Essay -
"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.
Colin Miller
© Copyright 1995, 1996, 1997
Secrets of Home Theater & High Fidelity
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