Sunday, October 7, 2012

SEOS Project: Box Construction Day 2

I started off the day by making a new, slightly smaller router base since the waveguide cutouts I was getting from my template were too small. Once that was fixed, I made a jig to align the front baffles underneath the template. Since the template is much larger than the baffle, I needed something to hold it up as I traced it, and also a way to make sure it was properly aligned on the baffle. To do this I took some scrap MDF and made a slot that I could slide the baffle into under the template. This made sure the template was aligned consistently for every cut and that the baffle didn't move. I simply clamped down the open end, and the baffle stayed put as I went to work with the router.

Below is what the baffle looked like after the first pass with the router. I made the recess about 5/16" deep and made sure to do the outline first. Then I went back in and chipped away around the edges until there was enough left over to seat the waveguide.

Next I used a jigsaw to make the rough cutout for the waveguide. It doesn't have to look pretty since the waveguide will cover it, so I made quick, rough cuts.

Once the waveguide holes were made, I got started on making the woofer cutouts. Comparatively these were extremely easy. Since I was using a 1/4" router bit, I had to make three separate passes, each one just smaller than the other to get down to the size where I would make the cutout.

First pass

Third pass

Final cutout
After all the cutouts were made, I went ahead and cut some more rings for the inside of the baffle. Since I cut about 1/3" for the recess I wanted to make sure there was plenty of stuff for the blind nuts to bite into on the backside. I did the same for the waveguide, but just cut some triangles for the corner. I originally meant for there to be a two layer baffle of 1/2" and 3/4" MDF, and had cut out the 1/2" pieces, until I realized that my flush-trim router bit only has a 1" cutting surface. A bit of an oversight, but I think this solution works perfectly well, and keeps the weight down a little.

Next up, I cut out some spare Baltic Birch to make some braces. These are 3/4" x 1 1/2", and are enough for all three cabinets. There will be two side to side braces, two front to back, and one top to bottom for each cabinet. After the boxes are complete I'll use a chopsaw to cut them to length and glue them in.

For the rings, I applied some glue, and rather than waste clamping time, I just shot it through with a few screws to hold it down while the glue dried. The glue is more than enough to hold it, but I'll leave the screws in.

I used a drill press to drill the holes for the mounting screws since I don't trust myself to drill the holes straight with a handrill and there isn't much space to work with.

Once the holes were drilled, I tapped in the hurricane nuts with a mallet after applying some Gorilla Glue to them. The nuts are supposed to hold themselves in, but I've spun my fair share of them, and it's no fun trying to fix it, so the expanding Gorilla Glue should do the trick. I'll also make sure to chase each one with a tap before trying to mount the drivers.

A row of finished baffles! The front baffle always takes up about 90% of the build time. The waveguide cutouts have 8/32 hurricane nuts, and the woofers have 10/32 nuts.

A quick test fit of the driver and waveguide in the front baffle. Looking good so far!

Clamping up the front and back. It's always a pain to get things aligned correctly, so I made them slightly larger than needed, so I can just trim them up with the flush trim bit and be done with it.

Friday, October 5, 2012

SEOS Project: Box Construction Day 1

I woke up this morning to get started cutting some wood for the speakers, and found a great surprise. My parents are celebrating my birthday early since I won't be home on the actual day, and when I walked into the garage I was greeted with this:

I haven't had a chance to use the orbital sander just yet, but table saw has already proven to be worth its weight in gold. When it come to making repeatable cuts, nothing beats a table saw, and this made short work of the cuts that I had to do. Thankfully when I went to Home Depot, a helpful employee cut a couple 15" panels out of the full 4'x8' piece on the panel saw, which also significantly cut down on my work.

Before making any cuts I did a few checks to make sure the saw was properly aligned. I measured from one of the teeth on the blade to the fence, then rotated the blade until the tooth was at the back of the blade and measured again.

Since the measurement was the same, the blade was well aligned with the fence, and I got started on making the cuts.

There was a small setback, since I found that I had misjudged the sizes of my material, and I'm currently short one 19 1/2" x 14 1/2" side pieces. I'll get some more MDF when I get the chance, and cut that last piece.

Once everything was cut out I clamped up the sides and the top and bottom for one cabinet, leaving a bit to hang over on the top and bottom so they could be trimmed flush with a router later.

While the glue was setting up, I got started making the template for cutting out the recess for the waveguide. I followed the instructions in this post on flush mounting irregularly shaped drivers and started by making the larger router base. In the picture below is the circle jig I made for my router, including the incorrectly measured hole for the cutout.

Without thinking, I measured to the far end of the bit, which resulted in an exactly 6 1/4" diameter hole, and a 1/2" smaller cutout, so I remeasured and finally cut out the 6 1/4" disk I would go on to use as the larger router base.

Once the larger router base was cut, I put the original base back on. Next I screwed the waveguide into the MDF (this time 1/4" thick) and traced around it with the router to make the template.

With the angle on the edge of the waveguide, the router base ended up tracing a little too close and made the template slightly too small, so I'm going to cut out a slightly smaller custom router base which will make the final cutout larger. Tomorrow I should be able to finish up the front baffle and hopefully close up one of the boxes.

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.