areas predisposed to smoother bass. As a rule of thumb, keeping listeners away from walls and out of the center of the room is a good start.
I am sometimes confronted, during friendly discussions with clients, about the relative importance of one or another "minor" acoustic or setup flaws in their home theater. Does proper setup and calibration really matter that much balanced against other priorities? In the final analysis, just what is missing due to any of these seemingly innocuous misalignments? I think the answer is best explained from my perspective as a music lover relating how I am occasionally teleported in time and space by the majesty of an amazing recorded performance. If never on a quiet evening in your home, a recording has astonished you with its realism and moved you emotionally, which among these few subtle acoustical flaws has robbed you of the experience? If you have not been surprised recently by your sound system perhaps Element 35 could be a contributing factor, of course, don't forget about the many other Design and Calibration elements in the mix. Next month another Element.
Gerry Lemay (Gerry is the Director of the HAA, President of Quest Convergence Systems, and writes the Home Theater Rx column for Home Theater Magazine)