The Triune
An All Dayton LCR Speaker
Frequency response plot
Click on drawings for a larger image
Horizontal off-axis response curves
Horizontal off-axis response curves
with tweeter connections reversed
Impedance plot
Woofer network
Tweeter Network
Vertical off-axis response curves
Triune: From 17th century tri and Latin unus one. Literally:
Three in one.
The name is somewhat appropriate, not only because it
indicates the number of drivers in the design, but also it's ability
to be used as a left, right, or center speaker with excellent polar
response. The idea for this design was sparked from numerous
posts on the PE forum wanting a small, spousal acceptable MTM
center channel using the Dayton
295-301 5.25" shielded
mid/woofer and the
275-075 Shielded silk dome. I wish to thank
everyone who responded to my posts on the forum and helped
shape this resultant design, which comes in at a parts cost of
$60.

Background:
The MTM configuration, while its vertical off-axis response is
perfectly acceptable in a vertical alignment of drivers, generally
exhibits unacceptably poor horizontal off-axis response when
orientated with the drivers aligned horizontally. This may be due
to the philosophies of the designer, or constraints of the drivers
and crossover topologies utilized. In other words, if the MTM was
not optimized for horizontal orientation, it likely will exhibit audible
combing effects off-axis. The scope of this design exercise was
to determine if good horizontal off-axis response could be
attained using the chosen drivers, and what crossover
topologies were more acceptable to this endeavor.

A little Theory:
The maximum crossover point in most speaker designs is
generally that frequency whose wavelength is equal to the
center-to-center spacing of the adjacent drivers, that is woofer
to mid, mid to tweeter, etc. This is an appropriate rule of thumb
for speakers with vertically aligned drivers as the vertical
listening angle varies very little from seated to standing. With
horizontally aligned drivers however, we must contend with a
much larger off-axis angle, and consequently for a horizontal
MTM design, I suggest we must be concerned with the maximum
center-to-center spacing of the two woofers, and use that criteria
as the acceptable minimum crossover frequency. The quandary
is that the woofers need a lower crossover frequency than most
tweeters are capable of, to produce a good off-axis response.

Design:
The results of this final design are very encouraging. As you can
see from the frequency response plot, the on-axis response is
reasonably flat with approximately 4 dB of baffle step
compensation. The modeled sensitivity is roughly 87 dB.  When
used as a center channel, the horizontal off-axis plots indicate a
maximum deviation of around 5 dB at 30 degrees off-axis. While
this is not ideal, it is quite acceptable compared to the other
topologies I investigated. The off-axis plot with the tweeter
reversed shows good phase tracking through the crossover
region, even off axis, something I was unable to attain with my
preliminary designs. The vertical off-axis plots show very little
variation even 60 degrees off axis, indicating very acceptable
performance when used vertically as a left or right main speaker.
The impedance plot has a minima of 4 ohms, but is above 6
ohms for a significant portion of the drivers passband. While I'll
call this a 4 ohm impedance, I suspect many commercial
designers would consider this a 6 ohm speaker.

A little more Theory:
I modeled many different crossover topologies during this study.
I found the best results for my criteria were with lower order
underlapped designs, but like most designs, the actual driver
responses, especially the woofer, determined the actual
crossover used. A crossover network using Butterworth filters will
sum + 3 dB when both low-pass and hi-pass filters are at the
same frequency. However if the woofer crossover point is moved
down in frequency, it is possible to obtain a reasonably flat
response, with only  minimal peaking. Setting the tweeter filter
network to model a 2nd order Butterworth acoustic target
response at 1900 Hz, I was able to move the woofer crossover
point down to 1200 Hz, and approximate a 3rd order Butterworth
acoustic response. This lower low-pass filter
frequency allowed significantly more spacing between the
woofers with acceptable horizontal off-axis response.
0 < Page > 2