Written by 3:00 am Analog

Part 3 of the Mono Series – No half-dollar? Need to make change?

Here’s the third of a four-part series by Roger Skoff – And now, the latest and greatest thing yet… Mono…

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In the last section of
this four-part series, I told you that, given “perfect” speakers, perfectly
positioned in the “perfect” room, what you should hear when you play your system
in mono – even if the music’s as big as a massed choir or a full symphony
orchestra – is an acoustic “image” about the size of a U.S. half-dollar coin (3
cm diameter), floating at ear level directly in front of you, regardless of
where between your two speakers you are sitting.

If that’s not what you
hear, then there’s a problem that – odds are – we can go at least a long way
towards fixing.  To make the fix, the
first thing you’ll need to do is to figure out the problem’s source; and in
doing that, the first thing to rule out is your electronics. Is your system
solid-state? If so, you’re probably fine. If it’s tubes, though, when was the
last time you set your bias or changed your tubes? Not all tubes (even of the
same brand and number) “wear” identically, so there could be some differences
between what’s going to one speaker and what’s going to the other. Check it

If it’s not your
electronics, the next thing to consider is your speakers. Regardless of what
the manufacturers claim, unless you’re using “omnis” (omnidirectional
radiators, like MBLs or Ohm “Walsh” drivers) or full-range single-driver panels
(like some electrostatics or “magneplanars”), it’s almost certain that there
will be “time alignment” issues with your drivers, at least at some listening
positions – especially if you are running a separate sub-woofer. Without
seriously re-engineering your speakers, there’s nothing you can do about driver
alignment, so just “shine it on” – all it can do is to make your “half dollar”
a little bigger, and even getting your mono image down to a consistent “basketball-size”
can make a great improvement if it’s already bigger than that or has no real shape
at all.

After your speakers, the
only things that remain to consider are your listening room and where your
speakers are placed in it.  The last part
of this series will go into speaker placement in some detail so, for now, let’s
just concentrate on the room you listen in.

When you’re in your
favorite listening position and you play your system in mono, if you don’t hear
a point source (of whatever size), what do you hear, and where does it sound
like it’s coming from? If what you hear is a localized source of any size, your
listening room is probably at least somewhat acoustically symmetrical already and
your goal should be to get it as much more so as you can.

Having a room that’s
physically symmetrical is a good start, but physical symmetry is no guarantee
of acoustical symmetry. As an example, consider this: What if your listening
room has a perfectly regular shape and your speakers are placed away from the
back wall and equidistant from the side walls, but one side wall has either a
large soft wall-hanging (making it absorptive) or is covered in
irregularly-shaped wood blocks or stones (making it diffusive), and the other
side wall is either smooth or a picture window (either one of which would be
highly reflective); will your room be acoustically symmetrical? Of course not,
and you’ll easily be able to hear it.

Even in a room that’s
otherwise terrific – perfectly symmetrical, with no hard/soft anomalies – you
can still have problems if your speakers are not symmetrically-placed in the
room: Having them a significantly greater distance from one side wall than from
the other will change what you hear at your listening position, and should be
avoided if at all possible.

An easy way to understand
room acoustics is this: Your speakers are, in fact, air pumps that create waves
of positive and negative pressure in the air of your room. Some portion of that
wave energy will go to your ears directly, but the rest of it will bounce
around off the walls and furniture, either to be lost or to be heard by you as
echoes. If you consider the air in your room to be the ocean; the room’s walls
to be the shoreline, the sound to be the ocean’s waves, and the room’s
finishes, furniture, and features to be rocks or byways that will direct,
reflect, or disperse the waves’ pattern, you’ll have a good analogy for what
happens when you listen to your system.

If you’ll keep this
analogy in mind, you’ll find that optimizing your room is actually a fairly
rational process: The primary goal is to get the wave energy bouncing around
your room not to interfere with your enjoyment of the wave energy coming
directly to your ears, and the way to achieve it is, on the one hand, to get
your room as acoustically symmetrical (relative to your listening position) as
possible and, on the other hand, to try (without making your room too “dead”) to
minimize that bouncing wave energy as much as possible. 

In making room changes,
always remember that there are only three options for dealing with stray
energy: You can reflect it, absorb it, or diffuse it. The first thing to do is
to look at the locations of both your speakers, and of your listening position
relative to them. Keeping your speakers’ dispersion pattern in mind and remembering
that “the angle of incidence equals the angle of reflection”, locate the “first
bounce” point for each of your speakers, on the nearest side wall, on the
floor, and on the ceiling. Are the textures or reflective/absorptive/diffusive
characteristics at those points the same for both speakers? If not, make them
so, using hard surfaces to reflect, soft surfaces to absorb, and irregular
surfaces (large, leafy plants are great for this!), to diffuse the sound
energy. Making the right choices in just that one thing will go a long way
toward both improving your room’s acoustic symmetry and (to the degree that you
use absorptive or diffusive surfaces) reducing stray energy. It will certainly
help to reduce the size of your mono playback “image”, maybe even down to
“half-dollar” size!

last section of this article will show you how to use an altogether different
mono technique to finish “dialing-in” your room and get your speakers placed
exactly where they should be. See you then.

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