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Acoustical Design Discussion
For the serious technologically minded.
Edition #3 - Listening positions
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 35: Listening positions should not be pre-disposed to extreme bass response anomalies.
One of the problems associated with small rooms is the effect of
standing waves or room modes on low frequency response smoothness. In
rooms which are generally rectangular, assumptions can be made on the expected response at any given location. For less symmetrical rooms,computer modeling must be employed to understand the response. Other acoustical issues such as boundary gain
and boundary interference also point to avoiding certain listening positions in the room. In
most cases, the least desirable locations are near any wall and the center of the room.
The concept that there are good and bad places to sit in a theater is
not earth breaking news to most cinema aficionados. Whether it’s in the
neck-breaking front row or eye squinting back row, most folks know
exactly where not to sit at the big show. Transferring this insight to
the home cinema is not straightforward. For one thing, most home
cinemas (excuse my honesty) are not exactly havens for sonic ecstasy.
The bass is usually turned up too loud anyway and someone is always
complaining that they didn’t catch the last line of dialogue. My
apologies to those amongst our fair readership that have excellent
sound; I’m merely pointing out that many folks are too involved in
suppressing the fatigue of poor sound to begin exploring the solace of
the best seat in the house. Besides, there is usually only a couch or
maybe a couple of Lazy Boys to choose from and the video display is not
(usually) hidden behind the bobbing head of the guy in front of you. In
the end, most live in ignorant bliss assuming the boomy bass is correct
and even applauding its excess.
For many audiophiles the best seat is a well defined spot diametrically
placed between the left and right speaker. On the occasions when they
are “forced” to use the center speaker for a movie, the dialogue blurts
out like its hanging from the car window at a drive-in movie theater.
(That’s not too obscure a reference for everyone is it? My town
actually still has a drive-in and the good old metal speakers have been
largely replaced by FM transmissions to the car radio.) The center
channel has been relegated to the collective mind set of the populace
as the “dialogue speaker” and it is best to be turned several dB louder
than the other speakers… right? Well, this Pearl is not about the true
value of a properly tuned and matched center channel changing the
proverbial sweet spot into a sweet area, but rather about smooth bass.
I will leave to a future discussion why a center channel is the most
important speaker in the house. It is mentioned in this article because
of our desire to make many listening positions good sounding and its
pivotal role toward that aim. Let’s examine why one would not usually
want to sit in the center of the room.
This heretical concept of not listening from the center of the room is
usually greeted with boo’s and hisses from my audiophile friends. The
truth is I have spent more that a few years listening from the room’s
central sweet spot drinking in the luscious soundstage so painstakingly
crafted from the careful repositioning of the left and right speakers.
Then I got a subwoofer, and became both enthralled by the majesty of
those elusive bottom two octaves and repelled by the insidious
boominess and loss of clarity. The truth is the center of the room is
where the rooms’ resonant frequencies, referred to as modes, are the
most schizophrenic. Certain modes cause narrow frequency bands to swell
up to boomy heights, meanwhile other equally narrow frequency bands are
suppressed to virtual oblivion. The end result is a roller coaster ride
response curve providing little satisfaction to the enlightened
listener. So how do I keep my soundstage and have my smooth bass? You
guessed it; scoot your Lazy Boy and your center channel (right and left
speaker in tow) out of the rooms’ center. The front soundstage will
still be intact if you symmetrically move everything to the right or
left a foot or so and similarly the listener forward or backward. The
soundstage remains symmetrical and the bass is not predisposed to poor
quality. Where else can we expect poor bass quality?
The typical placement of listening seats near walls in our living rooms
and home theaters is a direct result of the human mania for proper
space management. In some cases, we are trying to place 10 lbs of “doo
doo” in a 5 lb bag or alternatively we need that open “space” feeling
in the room. In other cases, the room is laid out such that the walk
ways enter near the center of the room; it is just darn hard to put a
sofa anywhere but against a back or side wall. Poor quality bass is
often overshadowed by the impression that more is better. If you want
more, sitting near a room boundary or two will give it to you (for bass
that is). While in the center of the room, modes sound either too loud
or too quiet. Against the wall though, they are all on full blast! If
you have ever listened to your system from one of your rooms' corners,
you’ve experienced the finest manifestation of this phenomenon.
Most folks, allow me to be more direct, most males do not verbally
observe excessive poor quality bass even if they sense it
subconsciously. On the other hand, most women tend to be brutally
honest about it. I have heard about more than one wife or girlfriend
refusing to watch a movie until it is “turned down”. Sorry for my
sexist observation, but I believe most folks will agree with my
assertion and it is a great complement to these frank ladies that they
won’t stand for bad sound. Men seem to gut their way through the loud
boomy passages yet over time succumb to the bass induced listener
fatigue and either turn the sub down or listen less. The proper
calibration of a subwoofer is an exercise in subtlety. A properly
calibrated subwoofer is sublime; a football linebacker that can ballet
to repeat a metaphor. Of the five basic elements of subwoofer
calibration, the first is listening position. ADR Element 35 embodies
this concept. Good bass is not wimpy, on the contrary, it is deep and
can be quite loud; it is above all smooth. Most folks find a properly
calibrated sound system to be quite tolerable and even inspiring at
high sound levels.
I’ll finish by pointing out a bonus to the common household rectangular
room. By maneuvering away from the rooms’ center and the boundaries we
find four areas of modal moderation. These moderate zones, as I have
taken to call them, provide not only the smoothest bass, but also as a
bonus, display similar acoustical qualities. This means that if we make
changes to the subwoofer position or equalization to smooth the
response, we’ll be generally improving all of these areas similarly.
The true challenge to designing a great theater is not discovering
where to set a listening chair but rather how to arrange seating to
place most of our guests in these areas predisposed to smoother bass.
As a rule of thumb, keeping listeners away from walls and out of the
center of the room is a good start.
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 35 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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