Loudspeaker Designs and Articles for the DIY Enthusiast
MTM Crossover
Links to:
Aviatrix Main / Design Goals
Driver Selection

Sealed MTM Cabinet Construction
MLTL MTM Cabinet Construction
MTM Crossover Design

MT Cabinet Construction
MT Crossover Design

Crossover Design:

MTM Sealed version

Due to the high Qts of the ND140, a pair would normally require a
large enclosure, about 100 liters, to provide a Qtc of .7, with an f3 of
roughly 80 Hz. I took advantage of the high Qtc of about .9 in the 15
liter box and capacitively loaded the ND140's with 1000 uF. This
reduced the response peak and consequently lowered the relative f3
of the system. Modeled f3 is 60 Hz, and f6 is 50 Hz. Due to the
capacitor, the low end response ultimately rolls off at 18 dB per
octave, instead of 12 dB per octave.

The lowpass, network consists of two large 500 uF caps in parallel,
along with the more traditional 2nd order electrical filter functioning as
a 3rd order acoustic filter with a crossover frequency of 1000 Hz. The
highpass is also a 2nd order electrical filter, and summed with the
response of the tweeter provides a 4th order acoustic response at
1000 Hz. The drivers sum in near phase quadrature.
Crossover Design:

MTM MLTL version

The crossover for the MLTL is identical to the sealed
version with the exception that the two 500 uF caps
are not utilized. The following comments concerning
center channel usage and baffle step adjustment are
applicable to both versions of the AviaTrix.

Center channel usage
The crossover for this design was optimized for use
as a center channel. When orientated horizontally,
the modeled horizontal off axis response at all
frequencies stays within +/- 2 dB up to 30 degrees off
axis. While the worst case response off axis response
is around the crossover frequency, and is -7 dB
down at 45 degrees off axis, all other frequencies
stay within 3 dB of nominal at 45 degrees. At the
more normal seating positions of +/- 15 degrees off
axis, the variation at all frequencies is about 1 dB.  

Baffle step adjustment
A parallel pair of resistors in series with the tweeter
network provide the necessary attenuation. The
value of one or both of these resistors can be
changed to suit the room positioning. For near wall
mounting I suggest a 10 ohm and a 12.5 ohm in
parallel. For more traditional speaker placement, i.e.
further out into the room, or if used for stereo music
reproduction, I suggest using 12.5 ohms for both
resistors. I didn't experiment with in wall, but perhaps
using a pair of 10 ohm resistors may be appropriate.
Some experimentation may be necessary to find the
best solution for a specific room position and
Conclusion/Listening impressions:

I’ve quite enjoyed having these in my
listening room, and feel we have been
successful in our goals.

Recognition should go to Wayne for the
gorgeous enclosures and crossover
voicing, as well as numerous
collaborations in the general designs.

While the AviaTrix can’t quite keep up
with my reference speakers, they come
very close, and are quite listenable for
extended periods of time with no fatigue.
They are very clean and articulate, with
surprisingly low bass for a 5” MTM
design, especially in the MLTL
configuration. Where some speakers
can come off sounding a bit rough
around the edges, especially when
pushed, the AviaTrix remains smooth
and ethereal, demonstrating its low
distortion levels. Those that prefer the
full, lush sound of paper driver woofers
may find the bass and lower midrange a
little too clean and articulate for their
tastes, but I think many will find the
sound quite enjoyable. Extended
listening sessions, some at high SPL’s,
have not resulted in any fatigue.