My Unity Horn Speakers - The Finale
My latest iteration on Unity Horn speakers in the home takes a bit of
an unconventional direction. The horns have been soffit mounted along
with eight Peerless XXLS 830847 drivers. The drivers are mounted in
pairs in a clamshell configuration to minimise structure born vibration
from the speakers. This was essential because the speakers are not
decoupled from the house structure, they are actually integral to it.
The 830747's are mounted in sealed enclosures of about 90 liters per
driver. Soffit mounting is a technique used regularly in recording
studios. It eliminates the baffle step associated with having a pair of
speakers sitting out in the room. The radiation angle at low
frequencies is confined to 90 degrees in the horizontal plane. The
Unity horns also have a radiation angle which is around 90 degrees at
the crossover frequency, narrowing to 60 degrees up higher. Having a
speaker system with essentially the same radiation angle at all
frequencies guarantees a flat power response. This can clearly be heard
when walking around the listening room, and indeed the whole house.
Apart from a reduction in exteme treble, the sound essentially doesn't
change as you walk around the house. The baffles are at an angle of 30
degrees from the front wall, and combined with the 60 degree radiation
angle of the Unity Horns, there are no reflecions off the walls in
front of the listening position. The rear wall is a long way back so it
takes a long time before any reflected sound reaches the listening
position. As a result, the sound is very dry, almost like being
outdoors. The dry sound makes the acoustics captured in a recording
much more realistic.
My little helper, Bianca, posing for a photo during the
construction of the enclosures. They are made of 25mm ply, except for
the baffles which were MDF, 25mm and 10mm laminated together to allow a
recess for the grills and provide a nice smoth surface to match the
finish of the gyprock walls. The enclosures including baffles and
drivers weigh around 250Kg each.
The depth of the boxes required that the speakers be recessed into
the wall, to achieve this I needed to move four studs. Being an
external wall on a brick veneer home, this left me with an extra 200mm
of mounting depth to play with. I needed to keep the baffle width down
to 1200mm because of the proximity to the window in the front wall.
Here you can see the back of the Unity horns mounted in the speaker enclosures with their associated crossovers.
Here is the crossover I am using on the Unity horn. As far as I can
tell, it's Tom's most recent development for my horn and combination of
drivers. I tried three other versions provided by Nick McKinney before
this one and was never happy with the results. This version sounds
excellent. Tom proposed a mod to the midrange ports which I was unable
to do when the speakers were mounted in their previous boxes. He
suggested that I change the 10R resistor in the mid section for 12R
when not using this port mod. The arrow between the 25R resistor and the 5mH inductor is showing that the DC resistance of the inductor is subtracted from the resistor. If your inductor has a 3R DCR the resistor needs to be 22R. Keep the DCR on the 2mH inductor as low as practical. The mids (Misco JC5RTF-B I believe) are wired in series / parallel. The DE25 is the 16R version. I'm not sure why this version was used as the top end sensitivity on the compression driver sets the system sensitivity. The 8R version would have lifted the sensitivity by 3dB. This could be done by scaling the high pass network components to suit. The B&C DE250 would be a suitable substitute now that the DE25 is becoming hard to obtain. These drivers have very similar characteristics. The BMS 4550 would be better yet, but would require a crossover redesign. The mid driver port holes are 19mm dia and 19mm deep and enter the horn where it is 75mm across.
Filling screw holes and blending the speakers into the existing (Out of square!) walls.
The final result, with grills on. It almost looks like the speakers
were meant to be there all along! The 42" plasma will be replaced with
a bigger one in the future. Throw in a built in cabinet for the
electronics and the job is done.
This is the frequency response of the system from the listening
position, after calibrating the Behringer DCX2496 which is in charge of
crossover duties. The plot was taken with my IMP/MLS and is two traces
joined at 400Hz. 10dB/div, 1/3rd octave smoothing. Thanks to the
controlled directivity of the Unity horns, the response above the 340Hz
crossover changes by no more than a dB or two anywhere in the room.
Room modes mess up the bottom end a little, but not as badly as
expected because of the large (80M^2) irregular floorplan, soft
absorbent walls and the distributed bass system. There is only a small
amount of eq though the pass band of the bass section and the horns,
but a fair amount of work was required just above the crossover region
for the bass drivers. They have a suspension related response
irregularity at around 500Hz which needed to be taken care of. The
final acoustic response slopes were fourth order HP and sixth order LP.
With a small amount of delay everything fell into place. Interestingly
enough, there is no padding required for any of the drivers. The
sensitivity of the four 830847's per side is the same as the Unity
horns, with the B&C DE25. The horns are better than +/-2dB up to
16KHz throughout most of the room. The slight rise in the low end is
intentional. The system sounded better that way. When it was truly flat
the bass sounded lacking except at very high levels. At normal
listening levels of around 90dB the system sounds great when it's
balanced like this.
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