Hello, Knuckleheads. I just thought I would fill the group
in concerning a special project I just completed. At the
very least it should be of academic interest. I also thought
this would be a good way to let some of you know that I am
still alive, although I am fully "retired" from audio
writing - and glad of it.

Anyway, my "main" system consists of two, ten-driver Allison
IC-20 left and right channel speakers, four Allison Model
Four "surround" speakers and two additional Allison RDL AV-1
minispeakers as "back" surround speakers. (The Yamaha
processor I use has both standard side/surround channels, as
well as a pair of front "effects" channels and two back
surround channels.)

The IC-20s are on a 22 foot wall, centered about 12 feet
apart and 5 feet from each side wall; the front "effects"
speakers are on the side walls, two feet out from the front
wall, mounted six feet up, and facing each other across the
room; the normal, "rear" surrounds flank the listening
couch, four feet from the back wall, facing each other
across the room, and are also six feet up; and the back
surrounds are four feet apart on the back wall about seven
feet up and face the front wall. Note that the side wall
length is 18.5 feet and the ceiling height 8.5 feet. This
room tops out at about 3400 cubic feet.

For center-channel work, given that I have a pull-down
screen and a front-projector video monitor (the screen is 8
feet wide and 4 feet high and pulls down in front of a
window that is the same size), I had previously built a
rather unique speaker that was made up of four AV-1
minispeakers set up in an array that resembled the top half
of an IC-20. (The array even had two radically shortened
IC-20 grills that I modified myself.)

This four AV-1, eight-driver speaker package had four
Allison RDL two-way tweeters and four Taiwan-built 4.5 inch
cone drivers to handle the midrange down to the 90 Hz
crossover to a Hsu subwoofer that just handled the center
bass. (The main subwoofer for all the other channels is a
Velodyne F1800RII, located in the left-front corner of the
room.) Note that the 4.5-inch cone unit were the standard
midrange driver used in the AV-1 and not something that I
picked up elsewhere.

The speaker was equalized by an AudioControl C-131
1/3-octave equalizer and the result was a measured room
response (exhibited by my AudioControl SA-3051 RTA) that
rivaled the unequalized IC-20s. To be truthful, I also
slightly equalize the IC-20s with a Rane THX-22, and so all
equalized up-front speakers measured +/- 1 dB from 80 Hz on
up to 16 kHz. I documented the IC-20 curves in issue 95 of
The Sensible Sound a while back and the center speaker had
an almost identical response. (The response below 80 Hz
ramps upward slightly down to 20 Hz with all three
speakers.)

Anyway, this home-built center speaker had several minor
defects:

1. It was a two-way system (using four AV-1 crossovers, of
course) and therefore the tweeters were working hard below 4
kHz and down to 2 kHz, in contrast to the tweeters in the
IC-20s, which cross over at 3750 Hz. This was of no serious
consequence, but I was still concerned, especially given
that I had to equalize (boost) a bit at 2 kHz to flatten the
center speaker's room curve. As John Stone might
acknowledge, any Allison tweeter is at its best above 4 kHz,
even if the two-way version has ferrofluid instead of the
silicone grease in the three-way version.

2. The midrange drivers exhibited a moderate peak (at about
800 Hz) that had to be equalized out. There is no such peak
in the IC-20 midrange, and so even though the response was
smooth after equalization I felt that the midrange drivers
were enough inferior to the Allison units in the IC-20s to
make them suspect. Indeed, when feeding certain midrange
test tones into the center speaker I heard some moderate
harmonics that I did not hear with the IC-20s.

3. The small midrange drivers were being run down to 90 Hz
and I felt that they were being pushed a bit harder down
there than they should be, even though the Hsu crossover was
high-pass filtering at 24 dB per octave. (The Hsu amp and
crossover are actually driving a modified SVS 16-46
subwoofer that handles just the center bass and which is
located in the right-front corner of the room.) Remember,
this speaker was being used for home theater (as well as
music) in a 3400 cubic foot room at a 14 foot listening
distance. It had a big job to do.

