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This speaker was not
designed but evolved into a design. I needed to build a set of small speakers to
complete a gear swap.
What drivers did I have on the shelf? The MCM 55-1855 mated to an ApexJr ribbon were my best bet.
What kind of box?
It would be nice to get better bass than the 95 Hz F3 of a 0.7 Q sealed
box. I had some 6" plastic pipe and had wanted to make some of this into a speaker.
So we build a toob TL.
Other details? Lets mount the drivers on an angled top plate (need room for
the tweeter after all). 45 degrees is fairly easy to cut so lets start there.
How long? As long as i could get from my piece of pipe ~ 25". Fb
= 135 Hz before stuffing. With stuffing and end correction it will be in the ballpark
of what is required.
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Many hours of filing
(those top plates have ALOT of file equity in them) and some routing & gluing
and more filing. The terminus shape is dictated by the tools I have to make the hole.
With the complex radius of the shape maybe we spread the Q of any across-the-port
resonances. The mid-woofer cutout placement was done so that the magnet would wedge
against the back of the tube -- i got close, I only had to shave away a millimeter
or 2 to make them fit.
Then some flat black
paint -- wow that brings out the elegance of the shape of these things -- and the
resemblence to a big cigarette has gone away.
1" of low-density foam lining the walls of the last 18" of the tube. 30
g of well-teased polyester fluff, then 40 g, then 50 g. Happy. Bass down to 75 Hz?
They are playing music and even with just a 3.3 uF cap on the tweeter they show potential.
Add an inductor to the woof and that cleans up some ringing in the top. These things
are gonna work out.
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Some measuring. Coat the
mid-bass -- that cleans up the ringing and we can toss the inductor. Better. Still
a bit of a dip near the XO. Evolve the XO to a lower cutoff 2nd order
(~4.5 kHz vrs 6k) on the tweeter complementing the (now) smooth roll-off of the woofer. |
Add a solid Garry Oak
(endangered & from a Heritage Tree -- it was a windfall) base. This is spaced
1.25" from the bottom of the tube with pieces of copper pipe with a lag bolt
thru the base into the medite on the bottom of the tube. Afix the XO and add some
terminals. They are going on carpet so I made some spikes by grinding pointy bits
to some bolts.
With my SO's help I (she did the sewing -- check out her web site) made up some grill socks recycled from
my old Accoustat frames (suffering a bit from kittens using them for climbing practice)
and they are ready to go.
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Chart
1: in-room pink noise response curve
(red-- 1/3 octave RTA,
green -- Spectrum Analyzer)
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| Line Configuration |
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Physical length |
25 inches |
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Physical fs
at 1/4 wl |
135 Hz |
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Sd |
11.6 in2 |
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Tube cross section |
26 in2 |
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Terminus area |
~12 in2 |
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Stuffing |
foam & polyester stuffing |
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Cost of Bits (in
USD)
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| MCM 55-1855 5" Aluminun
cone mid-bass |
$ 13.50
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less in quantity |
| ApexJr Ribbon Tweeter |
5.00
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| 3.3 uF cap + 0.1 uF cap
+ wire |
2.50
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| Nuts & Bolts |
2.00
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| 0.4 mH inductor |
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salvaged |
| terminals / 6" PVC
tube / Medite for bases & baffle / gary oak / grill cloth |
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salvaged |
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Total
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$ 44.00
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for the pair |
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Pictures
of a similar project
(added 25_09_06)
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