another Maelstrom sonotube build - Home Theater Forum and Systems -

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post #1 of 14 Old 03-28-09, 03:00 PM Thread Starter
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another Maelstrom sonotube build

my project is really not much different than what alot of other folks have done, but I thought I'd create a thread anyway, just in case any tips or tricks I discover may be of interest to anyone else. I will add to this as I go.

This one is sealed, and uses a 24" dia. x 25.5" length of sonotube, so net volume should be right around 6 cubic feet. I am using 3/4" plywood, it is fir core with birch laminate. I am very happy to not be using MDF for this project! No more MDF for any indoor projects...too much dust and fumes. The plywood will be doubled up on both ends, with the innner section fitting inside the tube and the outer section sitting flush. So the o.d. of the outer sections will be 25".

Really the hardest part of making a sonotube sub is cutting out the circles for ends. These can be purchased from Elemental Designs. I considered this but there was a long lead time when I checked with them and I really wanted to try cutting them out myself. I am using a Craftsman router, with a 1/4" x 1" straight carbide bit. I suspect the spiral/fluted bits do a better job of pulling the chips and dust up where the vacuum attachment will catch them. This particular router will accept a circle cutting jig with two parallel rods. I have no idea if this is typical to other routers. Anyway, the "stock" jig will not reach to cut out such large circles, so I made a larger one using a 4' section of 3/8" dia stainless rod. You could also use carbon steel rod, it would be easier to drill. I bent this using my vice and a length of pipe, when finished it is formed into a "U" shape, then I drilled a hole for the pivot screw. This jig would work for even larger circles. I learned you want to use a fairly heavy duty screw or it can break off, the one I ended up with is .175" dia. on the threads. Also the jig tends to want to climb around on the screw, making the router move up and down causing slight humps in the cut, I solved this by shimming up the screw under the jig with nuts, then cincing the screw down all the way so there is no verticle slop.

I recommend cutting the inner sections first, if you are like me you will get better as you go! Don't get greedy near the edges, leave a little extra when you lay out your placement...otherwise you may get a flat spot...and some router bits do strange things when going from a fill dia. cut to a skim at the edge.

The first one I cut out was a bit lumpy/wavy, still way better than a freehand job with a jigsaw though. The shimming I mentioned helped alot, also keep in mind any jig like this has a certain amount of slop. If ylet the router "walk" back and forth within this amount of slop, you will have a less than perfect cut. As you cut, you want to pull the router outwards away from the pivot, do not let it go inwards even for a moment. Avoid sitting in one place, keep it moving. Think ahead, when you get near edges, things could shift when you break through a section etc., if you don't stay steady it will create a hump in the cut. When you get near the end the circle you are cutting must be supported, and stop when you get down to about 1/4" thickness. Turn off the router, take out the screw and get the router out of the way. Then move your circle up and down a few times, breaking that last little thin section. It should break in such a way you can sand it smooth with little or no evidence where the start/stop point was. So I cut out the top circle last, this is the one that will show the most, and I am going to use stain and poly on it too. It ended up nearly perfect on the router cut, no significant waves in the cut.

I am planning on cutting an extra ring, 22" i.d. to put near the middle of the sonotube for support. I'm sure this isn't really needed but the idea is to help prevent the cardboard sonotube from creasing or getting dented in if it gets hit by something, and will only add a pound or two. When finished, I plan on also using the sub for a lawn roller, so it must be as sturdy as possible. Ha ha. Edit, I decided the extra ring/brace was overkill, so I skipped it. I still think this would be a good idea if the sub was, say, in a dorm room or was going to be moved a lot to where it could get beat up. Mine will be back in a corner behind another pc. of furniture.

After cutting out each 24" section, I sand down any high spots on the outside dia with coarse paper on a block, then I use a roundover bit (set very shallow) to make a slight taper on one side. This helps with insertion, helping to avoid boogering up the edge of the sonotube plus makes a little more room for glue.

The type of sonotube I bought has a waxy coating to make it somewhat waterproof. I used coarse grit sandpaper to remove this and rough up the surface on the first inch or so of each end on the inside, to promote adhesion.

Will add pictures later on.
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Last edited by random username; 03-28-09 at 08:29 PM.
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post #2 of 14 Old 03-28-09, 06:01 PM
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Re: another Maelstrom sonotube build

Good luck on your build! The waxy coating is a release film that can be peeled off. Looking forward to your pics!
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post #3 of 14 Old 03-30-09, 06:30 AM Thread Starter
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Re: another Maelstrom sonotube build

This project is going well, overall. I can't begin to explain the simplicity of sono with plywood, vs. building a flat sided enclosure with heavy bracing out of least considering I have no table saw.

