"Equalizers: Friend of Foe?" - May, 1997
By Colin Miller
Bobafett was, by far, my favorite bounty
hunter. He reminds me of a post apocalyptic Mad Max who refined
himself into a disgruntled Batman. Besides the funky spaceship
that looks like a Mr. Potato Head refurbished by RoboCop, he had
the coolest costume - all the tools of his trade but without the
fruity cape so rampant in his era. The new version of Star Wars,
I feel, greatly benefited from his personal appearance. New
visual and audio effects don't hurt either.
Sometime before I graduated preschool, my father took me to see the original space epic, sans mother and infant sister. As the star destroyer chased Princess Leia's ship down the opening scene, it almost scared me out of my corduroy pants. The recently improved version makes me want to wet them. As a former college roommate once said, "You're never too old to wet yourself, but it's still embarrassing." That's why, like any of you who have assembled a small cinema in your humble abode, I can't wait to get this refurbished gem home.
The picture, of course, will not be as nice on my television. I concede that there is nothing that I can afford which will provide the same resolution as 70mm film. I do, though, have half a chance at getting the sound right. For me, when complementing a classic story, this is more than half the fun. Regardless of whether I choose to use Dolby Surround, Dolby Pro Logic Surround, Dolby Digital AC-3 (DD), DTS, or some proprietary matrix directional enhancement circuit, there's one crucial step I must address before the galactic field trip. It is this crucial step which is the topic of this soliloquy.
Voice matching, voice matching, voice matching. Every surround sound demonstration you've probably attended has stressed the need to voice match your speakers. What does that mean anyway? Simply put, it means that if you do not do so, the sound reaching you from one speaker sounds different than the sound from another speaker generated by an identical input signal. If the speakers are perfectly voice matched, the voice of a person radiating from all the speakers will have the same tonal balance, harmonic characteristics, dynamics, and phase relationships. If the speakers array is at least close, the soundfield transistions easily around the room, without any identifiable handoffs from speaker to speaker. Fail in this regard, and the soundfield clumps to the individual front speakers without equal dimensionality between them, and the rear information becomes distractingly separate from the foreground information. If you want to hear sound effects jump from speaker to speaker, that's fine. If you want to sink into the moment, it's hardly helpful.
The general consensus is that if it's impractical to use identical loudspeakers all the way around, the best way to voice match your system is to buy speakers from the same manufacturer. If they're made by the same company, wouldn't they have the same general "sound?" Maybe. But probably not. However, that's not necessarily a disaster. Most reputable setups aren't perfectly voice matched, even those that stress it as a necessity, and most of them sound pretty darn good. But they could sound better.
The high-end entity circling my house in angst just shrieked. Doesn't THX mandate speakers with identical drivers in the front? Yes, it does, and wisely so. It also wisely mandates the limited vertical dispersion of high frequencies to minimize room interaction. Still, even with these precautions, a speaker on top of a television will not sound the same as one on a stand near the corner of the room. Nor will any of the front speakers sound identical to rear surround dipoles, bipoles, or monopoles nailed up on the wall next to the stucco ceiling. Even with extravagant room treatments, and careful placement necessary to optimize and/or minimize reflections and relative time delays, you're probably going to be disappointed to discover with a spectrum analyzer that their tonal responses are very different. Is there an obvious easy fix for this? Yes, and it is painfully obvious.
Very painfully that is. Excruciating might be more appropriate in the context of high-end audio. It's certainly more graphic. That is to say, graphic, a multi-channel graphic equalizer ("EQ"), or one of the parametic variety if you like, can easily take care of those nasty tonal response deviations. Whether we realize it or not, the majority of sonic differences we immediately hear have to do with tonal characteristics. On the other side of the fence, it is another step in the audio chain, introducing distortion by it's very nature. Most purists that I've met shun the use of an equalizer. I remain leery. Why? THX surround processors use equalization. An equalizer is only a collection of frequency specific boost and cut controls, just like the bass and treble knobs on most receivers. The more bands, the more precise the adjustments possible. With a parametric, you can adjust where in the spectrum the given control affects the signal. Even though tonal control seems like a good idea, how many high-end preamps have you seen with a ten band graphic equalizer?
The more circuitry you put in the signal path, the more it can affect the accuracy of the signal. Tone controls are filters. They selectively change the amplitude of whatever portion of the signal as a function of the frequency. They also induce phase shift, meaning that there is a relative time delay also as a function of the frequency. Dynamics should not be affected theoretically, but since perfect components do not exist, dynamics may or may not suffer, although the perception of dynamics may change due to the alterations in frequency response. And, as with everything, harmonic, IM distortion, as well as noise, make their contribution. Unless you have a specific need for it (EQ), leave it out. Personally, my favorite preamp has no tone controls, no balance, no gain, and no power cord. Just a switch with an attenuating potentiometer.
