February:
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 #7.
Acoustical Design Element 7: Front center speaker is tonally matched to left and right front speakers.
All speakers in an audio system should be tonally similar and nowhere is this more audible than the front three LCR speakers (Left, Center and Right). Failure to assure matched timbre here is a key failure in design. The stereo illusion depends on these three sound fields working together to give the illusion of a seamless and cohesive soundstage across the front of the room. Smaller modified versions of large speakers can be successful alternatives for identical speakers but the best results occur from matching the front speakers identically.
I open my discussion of element 7 cautiously. The element is one of the shortest on the list but its economy of verse conceals the depth of its connotations. On the face, the meaning is straightforward enough; use three speakers that sound the same across the front of the soundstage. This is a seemingly straightforward task which most speaker manufacturers rise to accomplish. The problem we face as calibrators is the number of ways this simple solution can be diluted or even devastated in practice. It’s also interesting to note what many consider to be “matching” in reality falls short of the mark.
I like to repeat the mantra that calibration begins with the design. In this case the design means choosing left, right and center speakers that sound the same. A matched pair of left and right speakers is a no-brainer; we’ve all been raised in the business to expect a matched “stereo” pair. It was only recently (late 80’s) with the advent of Dolby Pro-logic that the “center channel” began to assert its presence in the grand scheme. It was obvious; we needed a high-quality transducer to allow the dialogue to be properly played alongside the rest of the sound track. In those days I saw systems using the TV speaker as the center channel. This was such a mismatch that even the most detached listener couldn’t avoid noticing the need for a “better” center channel. The mismatch problem was highlighted by the extensive use of large full range speakers for the left and right. These large boxes did not lend themselves to center channel use since placement on-top, under, behind, or even in-front-of the TVs of the day was then impossible. A smaller and perhaps forgivably lesser version of these large speakers was needed to solve the problem. Enter the matched center channel speaker and the D’Appolito and MTM design.
The emphasis on the superiority of the left and right channels has more than just historical relevance. After all, isn’t stereo still the king of music sales? This is true despite all the time and money expended on multi-channel music (DVDA, SACD); Record Companies are still primarily and for many titles solely distributing two channel versions for music titles. This implies for some that the center channel and its surround sound cohorts (the subwoofer, side and rear channels) are there just to add spice to the occasional movie. I’d be willing to agree and leave the discussion here except for two realities. First, the movie via DVDV* has become the primary use of home audio systems for most users (and for many the primary way to listen to music in the home ala the concert DVDV) and second, the subwoofer is not just for low frequency effects anymore.
Allow me to deal with the use of DVDV* as a primary software source first. The emphasis on video in our culture and our industry has supplanted pure audio in all but the portable mediums. We don’t plop down on the couch and flip on a CD, we flip on the TV. Far be it from me to rally the audiophile faithful to pull back the scourge of video, I proudly point out that the 2-D video we all enjoy is pretty boring without our 3-D audio alongside. The future of high-end audio is as a partner with video. This means that the “critical listening” that marks the habit of a true audio enthusiast is now often mixed with video images of the artists. It is also a really fun way to experience a performance. This also means that those odd manifestations of speakers known as the center and surround channels are not only used to add effects but can greatly enhance the fidelity. After all, isn’t a live performance a surround sound experience? I’ll leave the discussion about whether the “in-band” mix is a bastardization of or a brave new art form for another article. So we need to plan on using the center channel in its new role as an equal partner to the left and right speakers… or do we? Why do we need a center channel?
I’m now looking at the original subject of this Pearl and realizing the “depth” I mentioned could fill a book not to mention a very long article. The need for a center channel is based upon its use in the sound mix plus its actual function in the system. A recording engineer will use the center channel liberally in most movie soundtracks. Its use is typically perceived as a”dialogue” channel. While in general this would be true considering that 95% of the movie dialog emanates from the center speaker this is not its function in the grand scheme. In a large theater the need for a centrally located speaker to anchor the soundstage is obvious. With such a wide stage and the potential for customers (audience members) seated at left or right extremes, there needed to be a way to keep the soundstage, and most importantly the dialog, linked to the movie screen. Enter Dr. Fletcher and the concept of stereo imaging (Blumlein’s contribution is duly noted). Harvey Fletcher used a center speaker to perform this critical role in his experiments dating back to the 1930’s at Bell Labs. In our home systems the role of the center speaker is identical. Without it the home theater reverts to a two-channel stereo and the listener is relegated to the very small “sweet spot” from which to enjoy the soundstage. Unless you are a lone listener comfortably positioned diametrically centered between the left and right speakers to hear a “phantom center image”, the center channel is required. Any theater designer who ventures forth without a center channel is not to be trusted.
