Measurement Set 23 APR 2004
a phone call with Rick Craig at Selah
Audio it was apparent that measuring one of these beasts is
a less than exact science. Rick and I both use LspCad. Normally
for an MTM design, a single woofer is measured along with the tweeter.
Each of the woofers is then located on the box front so the program
can accurately model phase and lobing effects. With seven drivers
on each half, where is the geometric location of the acoustic center?
suggested modeling as a simple TM with coincident acoustic centers.
Based on his experience, he recommended careful alignment of the
measurement mike on the tweeter center -- to minimize any cancellation/reinforcement
(CA/RE) from woofer interaction. He suggested the peak areas MAY
be artifacts of the CA/RE -- and that small changes in microphone
position will result in large measurement changes.
this logic a bit further, could one look at the array as a 2.5 or
maybe a 3 way? The resistance tapering introduced to reduce "sound
bloom" suggests that outer drivers should be a bit weaker in
SPL. Jim Griffin mentions the possibility of treating those peripheral
drivers with a separate crossover. If one rolled off those outside
8 drivers on a steeper slope than the middle 6 drivers, could you
use that compensate for baffle step and control the bloom? Seems
like something worth checking out.
the arrival of the Morel DMS37 tweeters, I built a new tweeter insert;
finished damping/sealing of the second cabinet; and made a large
base for a stand. This would allow me to plumb and level the array
for the next set of measurements.
erecting the cabinets, I hit on easy way to align the microphone
with the tweeter center. First I set the cabinet plumb and level.
Next, I set a chalk line on the ground perpendicular to the baffle.
About 2.5 meters away, I set up a piece of plywood with a tape measure
hanging over the front. Next, I retrieved a small laser pointer
I use in the classroom. I carefully taped it to my 4 foot carpenters
level. Now I simply, placed the edge of the level on the tweeter
axis, aimed it at the tape, and read off the height. Instant laser
level! After a couple of hours, I had a new set of measurements.
went ahead and split the woofers into the "Inner 6" and
"Outer 8" based on my wiring. I noticed the peak centered
at 700 hz is still present. It seems to be more pronounced in the
Outer 8, less so with the Inner 6. I have no idea why.
Bogus Measurements and Initial Xover 25 APR 04
spending time trying to model my second set of measurements, I was
concerned that the Morels couldn't play loud enough. The all driver
spl looked like I would have more than enough headroom. But when
I loaded the driver specs, it looked grim. I went ahead and designed
a simple crossover to try out. If the Morel wouldn't play loud enough,
I would have to do a major revision of the design -- or possibly
scrap it altogether.
the test Xover, I wired up the pieces, slid in the test CD, and
fired up the prototype. Much to my surprise, (and delight) I wound
up with a 4 ohm series resistor on the Morel. So, something was
funky with my last set of measurements. I traced it to a bad jack
on my sound card. After fixing things, I measured TBLA again.
to the quick and dirty crossover, the image is definitely different
than the traditional TM. I liked the DMS 37 a lot, not as beamy
as I had feared, very smooth and warm. The box sounded a bit boomy,
so I tried plugging the vents, this smoothed out the response quite
a bit. The crossover wasn't bad, but wasn't quite right. Some phasing/sibilance
issues and overly subdued female vocals. I will also experiment
with stuffing and bracing.
HERE FOR REVISED "FINAL" MEASUREMENTS
new measurements looked much more reasonable with respect to SPL
and tweeter responses the high end tail-off noticed earlier. I went
back to LspCad and introduced the new measures. By using the "Combined
Array SPL2" as a target curve, I was able to match the individual
driver responses to the combined curves. An encouraging sign for
get this match, I had to add 5db to my measured response for the
Morel. I'm not sure why this was necessary, but it explains the
need for padding down the Morel in the first design. Using the basic
topology of the first crossover, I was able to construct a fairly
smooth TM crossover. The gentle slope from low to high is intentional,
I find it easier on my ears.
three tries, I finally hit on a crossover design that seems to work
well (or at least serve as a basis for tweaking). I had to cobble
up coils and caps from supplies on hand, so the prototype looks
pretty gross. Click here
for Initial Crossover
a driver spacing of less than 5 inches, I needed to keep the crossover
under 2800 Hz to minimize lobing effects. With the 2000 Hz point,
the transition from woofer to tweeter seems fairly seamless.
though I have only one box completed, I sat down in the garage and
listened for about an hour. Overall, I liked what I heard. Bass
is clearly available. Good slam and attack -- though just to the
mid 60s. (I would have no trouble living with these in standalone
mode) A bit of box resonance is noticeable so more damping and bracing
is in order. I will try light stuffing with dacron and a bit more
Morel is lovely. While carrying most of the 2K+ information, it
reproduces highs effortlessly with very good dispersion. I ran it
a bit hotter than I usually do, but never felt like I had to turn
the volume down. There is a little bit of a sweet spot, but off
axis is quite clear and clean. Moving from sitting to standing brings
the image with you.
female vocals, piano, and horns. Male vocals seem a wee bit subdued.
