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Acoustical Design Discussion
For the serious technologically minded.
Edition #2 - Subwoofer Placement
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.
Uneven
bass response means too much or too little bass at various frequencies
and often simultaneously different responses for each listener in the
room. This distortion results in a lack of articulation and clarity and
at worst, the result is often fatiguing. In either event, it ruins the
performance potential for a high end sound system. Optimizing the
location of the speaker and listener in the room and the correct use of
parametric equalization is the key to solving the problem. Interior
Designers should take heart; good sounding subwoofer placement can
still be highly flexible for modern interiors.
I wrote an article a few months ago about seven acoustical myths. In it
I point out the issue I have with the oft stated belief that a
subwoofer can be placed anywhere because humans can’t discern the
source direction of deep bass. I also point out that while it is a
powerful and necessary tool, equalization cannot solve all acoustical
ills. I wont re-write it but I will restate the basic premise; the
position of a subwoofer is a calibration element in small rooms. I add
the “small room” caveat because first of all, that where most of us
keep our subwoofers and also because the behavior of low frequencies in
a small room predisposes terrible frequency response. Allow me to
insert a short excerpts of the article of which I spoke:
Myth 1: It doesn’t matter where you place the subwoofer.
This is a story retold everyday by enthusiastic A/V salesmen eager to
please appearance conscious customers. The sub is discretely hidden
behind the TV in the corner, behind the couch or even built into the
floor. In many cases, an otherwise successful installation comes up
short because of poor bass quality. Small rooms do strange things to
long wavelength bass frequencies. In fact, the typical room becomes a
virtual topographic map of mountainous bass peaks and hollow bass nulls
and the smaller the room, the worse the variation. Most installers
don’t always have the option of placing any speaker much less a bulky
subwoofer in the perfect sonic position. It is worth the effort,
though, to choose the speaker location wisely rather than assuming it
doesn’t matter. It doesn’t make sense to spend big bucks on a top of
the line subwoofer then leave its calibration to chance.
Calibration of a sub means an alignment of 5 basic elements:
• The sub’s relative volume compared to the main speakers
• The phase relationship between the sub and the main speakers at the crossover frequency
• The position of the listener’s chairs in the room
• The position of the sub based upon the chairs
• The parametric equalization of any remaining response anomalies
An additional consideration is the production of artifacts by the sub’s
woofer or port. These noises are not in the soundtrack and are
unwanted. They are not always obvious but they create an annoying side
effect. While low frequency sounds are almost impossible for humans to
localize (that’s where this myth originated by the way), these higher
frequency artifacts are easily localized and often draw attention to
the sub’s location. The first step in stopping this is usually to turn
the subwoofer by pointing the port or subwoofer away from the listener
until the sub’s bass sounds fully integrated with the main speaker’s
sound. The important thing to remember is that a sub can be behind,
beside or in front of the listener and remain undetected. But the
location and position needs to be scientifically chosen and the sub
properly calibrated. Room modeling software can help an integrator
determine where a room’s peak is and where null modes are located.
Myth 2: Equalization can fix all acoustical problems.
Equalization is the process of electronically correcting the frequency
response of a sound system. Recently, there has been a sudden increase
in the number of manufacturers with parametric equalization built into
their processors and subwoofers. This is a good thing. It demonstrates
consumers the need to calibrate home-theater systems in order to
produce the best performance. The proper use of equalization can
provide an outstanding improvement in sonic fidelity if the EQ is low
noise and low distortion. The acoustical distortion of bass in a small
room can be greatly reduced by a thoughtfully set up parametric
equalizer that minimizes room-mode peaks and smoothens frequency
response. Unfortunately, the impression some have is that this fix
results a cure for all acoustical ills. This is not true. Some response
distortions cannot be equalized away. Frequency response dips due to
bass mode nodes or boundary interference will not be changed by EQ. In
fact, a poorly set-up equalizer can make matters worse if the operator
does not understand the nature of these cancellations. Many response
distortions are only solved by movement of the speaker and/or listener
as fix. Some problems are too severe for an equalizer to repair. Other
acoustical problems are not directly related to frequency response.
Excessive reverberation in a room, for example, can only be solved by
physically changing the interior surfaces of a room, making it more or
less reverberant. An improperly placed speaker can cause a disjointed
and in-cohesive soundstage, or a lack of surround envelopment, bad
imaging and acoustically mono presentation among other things. None of
these problems can be cured by EQ. Equalization, particularly with a
high quality DSP parametric equalizer is a powerful and necessary tool
in the hands of a competent calibrator, but it is only one tool among
many that are needed to calibrate a sound system.
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 22 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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