So the purist may say, "Why bother with it? Leave it out and forget about the mess." The "straight wire with gain" approach doesn't accommodate scores of sliders, each of which can cause a deviation from the "pure" source signal. In fact, tone controls seem to be an absolute stigma when it comes to high performance consumer electronics today. Well, if additional equalization isn't needed, I wholeheartedly agree with the purist. Less, indeed, is more. On that other grimy hand we have the everlasting real world situation where "pure" usually isn't. A world that the professional world has accepted before my eventful birth. Do you think your system or listening material is pure? Think again. Unless you have single transducer full-range speakers, the speakers you have use crossovers. If they do use a single transducer, chances are that they have limited bandwidth and/or limited dynamic range with beamy dispersion characteristics. The use of specialized transducers facilitates these shortcomings, and crossovers facilitate their operation. Electronic or passive, crossovers are filters that attenuate output based on frequency- phase shift and all, i.e., your speakers have EQ built-in.
Do you ever listen to any recordings where the engineer used a mixing board? ALL audio present on major film soundtracks has gone through mixing equipment. Guess what. It's undergone EQ and reverb. More filters. You can't get away from it. Finally, no matter how pure the sound coming from your loudspeakers, unless you live in an anechoic chamber, the room is going to stick it's tongue at you with a different twist from every position. Because of this, the use of equalizers, drawbacks and all, are especially practical in home theater applications. It really makes sense when you realize that the best movie houses use them, and for good reason.
Up until a few years ago, I always used an equalizer, though admittedly not very exceptional ones. I won't name brands, but most that I tried that were carried by chain retail stores always took away the subtle details under a generalized fuzzy veil accompanied by a slight layer of mud on the bottom. Because of this, I dismissed equalization as a must avoid process. I did the best I could with placement, and took what came, which wasn't all that bad. However, in the last few months, as most audiophiles, I have noticed points of lacking in my home system despite predominant satisfaction. I've got room shaking extension, but some of the fatness that should roll with the bass seems lacking. The lower mid-range is warm and pleasant, but just too throaty. Other than that, I like it just fine. Now I could just keep swapping components indefinitely, or I could do what I did, get an industrial Audio Control 1/3 octave spectrum analyzer and measure the damn thing. My results? Very coincidental.
The bass, most likely because of room conditions, after a healthy quaking 25 and 30 Hz, sucks out about 6 dB from approximately 40 to 50 Hz, frequencies which give weight and punchiness to the higher harmonics some of us love in mid-bass. The lower midrange, possibly for the same reasons, peaks to a similar extent at 160 Hz, which explains my other observation. Along with my otherwise content evaluation, response settles down into a nice flat stroll all the way up to 16 kHz, after which it drops 6 dB at the 20 kHz extreme at the listening position. The 20 kHz drop never bothered me, as I'm not sure that I can hear much up there, or that there really is much there in musical passages, but I wouldn't mind having that corrected either.
My conclusion: I'm going to get the best equalizer AudioControl makes for under $800 or so, (they do have a well-deserved reputation for building quality, affordable equipment,) and wire it through my preamp after the potentiometer with external connections which will allow me to switch it out of the signal path with one toggle. I was considering adding a buffer stage to my home-brew passive unit anyway to compensate for the output impedance induced by the potentiometer which might cause slight problems when driving the inputs of multiple amplifiers with low input impedances. This solution not only gives me optional frequency response correction, but saves me the effort of installing a power supply and extra circuitry. It will use more amplifier power to boost the middle of the deep bass up to par. This is not an efficient use of power to derive maximum SPL, but if that were my main goal, I would still have Klipsch Forte IIs. Besides, 105 dB as a continuous reference level is enough for my ears. They don't need anymore abuse, and a more powerful amp is in the works, not for output levels, but for the sound quality associated with the mighty beast.
I could, on the other hand, position the speakers for hours, probably end up worse than I am, and anger my roommate in the process. Or, I could do what will probably make me happiest in the long run, even if it does exclude me from the upper ranks of the most elite audiophiles. I can't afford to go there anyway, and I usually don't like their taste in cuisine. I don't like bull on my plate. Not a religious or ethical decision, just a preference for non-bovine nourishment.
The drawbacks of good equalizers really are minimal. Quality multi-channel equalizers do not have to cost fortunes. I'm not advocating that you go out and buy a piece of junk and thrust it into a previously pristine setup. Garbage is still garbage. But low noise floors, low distortion, and practical flexibility can box itself into a very reasonable package. It's up to you to find one that fits your personal requirements. The benefits, however, are far from minimal, especially in a surround sound application. In a conventional stereo array, two identical speakers have a chance at voice matching, as they can be placed symmetrically in a symmetrical room. My stereo speakers, are, by the way, very voice matched despite the room. This doesn't usually happen, but it could. Even when it does, you may find, like I did, that problems may still surface. A setup utilizing five or more speakers, though, has much more against it. Room boundaries are far from equal. Even without a television to provide anomalous reflections, the center, left, right, and surrounds will hardly share the same distance to walls, corners, ceilings, floors, curtains, windows, couches, coffee tables, and so forth. Reflections will be significantly different, both in initial delay and in tonal balance. Room treatment and good placement will help, particularly to minimize the time smear of reflections which mauls good imaging, but it is impractical from a decorating perspective to completely optimize a listening environment. Comprehensive room treatment can also be very expensive. An equalizer takes a direct approach to the problem, which is not perfect, but very practical, and extremely beneficial should you have the need for one. If you're into high performance home theater, chances are that you probably do.
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