As I weave my matrix of arguments I must again reiterate the premise of element 7; all three speakers should sound the same. Understanding this is no less difficult than understanding the need for those infamous right and left speakers to be matched. What would happen if the left had a clearly different tonal balance that the right? Suffice it to say the sound stage would suffer. We are seeking a cohesive and balanced illusion; a wall of three-dimensional imaging. The interaction between speakers is critical in crafting this illusion. The more closely matched the sonic qualities of the front three channels the better the illusion. This is why even the most minor discrepancy between the three front speakers can reduce the effect. This is also why matching not only the type of tweeter and woofer is relevant but also the alignment of these drivers.
Issues such as the alignment of the drivers become relevant because the presentation of the sound from each driver can have a major impact on the frequency response and stereo imaging. Let’s assume we have three”good” speakers with reasonable off-axis response, setting in similar acoustical environments and installed at the same height. We’ll hear the same sound from all three. The vertical alignment of drivers, tweeter centered above midrange etc, further allows the presentation of such speakers to be the same regardless of the horizontal position of the listener (assuming the same listening height). Besides the high or low placement of the center channel (ADR Element 4), the most common deviation from the sameness theory is the use of the afore mentioned D’Appolito design for the center speaker alone.
Dr. Joseph D’Appolito is a talented speaker designer whose process for controlling the directivity of sound and providing a big punch by the use of dual smaller woofers has been copied and in many cases imperfectly imitated over the years. The big attraction to such a design is the option of a horizontal arrangement of drivers thus trading height for width; a natural for the speaker that needs to set upon a TV. The flaw in this idea is that despite the best laid plans, the presentation of sound from the horizontal center channel does not match the vertical left and right pair. In many cases, the use of two horizontally displaced woofers in the same speaker means that for all except the listeners directly on-axis with the center speaker there will be significant and audible comb-filtering of the response.** As a final drawback, the wider horizontal source caused by the wide woofer placement increases the smallest image size and thus reduces focus. Why take the chance… why not use identical small and vertically aligned speakers across the front soundstage allowing the subwoofer to fill in the bottom two octaves? Because a subwoofer used with a small satellite speaker cannot sound as good as a full-range speaker… nonsense!
New vistas of excitement have accompanied the arrival of the modern subwoofer in the home entertainment system. Enthusiasts cheer the thunderous exclamation point it’s added to the explosions, bass drum hits, and various other geophysical sound effects. At the same time, its ability to accurately and in many cases more effectively reproduce the bottom two octaves of bass has made us rethink the need for those “large” left and right speakers. The opportunity this brings to ADR element 7 is obvious. We now have the unfettered ability to use smaller matched speakers for the front three channels without compromise since the low frequencies and the need for a large cabinet has been supplanted by the sub. I leave the integration of bass imaging between a single or multiple subs and the main soundstage for a future discussion. I have found no reduction in performance between a good full-range speaker system and a properly calibrated sub-satellite system. The compromise in performance caused by the improper placement or design of a center channel with reference to the right and left speakers (element 7) is reason by itself to use a subwoofer and bass management.
At this point I’ve made the case for three identical front speakers and why small ones work better; especially around a video display that needs its space as well. I don’t mean to imply that the use of a center channel eliminates the traditional stereo sweet spot. We all will want to sit in that magical place diametrically between the speakers; it’s still the best. We do want make sure all listeners are satisfied and even thrilled though and the center speaker is a vital part of that solution. I also don’t wish to impugn the honor of any full-range speaker out there. I currently own three pair of those big beautiful boxes. I have my share of prejudices regarding the level of difficulty of calibrating a full-range speaker but conversely I must point out the absolute necessity of calibrating a subwoofer to achieve great results. Without the careful and expert calibration of the response of a subwoofer with its small LCR (left, center, right) compatriots up front the sub can sound like it’s disjointed and unconnected with the system. No solution is foolproof and none achievable by luck. With the wise stewardship of a thoughtful calibrator, the sub-sat arrangement with identical LCRs can herald greatness.
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 7 could be a contributing factor, of course, don't forget about the many other Design and Calibration elements in the mix.
*We are all hopeful that the multi-channel formats of DVDA and SACD are retained and improve in the music marketplace. The higher resolution of these formats and the great care taken in their production is a valuable resource for the audiophile. Despite the level of apparent conflict over the placement of rear channel speakers for these formats over the traditional DVDV or movie soundtrack the center channel argument is valid for all formats.
** A modified and more suitable approach is to crossover the two small
woofers to operate in different bands. This two-way dual woofer design
prevents the comb-filtering since the horizontally displaced woofers are
no longer playing the same frequencies.
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)