November:
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 #40.
Acoustical Design Element 40: Listeners should not be placed too close to any direct radiator speaker in the system.
The integrity of the soundstage depends on a balanced sound level from all speakers. Sound level matching is a calibration element, but becomes compromised when systems have more than one listener. For anyone too close to any speaker the soundstage effectively collapses. A speaker to listener separation of 8 to 12 feet overall is sufficient to allow a group of listeners in two rows and four to five seats across to enjoy a well balanced soundstage from all speakers. This separation is typically more difficult to achieve from the rear surround field for off-center listeners. Use of dipole speakers for surround channels can be a partial though reasonable solution for home theater but is prone to response distortions and limits rear sound field image localization.
Of all the ADR elements, none would seem to be more self-evident to the conscientious listener as element 40 yet I believe it may be the most commonly failed. I’ll address this discussion to those among us who have witnessed the ghostly majesty of a stereo image floating in mid-air seemingly without the support of a speaker inline to create it. For those few who have not heard this age-old parlor trick called stereo imaging (actually it’s only a little over 70 years old), please try to imagine staring at a blank wall and being startled by the sound of Shania Twain in your face. Actually, I’m counting on the experience of not hearing this startling illusion to prove my point. The function of a multi-channel sound system is to surround the listener with a seamless façade of sound from all angles. If you aren’t sure what that means or you know you aren’t hearing that in your system, consider element 40 as a potential explanation.
I recall my early days of audio experimentation; the countless hours spent tweaking the alignment of my two speakers to create the most convincing illusion of phantom imaging. My enjoyment was only interrupted by the occasional attempt to squeeze a friend into the sweet spot to share my experience. (Of course this had some advantages on dates.) While there are other reasons why we have such a tiny sweet spot, the most obvious was that I needed near equal contributions from both speakers’ signals to generate a stereo image. As I shifted from side to side, the closer speaker took precedence and shifted the image its way. At an ultimate position off-center, the stereo soundstage actually collapsed into a single speaker playing. The problem is mitigated somewhat by adding more speakers. I say mitigated because while the center channel and side channels fill in the gaps in the sound stage, the lack of a cohesive and enveloping soundstage can be masked by simply hearing a speaker in each general direction. It’s not uncommon for me to audition a system with five speakers clearly localizable in each respective direction when in fact we should be hearing a seamless phantom image of sound in all directions. Translation: I should be hearing the sonic imaging not the speakers. I suspect many of my fair reader’s may be realizing what they are missing at this point.
Is this important? No, world peace and cheap gas takes my vote before seamless enveloping soundstages for all. However, the manufacturers of all modern home theater processors have placed the ability to match speaker levels in every unit I’ve ever seen; a definite sign of importance. Of course a little cajoling from Dolby, DTS and THX helps. All the fuss about matching speaker levels, test tones and Radio Shack SPL meters is about balancing the sound level of all the speakers in order to create this cohesive soundstage. The departure point for element 40 is that one cannot possibly balance the speaker levels for everyone even using these controls if any listener is too close to one of the speakers.
So what are the solutions? I wish there was a tweak I could think of which eliminated this problem. The good news is that if you are alone you get to sit in the sweet spot and soak up the seamless envelopment and glorious imaging without compromise even in a small room. If you are a member of a large family or have many friends, the issue becomes less clear as someone will doubtless get stuck in one of the seats near the side channels. This problem, commonly referred to in the extreme case as the “exit door effect”, means that contrary to the lucky sot resting in the sweet spot, the off axis listener will always be aware of a speaker nearby and less occasionally aware that there is a soundstage. Thus we can make things better for everyone if we limit the size of the audience and or move the speakers to be distant from all listeners.
I could sum up element 40 by saying don’t try to put 10 lbs of material into a 5 lb bag. Despite purchasing the finest audio gear and the most elegant décor; the overstuffing of theater seats into a small room is a bad thing. I believe it is better to limit the number of folks in the theater and give everyone a thrill than to push folks out of the good seats. This concept becomes even more important when you factor in several other relevant ADR elements into the mix that also focus on where the listener is seated. Larger spaces are required for larger audiences. The 8 to 12 ft measurement that is mentioned above is based upon the exponential decay of sound level from a source. Near the sources (each speaker) the drop of in sound level is rapid as you move away. Once you are farther away from the speaker the drop-off rate is much slower. Bottom line is that the difference in intensity from listener to listener is less dramatic. For a system designed with element 40 in mind, the careful matching of speaker levels will be effective for everyone. Everyone gets to hear the music and forgets about the speakers.
I am occasionally confronted in well meaning discussions about the relevance of one acoustical flaw or another. Does it really matter that much? At the end of the day, what is missing due to such tiny misalignments? Perhaps the answer is only fathomable from the perspective of the music lover teleported in time and space by the majesty of an amazing performance. If never on a quiet evening in your home, a recording has astonished you with its realism and moved you emotionally, which among many subtle acoustical distortions has robbed you of the experience? If you have not been surprised recently by your sound system, perhaps Element 40 could be a contributing factor. Of course, don't forget about the other 84 ADR and ACR elements in the mix.
Gerry Lemay (Gerry is the Director
of the HAA and writes the Home Theater Rx column for Home Theater Magazine)