I decided to take some time off from checking out the amp
that was sent to me to review and submit a draft of an
article I published a while back. I figure that maybe some
of you goofballs will be able to understand it. The rest of
you will carp and rant and say that it is unreadable,
boring, etc., because the topic it covers will be heading
right over your heads.

The draft:

Speaker-Room Suckout and Other Tidbits.

I've mentioned this before, but I will mention it again: in
typical home-listening rooms, spaced-apart woofer (or
subwoofer) systems will generate a cancellation notch at
some bass frequency that is dependent upon both the distance
between woofer (or subwoofer) driver centers and the
frequency.

Depending upon the spacing between the systems (between
woofer or subwoofer driver centers), at some bass frequency
the rarefaction wave from one woofer or subwoofer will reach
the other woofer or subwoofer just as it is generating a
pressure wave. (More on this up ahead.) The two cancel out
and you get a power-response notch.
There is no way to get away from this with spaced
woofer/subwoofer systems generating identical or
near-identical bass signals.

A similar thing happens with single woofers and subwoofers
interacting with stiff, large-area wall, floor, and ceiling
surfaces. The large surface area will reflect back the
signal to the radiating driver as if it were being radiated
by a second woofer or subwoofer at twice the distance from
the single driver's center to the boundary. The boundary
creates a mirror-image situation that mimics a second woofer
or subwoofer driver.

For example, a situation where you have two spaced woofers
or subwoofers 12 feet apart or another situation where you
have one woofer or subwoofer 6 feet from a large boundary
will each generate a suckout notch centered at 56.5 Hz. With
one system you have a boundary and with two systems you have
a faux boundary exactly between the two sound sources.

Note that this phenomenon is unrelated to standing waves,
which involve boundary/boundary interactions. The suckout
effect is quite different and involves either
woofer/boundary interactions or woofer/woofer interactions.
There is a formula to calculate this notching as it relates
to woofer/boundary interactions:

1130/d x 0.3

Here, "d" is the distance in feet from the woofer center
(measured by the shortest route possible) to the closest
part of the boundary, and 0.3 (three tenths) is the
multiplier that calculates the frequency of the dip.
Actually, the true quarter-wavelength multiplier should be
0.25 and not 0.3. However, because the boundary surface is
not equidistant over its entire surface from the driver
center, it has been found that 0.3 works better.

When calculating the suckout notch between woofer (or
subwoofer) centers you would use half the distance (1/2d)
between them as d. You would still use the .3 multiplier,
because the spaced woofers are generating a faux flat
boundary between them.

The big problem occurs when you have multiple boundary or
inter-woofer interactions. For example, if the woofer (or
subwoofer) centers are 10 feet apart and one or more of them
are also 5 feet from a large room boundary the suckout notch
will be augmented - in this case centered at 67.8 Hz. Note
that the distances do not have to be exact. Woofers ten feet
apart will still have additional attenuation applied if one
or more of them are, say, 4 foot 10 or 5 feet 2 inches from
a room boundary. The notching is not so abrupt that slightly
different distances do not count. The suckout slope will be
gradual enough for close fairly distances to still add to
the effect. Obviously, it is a good idea to get as much
asymmetry as possible when it comes to dealing with
bass-range cancellations.

Actually, at least with full-range systems placed in typical
locations, this suckout phenomenon is more likely to be a
problem in the middle bass, instead of in the low bass,
because the woofers in such systems tend to be fairly close
to room boundaries. "Fairly close" in this case means less
than, say, three feet. However, when woofer/subwoofer
systems are placed large distances apart (more than eight
feet) or large distances from room boundaries (more than
four feet) it can happen fairly far down in frequency, too.

With single subwoofers placed in corners, the issue does not
exist, because at such close distances any boundary-related
notching would be generated well above the operating range
of the system. Indeed, one of the advantages of
subwoofer/satellite systems that use only one subwoofer (at
least as it relates to suckout notches in the range below
middle bass) is that one can position the satellites so that
any potential suckout effects they would generate would be
below their crossover-controlled operating range. And as
noted, any that the corner-located subwoofer might generate
would be above its crossover-controlled operating range.

For example, if you have a sub/sat system with the sub
located in the corner it is likely that any three-boundary
suckout notches will be between 200 and 600 Hz. Obviously,
if you have the sub/sat crossover set at 80 Hz. these
artifacts will not be reproduced by the subwoofer. At the
same time, the potential inter-woofer and some (but not all)
of the woofer/boundary artifacts that would be generated by
the satellites will be in the 50 to 70 Hz range, which is
below the 80-Hz crossover point.
This situation still does not solve any middle-bass,
closer-boundary suckout problems with the satellites, but it
does eliminate any for them that would involve
longer-distance inter-woofer or woofer/boundary artifacts.

A lot of people are still confused about just what the
suckout effect (often called the Allison Effect, after Roy
Allison who documented its existence years ago) is all
about. Many people will mention "floor bounce" when
discussing cancellation effects and speaker measurements,
but the phenomenon happens with all large room boundaries
and not just the floor.

This cancellation artifact impacts the power response of the
system, whereas your typical floor-bounce artifact (where a
second, reflected signal arrives later than the original
after hitting the floor between the listener and the
speaker) involves first-arrival signals. While the frequency
of a floor-bounce notch will be effected somewhat by the
listener's location, the much more important power-response
suckout will be the same anywhere in the listening room.
Below is an explanation of why the effect happens at all
with woofer/boundary interactions, with my example primarily
dealing with the effect in the middle-bass region. As noted
above, at greater distances the suckout will happen at lower
frequencies.

Let's look at a typical box loudspeaker system positioned in
a room so that its woofer cone center is about two feet from
each of the three nearest room surfaces (floor and two
intersecting walls). When the speaker is radiating a very
low frequency the cone moves relatively slowly and over a
relatively long distance. If the radiated frequency is 40
Hz, for example, it takes 1/40 second (25 milliseconds) for
the cone to execute one complete forward-backward cycle.
Each half cycle takes 12.5 milliseconds (ms).

As the cone begins a forward movement it generates the start
of a
compression wave. This impulse travels at the speed of sound
(approximately 1130 feet per second at sea level) to those
nearby room boundaries and is reflected back toward the
woofer cone, arriving there some 3.5 ms after it left, while
the woofer is still generating the compression half of the
sound cycle. The reflected waves increase the instantaneous
pressure seen by the woofer and enable it to radiate more
power than it could in free space. This is why placing
woofers (or subwoofers) close to boundaries augments their
outputs.

However, as the woofer tries to radiate at higher,
middle-bass frequencies, it must reverse its motion more
quickly. For example, at 140 Hz. (the middle bass, for
sure), the cone reverses direction every 3.5 ms. It begins
its half-cycle of motion (attempting to create a
rarefaction) just as the compression-wave reflections from
those two-foot distant room boundaries begin to arrive back
at the woofer. In this case, the reflected signal is out of
phase with the cone motion, decreasing its radiation
efficiency. The result is a suckout notch in what would
otherwise be a flat woofer-output signal.

As I indicated before, this phenomenon will exist in all
parts of the room, since it deals with the actual power
input of the speaker to the room. That sets it apart from a
standing-wave artifact, as well as from your standard
floor-bounce anomaly. It is also much more influential than
the latter, because power response is a much larger
percentage of the total output than the direct response.

Howard Ferstler