I applied matching birch veneer around the two outer sections, this is the iron on type. Trimmed it with a razor blade. Bend one corner of the blade upwards to prevent it from digging in to the flat face. Applied with the wife's iron and a sheet of aluminum foil. When approching the end, I stopped about an inch short, rough cut to allow overlap. Then I used the razor blade, tapping on it with the hammer, to trim both ends at one time. The other trick I learned is using automotive brake cleaner to clean off any glue smeared on the wood, and you can also clean fine sandpaper with it. This was nearly my first time with veneer. The only other time I used it was inside an worked great on another project I had for building up a bracing board that was slightly short...sure beats trying to fill a big gap with glue! You can layer it up, and it down, whatever works.

After completing the veneer, I moved on to staining. This is the only part of this project that has been disappointing so far. Hard to see in the picture, but it has an uneven, wavy appearance. The plywood was already sanded pretty well, I just did a quick 600 grit. I think my problem is that the factory sanding job caused this effect, probably should have taken it back to coarse and then gone from there. No big deal, can always re-finish it later.

As you can see, I went ahead and cut out the inside of the rings. This is a little risky without having the driver yet. If the sizes were too small, the only way I would have to enlarge, would be a sanding roll on my die grinder. I may wait to do any gluing until I have the driver. I went just over 18.5" on the outer, and between 16.75" and 16.875" on the inner.

The wife and I went and picked out the she "hesitantly" participated a bit. I let her make the decision on this, she picked out some thin carpeting, sort of a dark brown color.

another Maelstrom sonotube build-sono3.jpg
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post #4 of 14 Old 03-30-09, 10:33 AM
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Re: another Maelstrom sonotube build

Looking good.... you have to be careful letting the wife participate. You might end up with little flowers and birdies on your subwoofer or she will want a lamp or plant on top of it.

Kevin Haskins
Exodus Audio
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post #5 of 14 Old 04-02-09, 10:20 PM Thread Starter
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Re: another Maelstrom sonotube build

Lol...she has to pretty much force herself to be interested. My wife is about as dis-interested in sound as anyone could be. That's not all bad. While she can "rock out" with the little speakers cranked on her laptop and think it sounds great, coversely I have trouble these days going to see live music...because there's usually something about the sound I don't like, and I fixate on that instead of just enjoying the music. And I'm probably not nearly as particular as many other entusiasts. Sometimes I think being so intent on having music and movies sound a certain way, is a curse. An expensive, but very fun curse!

Well, my driver got held up in a blizzard on its way here, and the next step on the project was gluing the other end on. So I "rolled the dice" and glued it in...if the driver doesn't fit, I'm going to have some very creative routing and/or sanding to do! Oh well...

I will post some more "tips from a noobie" and pics tomorrow.
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post #6 of 14 Old 04-03-09, 09:25 PM Thread Starter
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Re: another Maelstrom sonotube build

The other day when I glued the bottom on, I forgot to line up the sides with the seams in the trim veneer, but just by chance I was close.

When gluing the ends on, I used Liquid Nails heavy duty construction, the normal type (not the low voc/slow drying type). I used masking tape on the veneered trim edge. Then ran a bead in the corner, and went around with my finger smearing it up towards the taper. It doesn't hurt to smear some on the inside edge of the Sonotube as well, but keep in mind this stuff has a pretty quick set time (especially smeared thin) so don't fool around too long. IT may be helpful to use weights to hold things down. I followed up with putting a bead in the inside corner, pushing it into the corner with a finger while wiping the excess onto a paper towel. This is for sealing as much as anything.

So, this is my first woodworking project where I did staining and varnishing.

Staining and prep notes-
the plywood came veneered and sanded. Naturally, I thought it would be a good idea to make it even smoother. This was a bad idea. What happened was, when I stained it, the stain was absorbed unevenly for two reason is the sanding done at the mill was uneven, and the second was that finely sanded wood doesn't want to take stain. What I probably should have done, is taken it back to very coarse, preferably with a power sander sanding the entire area of the sheet prior to cutting. Then take it to no finer than a 150 from there.

Varnishing-when I got ready to apply, I shook the can vigorously...then proceeded to read the instructions which said something about "do not shake under any circumstances"

The varnishing went ok. I learned it is very possible, upon sanding the first coat with 220 grit, to burn right back through to the bare wood. Don't do that. Super fine steel wool seemed to have it's place here too, instead of the 220 grit. I only did two coats, it was amazing how much nicer it looked after the second coat. If you are a beginner like me, the smart way to do it is finish the more hidden section first (in this case the underside) as "practice". As it worked out, the underside looks great and the top looks ok but not as nice.

So the driver finally came today. I knew it was big, had looked at the pictures etc. but in person this sucker is substantial! See photo of it next to my old friend, a 15" Alpine. Back in the day, this baby was a serious unit, with its four inch voice coil...800W max, was supposedly "dealer/competition only". These days it's used as a weight to hold things down. Check out the contrast between the surrounds on the two drivers!

