Thursday, October 4, 2012

SEOS Project: Preparing to make sawdust

With both drivers in hand, I got started taking some measurements needed to being work on the enclosure and the crossover. First I measured the impedance curve for the Eminence Deltalite-II 2512 in open air and on a 1 cubic foot box. The equipment in the lab output a text file with the impedance data that I imported into REW to produce these graphs:

Off Box

On 1 ft^3 Box

REW also has a handy tool that will derive the driver parameters from the impedance graphs, so using that I got the following parameters:

TS Parameters file
Room EQ V5.01
Dated: Oct 3, 2012 8:29:47 PM
From measurement Eminence 2512 off box.
Zmin 5.41 ohm
fmin 213 Hz
f3 1036 Hz
Le(f3) 0.508 mH
Motional impedance parameters
RES 137.61 ohm
LCES 35.003 mH
CMES 373.5 uF
RAMS 137608.424 mohm
Blocked impedance parameters
RDC 4.50 ohm
dR 0.26 ohm
Re 4.76 ohm
Leb 131.0 uH
Le 2.001 mH
Rss 1503.6 ohm
Ke 0.0619 S-H
Thiele-Small parameters
fs 43.5 Hz
Qms 4.355
Qes 0.485
Qts 0.437
Fts 99.6
Mms 61.15 g
Cms 0.219 mm/N
Rms 3.836 kg/s
Vas 83.90 litres
Bl 12.796 Tm
Eta 1.39 %
Lp (1W/1m) 93.58 dB
Dd 25.72 cm
Sd 519.5 cm^2
Vb 28.317 litres
Secondary measurement: Eminence 2512 on box 1
Air temperature 20.0 C
Air pressure 1013.25 mB
Air density 1.2041 kg/m^3
Speed of sound 343.2 m/s

The data seemed quite different from the published specs (2512 Specs), but when I modeled it in WinISD, there actually wasn't a huge difference.

Below are the modeled system responses for both the published specs, and the measured parameters with a single driver in a 2 cubic foot box. I opted to go for a sealed box for several reasons: I'd like to keep the size manageable, going sealed simplifies the box construction, and finally since I plan to cross these over with subwoofers, I get plenty of extension (70ish Hz) for that purpose.

Going ported would allow a flat response down to around 40 Hz, which still wouldn't be full range, so subwoofers would be needed, and the crossover would be around 80 Hz anyway, essentially wasting the low frequency extension gained with the ported design.

Yellow: Given  Blue: Measured

Once the size of the box was chosen I went ahead and played around with the Enclosure Volume Calculator to arrive at the final dimensions. The only starting dimension was 15" wide for the front baffle to fit the SEOS waveguide with a little room to spare. I could have made it much taller and less deep, but it still wouldn't be tall enough to be floorstanding, and I'd rather just make stands for them later anyway. The .2 cubic feet entered for the driver volume is a rough estimate of the woofer, waveguide, and bracing. The final volume will not be exactly 2 cubic feet, but I tried to err on the larger side, since a slightly larger box will give a little more extension.

Finally, with the dimensions in hand I made up a cut list using a free program aptly named Cut List. It will try to find the most material efficient layout of panels, which can be nice, but in this case I just entered the panels and manually arranged them (which can be done just as easily with a sheet of paper and a pencil). I laid out the panels so that I could rip down the material and make sure that all of the panels had the same width (especially the 14 1/2" pieces since they must be identical). The 15" pieces aren't as critical since they'll be going on the front and back and I can just trim them with a flush-cut router bit.

Finally, I had to decide on what type of material to use. I'm very fond of the Baltic Birch plywood I used for my subwoofer project, since it's extremely strong, void-free, and fairly lightweight. It also has some decent looking grain, so with a little finishing work it provides a handsome natural wood finish. However, since these speakers will eventually go in a home theater (hopefully with a projector) they're going to get a more functional-- and less flashy-- matte black paintjob to reduce light reflections. And there isn't much that takes paint better than MDF, so that's the material I'll be going with. I'm not a fan of the ridiculous dust from the stuff, though, so I'll probably be picking up a respirator or at least a heavy duty dust mask when I go to the hardware store.

No comments:

Post a Comment