I can't say much about imaging from a single channel. Overall, I'm
very happy with the sound, and I'm certain this combo can be tweaked
into submission. The RLC leg in the tweeter allows some shaping
of response in the 3-6000 range (diffraction effects). With the
parallel resonance filter (for the 500-800 peaks) and the zoebel,
the woofer response can be formed. Impedance drops to a low 6.6
ohms at 2000 Hz -- so no problem driving the beast.
I'm out of play time for awhile. I need to build the other two box
halves, design a mechanical coupling system, and a base. With temps
already in the 90's, my shop time will be limited. So until I have
working pair, there won't be much to report for awhile. I'll update
when I get there.
5 2004 Crossover Revision
finally got out to the garage to put together a revised crossover
that addresses a few of the limitations of the first pass and uses
single coils and caps.
the primary inductor to 1.2 seems to be the best balance so far.
The move to 15ga also slightly increases sensitivity. With minor
revisions to the pseudo zoebel and a lower cap on the tweeter, male
vocals are clearer, and the 2000 hz area is slightly depressed.
(A bit of BBC dip) Crossover is now about 2200 hz. Click
Here for Crossover 2
this topology, most of the shaping is best done with the woofer
zoebel and the resonance filter on the tweeter side. With the padding
resistor ahead of the tweeter circuit, changes there affect both
volume and shape of the tweeter curve. The advantage is a smoother
impedance profile. With temps here approaching 100, I need to work
in the mornings. Hopefully the next entry will work with a stereo
16 Crossover and Boxes
learned something about Baltic Birch. Don't cut it and leave it
leaning. Warp city. (It might have something to do with climate
changes from northern Europe to Southern Arizona.) I was able to
finish the second set of boxes with lots of biscuits and clamps.
I had used hurricane nuts to allow for removal of back panels. Don't
cross thread those babies! They hold great but can be pushed out.
I'm still wrestling with how I will attach all three sections to
allow easy breakdown and reassembly.
listening to Crossover 2 for awhile, (during construction of my
Seas Coax CC) I just didn't think
it was quite right. Part of this may have been lobing effects. It
just didn't seem like vocals were smooth. Nor was I happy with piano
played a bit more and revised a bit. I restored the 8.2 uF cap in
the tweeter circuit to bring the xover point closer to 2000. With
some shaping of the zoebel, things looked good again.
came up with an easy way to experiment with tweeter padding. I added
two shunt resistors just ahead of the tweeter. Using a SPDT switch
with center off, I can now simply flip the switch to change padding.
I will probably leave this in the final design with a cup. I tried
using a conventional L-Pad, but the resonance trap really needs
the resistor to stay at the front of the circuit to maintain the
shape of the tweeter response. The switch allows about 1.5 dB of
6 Box Design Changes
anyone ever tries to convince you to do a 2-3 piece cabinet, hit
'em with a hammer. Because of weight and size issues (baltic birch
comes in 5' x 5' sheets) my three piece cabinets are a real b*tch
to get aligned. If I were to do this again, I might choose to use
a 1/4" plywood skin, or hardwood sides.
chose to use 3/4 stock for the base plate on the second set of boxes
and to glue in the back for additional stiffness. I did set up a
removable insert for the terminal cup with hurricane nuts. This
should allow me to change the crossover easily. The large cup allows
use of the SPDT micro switch for selective tweeter padding. I just
drilled a hole in the cup body.
wrestled with methods to attractively attach the three pieces. I
built a tweeter insert choosing to laminate a second layer of 1/2"
plywood. After discarding the use of long bolts and various knock
down fittings, I talked to my neighbor and had him weld up four
U shaped steel straps. He welded 1/4" bolts thru the top of
the strap. I mounted these inside the tweeter boxes with #10 machine
screws and nuts.
will glue the tweeter box to the base cabinets; drill holes in the
bottom plate of the top box; and tighten down the nuts through one
of the driver holes. Once I bondo the countersunk screw heads, and
set the nuts with locktite, The final look should be clean and permanent.
down side of this method is pre finishing. I will put a chamfer
on the matching edges to hide any misalignment, but I will need
to veneer the big boxes before final assembly (to get all of the
final thickness issues hammered out.)
had considered using a 3/4" roundover on the vertical edges.
After measuring, I figured out it would take two large sheets of
veneer. The roundover would also make it virtually impossible to
build SWMBO-approved grills. There just isn't enough space between
woofer frames and the edge. As I may keep these, four dogs almost
necessitates some driver protection. So the plan is now black, square-edge
fronts with the remainder veneered. The tweeter box will be black
all around -- to hide any misalignment. I may bite the bullet and
try to do a one piece grill out of 3/8 or 1/4 MDF.