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Acoustical Design Discussion
For the serious technologically minded.
Edition #5 - Sound Imaging
White Paper
- 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.
Acoustical Design Element 40: Front center speaker effective sound path is identical to right and left front speakers.
A proper soundstage requires a balanced presentation of all channels of sound. The front channels in particular must convey an accurate perspective of the recorded event by placing individual sonic images in the proper position within the soundstage. A left or right speaker that is not symmetrical with respect to eachother and the center channel disrupts this symmetry and detracts from a convincing recreation of the recorded event and also results in uneven frequency response due to comb filtering.
This element comes in under the heading of audiophile 101. The two-channel corollary of this element is that the right and left speakers must be equidistant from the central listening position or sweet-spot. Anyone who’s more than casually sat in the sweet spot and heard the three-dimensional illusions of a properly set up pair of speakers knows that if you shift significantly to one side or the other, the illusion begins to dissolve or becomes marginal. In the world of home theater, adding a center channel and multiple listeners complicates the problem but there is a solution.
There are several factors at work here. They include speaker intensity variability, frequency response distortion, and the complexity of the sound field. The first two of these effects are dependent on the position of the listener in correlation with the position of the speaker. So when your HAA Calibrator has a fit because you want to put the center channel closer to the left speaker so your Ming vase is properly displayed, understand he’s not a perfectionist but rather trying to save your sound system investment from one of the most fundamental sonic errors.
The intensity of sound decreases with the inverse of the distance traveled squared (see figure 1). Sounds pretty impressive but what this scientific truism from high school means is that near the speaker, sound level changes rapidly, meanwhile as we move away from the speaker, sound level changes begin to occur more slowly. This issue confronts the theater designer because if the speakers are too close to the listeners, the “sweet-spot” where the distance and sound level of each speaker is equal, becomes a dime-sized point. This is true because one of our most revered calibration elements requires equal sound levels from all speakers. If moving your head to the side by six inches changes the left speaker to be obviously louder than the right the sound stage illusion suffers. This issue obviously becomes a problem anytime there is more than one listener. Suddenly the wonderful family home theater becomes a thrill ride for one. How do we eliminate this problem? Distance. As you move farther from speakers, the rate of change in intensity is reduced. That means that if the speakers are on the order of 10 to 15 feet away from the listening area there will be less variation in intensity from speaker to speaker. We’d like to see only 2 dB variation for any listener but that normally would require a room larger than the space most of us can allocate to the media room. Moral of the story; choose to keep speakers (including the side and rear channels) as far away from the listeners as the room (and acoustics) allows and to keep the number of listeners to a minimum so none are too close to a speaker.* This means that anyone out of the sweet spot won’t ask for their money back unless of course the frequency response is bad.
SPL loss versus distance
Frequency response distortions are audible as poor tone quality and are caused in this scenario by comb filtering. Comb filtering is another fancy acoustical word that is a description of the shape of the room response plot; up and down in a repeated regular pattern instead of the preferred flat response. This problem is only kept in check if the phase of the signal from all sound sources is aligned. That’s simple right? Just make sure the black and red speaker connectors are hooked up correctly? Actually, Element 2 is specifically designed to limit comb filtering due to speaker distance differences. The phase of a sound wave changes as it travels across the room with distance and time. That means that the phase of a speakers signal is different at all points in the room. If two sound waves travel different distances to arrive at a listener, they are by definition at different phases. That means, you guessed it, comb filtering and an unnatural reproduction of recordings. The elimination of this effect begins most importantly with setting each speaker the same distance from the listener (see figure 2). If you’ve ever tried to draw a circle in the dirt with a stick you can imagine the difficulty of putting all speakers on the edge of such a circle drawn inside a rectangular room. It’s interesting to note that even though the center channel lies on the circle, it appears like it is actually farther away than the left and right to meet this criterion. What if you can’t get all the speakers on the arc? Another solution found on any Dolby Digital, DTS, or THX certified receiver is the addition of time delay as a correction. The distance a sound travels can also be viewed as the time it travels. If the sound waves travel the same distance, then by definition, they have traveled the same time. If we look at a speaker that is too close to the listeners, its distance traveled and thus time traveled is shorter than the other speaker’s signals. We can simply delay the closer speakers signal by a time that corresponds to the difference in travel-time against the other speakers. For the sweet-spot, we eliminate comb filtering. But what about all those folks who are not in the sweet spot? They certainly will still hear comb filtering; correct? Here’s where we begin to understand the benefits of the complexity of the sound field.
Equidistant Speakers
The saving grace in most sound systems is something that many folks would normally erroneously conclude is a serious problem; the high number of possible interactions between the many sound waves flowing past each listener. If one set of misaligned speakers creating comb filtering is bad, several speakers and their reflections should be horrendous; right? As it happens though, this complexity can actually have the effect of concealing or, better put, diffusing many of these problems. The phase cancellation issues still exist, but the response distortions created by one pair of sound waves can be hidden amongst the large number of others. An analogy; one way to make any individual nail in a bed of nails less annoying is to increase the number of nails. The effect of the many nails reduces the impact of a single nail. If the response distortions caused by one set of sound wave interactions are offset by a myriad of slightly different sets of other interactions and so on, the bad effect is rendered less audible. The reason we can’t simply leave this problem to be solved by the complexity of the sound field is because even nature can become symmetrical or less complex; using the analogy, if the nails are not evenly spread out but rather lined up in rows. By aligning the speakers to the center of the listening area by distance we are reducing the error for listeners near the sweet spot and simultaneously moving the frequency of such errors higher into the complex high frequency portion of the sound spectrum. What this means is that even those seat away from the sweet spot can be excellent listening positions if Element 2 is true.
* Don’t forget about Element 3 which keeps the separation between speakers between 45 and 60 degrees from the perspective of the central listening position.
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 2 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.
From "The Home Acoustic Alliance" by 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)
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