January:
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 80 elements. This month we look at ADR Element #19.
Acoustical Design Element 19*: Front LCR speaker sound paths are unobstructed, clear of furnishings or objects other than the walls within 180 degrees and 4 feet of speaker face.
The presentation of sound from the Left, Center, and Right (LCR) speakers if balanced and undistorted creates a three-dimensional soundstage for the audience. Reflections of sound from walls or nearby objects act to disrupt the accuracy of that presentation. Objects, furnishings, and even plants act to create diffraction ripples in the waves even if not completely blocking the direct sound path. An open and uncluttered area between the speaker and the listener is best.
As I read through the Acoustic Design Review (ADR) Checklist or discuss it with a fellow calibrator the words sometimes seem to dissolve into abstract or even arbitrary rules. It’s those moments when I am confronted with a real stinker sound system that new life is briefed into these axioms. I recently was asked to assist a fellow calibrator to smooth the frequency response in a relatively small room. The final result, as measured by my trusty RTA was very good, but my ears told me another story. I really couldn’t listen to it, it was that bad.
Well, bad is a relative term. The client was very pleased with the improvement. The bass was decidedly tight, having lost the blurring boom that was the principle complaint. Yet my senses were still left searching for the music. Movie dialogue was intelligible but decidedly distant and uninvolving. It’s the soundstage stupid (I said to myself)! It seems the speakers, while carefully placed behind an acoustically transparent (AT) wall and occupying a satisfactorily spacious area away from the front wall, did not have unimpeded access to the listeners. In the systematic process of scooting the speakers around to smooth the response; the rather large (too large) movie screen began occupying a greater and greater section of the projected sound field. We were finding better response but handicapping the direct sounds unfettered path to the room. Element 19 was ringing in my head.
This discussion could dissolve into a treatise on “right-sizing” movie screens, but the video image was not my principle issue. My friends at ISF go to great lengths to get clients to use appropriately sized screens but not because they can get in the way of our speakers. They are insisting on creating a surface right-sized for the projector resolution, light output and ambient light in the room. The issue of a screen partially blocking a speaker will increasingly raise red flags since the popularity of the 2.35:1 screen is rising. Do I then insist that customers use impossibly small screens? Another choice has risen to meet the challenge.
Despite my discussion above, the inspiration for Element 19 is not solely based on avoiding large movie screens. Traditionally, Element 19 means we are advising clients to remove obstructive furnishings or to move speakers to be the farther most point in front of any potential obstructions. Getting the speakers even slightly out in front of an entertainment center or RPTV can be a sonic revelation for an audiophile. But what happens when a client follows my advice, uses an acoustically transparent (AT) wall, and still ends up in violation of this seemingly straightforward rule? Oh yes, I had mentioned another choice. What if the movie screen was there but not there; I mean acoustically? You see the AT wall is physically there but is acoustically transparent. What about an AT projection screen? I suspect many of you were way ahead of me on this conclusion. (I’ll suppress my urge to address the dreaded Moiré issue with AT projections screens. Suffice it to say, companies like Stewart Filmscreen and Screen Research have tackled that problem nicely.)
The solution to element 19, besides moving the speakers out front, is to render acoustically inert any remaining obstructions. In the case of cabinet encasements or nearby surfaces we naturally line them with sound absorbing foam or fiberglass. A screen can become a transparent medium as well. I can deal with the AT wall bracing and screen frame by careful alignment of the tweeter and in some cases the addition of an inert wave guide (donut) around the tweeter to squeeze the precious high frequencies though the available opening. (This also helps to keep the angle of incidence of the sound waves on the AT fabric closer to 90 degrees; lesser angles of incidence are more inclined to reflect off of even a very transparent cloth). My suggestion is to make the entire wall including the screen acoustically transparent. Be careful of a further temptation once the screen is AT; i.e. don’t forget about elements 3, 4, 5 and 8.
Before you reach for the latest copy of the ADR, elements 3, 4, 5, and 8 simply outline the proper geometry of the front LCR (left, center, right) speakers to achieve a good sounding sound-stage. I’ve seen meticulously designed systems utilizing an AT screen where the LCRs are squeezed into the width of the screen. Of course this is the way the movie theaters do it, but they have screens occupying the entire front wall now don’t they. The movie screen should not exclusively dictate where the speakers are placed. A good sounding design will allow the speakers to occupy the best sounding location; period. Isn’t that why we chose an AT projection screen in the first place? Oh some compromise is inevitable, but we should place the LCR speakers for best response and best sound-staging then avoid ruining it by blocking them with that pesky screen.
One last note about a properly designed AT wall and screen; the distance from speaker to fabric must be right-sized as well. What this means is that the LCRs should be very close to the wall’s fabric or the screen material to minimize reflections from the AT material itself. Reflections from an AT material? You’ll recall my comment on the angle of incidence of the sound wave on the AT fabric; a speaker positioned near the fabric keeps the density of energy across the cross-sectional area that sound passes through to a small circle (see drawing). Thus the angle for more of the sound is closer to 90 degrees. In addition, the remaining bounce of sound from the fabric; yup some sound still reflects off of the material, immediately encounters the speaker face and is less prone to remaining behind the AT wall creating other audible problems. Some calibrators will cover the face the speaker with sound absorption to keep a second bounce to a minimum. Generally, a distance of 6” to 12” is a good ballpark figure. Wouldn’t it be nice if the AT wall and screen could be repositioned after the speakers had been calibrated? (See www.questai.com.)
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 19 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.
Sign up for the HAA Email Club to be alerted to the arrival of the next Pearl!
* Yes, to the avid Pearl readers out there, I already did Element 19, but it then was a different element. You see the ADR is periodically updated and the ADR numbers get reshuffled. It will happen again but only in the interest of furthering the cause of great sound through science.
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)