I test-fit the driver, lowering it in using some stainless wire. The driver has a unique hard rubber sealing ring. I'm assuming this is all I need, that I don't need to use anything else to seal it up? I guess I'll find out tomorrow when I put it in. I can't wait! Also I'm anxious to see how my Onkyo output works with the EP. I did a little testing today using a test tone, my IPOD put out 270 mV max, my computer's system hooked to BFD put out 470 mV (limited by the source, not the headroom of the BFD). If I really pushed it, I could get over 2 V out of the Onkyo which seems pretty good so we'll see...

By the way, the cutout dimensions I used turned out just right, 18 5/8" and 16.75"-16.875". It could be possible to go a little smaller on the small ring, if you wanted to "hand fit" with the driver available during cutting. Keep in mind you need accurate allignment between the two rings or the outside of the driver will hit.

To mark the holes for the driver, I used a large phillips screwdrvier, tapping on it with a hammer..this worked great. Using the wire to lower the woofer down, I had to rotate it, allign with marks already made then mark the last four (can't fit the screwdriver in the holes along with the wire).

I had bought special screws just for mounting the driver, but they would be way too short and small. I decided to use long, heavy duty drywall screws. I will use washers to back up the heads to prevent them from pulling through the hard rubber ring on the driver.

another Maelstrom sonotube build-almostdone.jpg

another Maelstrom sonotube build-comparison.jpg

another Maelstrom sonotube build-overall.jpg

Last edited by random username; 04-03-09 at 09:40 PM.
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post #7 of 14 Old 04-04-09, 11:45 AM
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Re: another Maelstrom sonotube build

I would think you want the screw heads/ washers butted up on the metal of the driver, not the external surface of the gasket.
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post #8 of 14 Old 04-04-09, 04:10 PM Thread Starter
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Re: another Maelstrom sonotube build

good call! I sorta figured that out as I went. I thought the ring was maybe hard enough, with washers to hold up, but it just compressed. That ring is made of some kinda impressive stuff! After removing the one screw that I had put in the wrong way, there was a big gouge/ring where the washer was. I walked away and came back a few minutes later, and the plastic had "healed" itself...hmmm...magic!

I ended up using the same screws, but putting the washers between the ring and the driver's frame. The heads of the sheetrock screws are barely larger than the holes in the ring, so as you tighten them down they pull right down in where you want them. I didn't have any problems after that, with the screws but...I think next time I build one I will glue some chunks of hardwood under the perimeter where the driver give more support...maybe overkill, but couldn't hurt.

I stuffed the sub with five pounds of polyfill. If I added any more, I'd really be compressing it in there.

so the sub is done, and wired to EP2500+BFD. I temporarily placed the sub in my listening/computer room, 9x15", starting in a nearfield position, right in the center of the room. At this point, all that was loaded for filters is Bruce's low pass 80 Hz combo (thanks, Bruce). With this rig, I have enough gain to run the amp, but not real hard without boosing/cheating with the BFD. How does it sound? First off, I'll say that this driver seems more sensitive/efficient at higher freqs than some...but this may be somewhat of a subjective comment since I don't often run without a low pass xover. I ran it without the filters for a bit and the low bass was really overshadowed by the higher stuff coming out. then I listened to music, with the HP 80Hz on, and no other filters. I'm not sure if "tactile" is the word, but this sub gracefully, effortlessly puts out bass you can feel, shaking my chair, without sounding too bloated or boomy. Even at low levels, the kick drum can be felt as much as heard. Running some quick tests with my sound meter and a sig. gen, it is very strong to 20 hz, then starts to taper down below that.

We'll see how REW looks, and I will post. I love weekends where you have no real obligations and can play around with hobbies!

Last edited by random username; 04-04-09 at 06:25 PM.
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post #9 of 14 Old 04-04-09, 04:48 PM Thread Starter
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Re: another Maelstrom sonotube build

At this point, the sub is located in the far corner of the room, about 10' from my chair. This is with no eq/filtering whatsoever. This shows as much about the room as it does the sub. The only thing I changed between the two runs, is I moved the mic about three is taped to a mic boom stand. One run was on the left side of my head, the other on the right. I wanted to see what would happen to that nasty dip at 59 Hz...sure enough it came right up. The sub is downfiring into the higher freqs are dampened considerably.

another Maelstrom sonotube build-maelstrom-noeq.jpg

Last edited by random username; 04-04-09 at 06:26 PM.
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post #10 of 14 Old 04-04-09, 07:44 PM Thread Starter
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Re: another Maelstrom sonotube build

Heres where I am at, my goal for this session was to make it as flat as possible within the workable range, no "house curve".

another Maelstrom sonotube build-maelstromeq1.jpg
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