4. While I am no imaging fanatic, I felt that the dual,
angled-panel arrangement for a center speaker tended to make
dialog clarity a bit less than optimal. This arrangement
works fine for left and right main speakers (the IC-20s, or
Model Ones), where spaciousness and side-wall reflections
work to advantage, but it seems to be a compromise for
center use. The speaker is also located in front of a large
drape (that is behind the pull-down screen that attaches to
the top of the speaker cabinet) and the off-axis signals
were obviously being absorbed somewhat. This caused the
direct-field signals to be more significant than normal with
standard speaker placement.

The solution to these dilemmas was to build a new center
speaker. The job is now complete.

This new speaker makes use of a vertical MTTM array on one
forward-facing panel. Basically, the arrangement looks like
one side of an IC-20 mounted in a standard, forward-facing
rectangular enclosure. The tweeters are stock three-way
Allison versions and the midranges are Allison units as
well. The system has two woofers: 6.5-inch Allison jobs
mounted on opposite sides of the enclosure at the bottom.
The grill is a "shortened" IC-20 screen (not as hard to
accomplish as you might think) and the side-mounted woofer
grills were cut from grills that came from a pair of Allison
AL-125 systems I had a few years back.

Rather than mount the tweeter and midrange drivers in the
prime cabinet and therefore make it important that the
gaskets around them are air tight, I built a shallow, sealed
baffle behind them so that the woofer enclosure part of the
cabinet is air tight and independent of the part holding the
mid and tweeter drivers. The wires leading through them go
through a small pair of holes in the baffle and lead into
the woofer-enclosure cavity. The small holes are sealed with
hot-gun glue.

The system has two identical crossovers: each was taken from
an AL-125 and modified to handle just one woofer. (Normal
AL-125s have two woofers.) The crossovers are mounted on the
interior side surfaces of the woofer cavity, and the
interior input cables to those crossovers run to two
five-way jacks on the back panel.

The speaker unit is therefore required to be biamped, with
each identical amp driving a woofer/mid/tweeter array
through a crossover. The crossover frequencies are at 4 kHz
and 450 Hz and are electrically second order in all
sections.

The amps used to drive the center speaker are the
main-channel units in my Yamaha RX-Z1 receiver (these can be
electrically disconnected from the main pre-out jacks on the
back panel), with the actual, left and right main channel
signals eventually going to the flanking IC-20 systems
routed from the main-out jacks on the Z1 to an outboard
Carver M500.

The power available to the center speaker is in excess of
250 watts (probably closer to 300), with an additional 250
watts available to the subwoofer below 90 Hz. Each IC-20
also has 250 watts available, with 600 more available from
the F1800RII subwoofer below 90 Hz.

The speaker is 38.25 inches high, 12.5 inches wide, and
11.25 inches deep (10 inches actually, but with a 1.25-inch
overhang at the top and bottom to handle the modified
grills). The box is made of 3/4-inch oak veneer plywood and
MDF and the full unit weighs in at 57 pounds. The surface
was finished with dark stain and given four coats of
polyurethane, with sanding by 400 grit paper between each
coat.

The center of the MTTM array is 24 inches off the floor.
Given typical seating height this could be a problem, since
there will obviously be direct-field lobing. However, to
keep the vertical direct-field signal as coherent as what we
have with the IC-20s (which have the center of their
mid/tweeter driver array at 36 inches), the speaker is
tilted back slightly.

I continue to equalize this speaker, although it does not
require much of that above 400 Hz. Indeed, above that
frequency it is inherently nearly as flat as an IC-20. Below
that frequency equalization is applied to take care of the
usual room anomalies. Like the previous speaker (and the
IC-20s) in equalized form it is +/- 1 dB from 80 Hz on up to
16 kHz, but it gets that way with much less fuss than the
earlier center speaker.

Anyway, I just thought I would let the group know about this
excellent sounding speaker that is made up of Allison
hardware. Well, the dual five-way post connectors on the
back were from Radio Shack.

Adios.

Howard Ferstler