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Discussion Starter #1 (Edited)
Well, the day finally came so here is my build thread. Thank you to everyone in this wonderful forum for all the help provided, both direct and indirect.

As you can see from the title I chose an 18” Maelstrom-X and 2 18” passive radiators. The form factor is a 25” cube and the tuning is somewhere between 15.5-16.5Hz depending on how much fill is added (according to BassBox). Double-thick front and side baffles give it an internal volume of 6.4ft^3 after taking into account the drivers and braces.

The main reason I opted to spend the extra money on the PRs vs. a sealed design is explained by the following figure.


I chose AE PR18s with a 2100g mass. Out of curiosity I calculated the equivalent port dimensions and it is 22in diameter by 48 feet long!

Here is a excursion sim from WinISD including a SSF (4th order 13Hz) and the BFD and EP2500 rolloff. This is my approximation of worst case conditions as it is 2000W and the coil temp rise is 100K.



Here is the design as drawn in google sketchup. I used excel and measurements of the driver and PR frames to make sure there was enough internal clearance. The spreadsheet allowed me to vary the back-to-front offset of the passive radiators and it would redraw the figure shown on the right.

I settled on moving the PRs 1” towards the back w.r.t. being internally centered.

And the cutsheet diagram


Due to workspace and skill limitation I decided to have the box built for me rather than go the tool borrowing/rental route. I had the box made by Nik Brewer from this thread.

The box arrived last weekend and it was a beauty. Here is a picture of it as I was putting the hurricane nuts in.


inside the box showing the driver/pr and braces clearance.


For fill I purchased standard cheap pillow polyfill. I think it may be a little less dense than “speaker grade” polyfill. In a rush to hear it for the first time I just put two bags in for a total of 40oz. I will be adding more soon but will need to find a way help it stay in place, particularly at the top of the box, while keeping it away from the moving parts.

For initial testing I mounted the drivers without the rubber gasket because I’m saving my socket head screws for the final product. Also the machine screws I’m currently using make it difficult to use the gasket because of the tight flush-mounting.


Here a link to a post where I show my final mounting solution
http://www.hometheatershack.com/forums/exodus-audio/16611-maelstrom-2xpr18-prebuild-questions.html#post160267

Measured in-box impedance (including about 0.1ohms total between amp and driver)


This was pretty close to the BassBox and WinISD predictions. The two peaks are just a little closer to each other than the simulations. The result shows that I really shouldn't have to worry about thermal issues over much of the low end because of the high impedance and cooling benefit of high excursion.

This graph places the tuning at about 15Hz (0 degrees phase crossing). But eyeballing the excursion on test tones makes me think it is closer to 16Hz.

Here is my first REW measurement. It is not in the final room. It is placed 1' from the wall and 5' from the corner in a 11.5'x25.5' room in the basement which opens up to a hallway and stairs. I used a mic calibrated down to 10Hz

It was quite easy to smooth out to fit a 60Hz LPF target using the BFD.

Listening impressions.... I don't know what to write here, it's just ridiculous - particularly stuff at 30Hz and 16-20Hz.


edit: shortcut to the finished result
 

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Nice documentation. The dual PR + Maelstrom is my favorite build. It can be done in a relatively manageable box size and gives you just massive output.

Keep us up to date once you get it in room and finished.

Kevin Haskins
Exodus Audio
 

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This looks nice. Nik did a good job on the box. What kind of finish do you have in mind? Grills or no grills? Very impressive output below 20Hz! I know how difficult that is to achieve with a sub of this size. Congratulations!

Mike
 

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Discussion Starter #4
Thanks. Nik is also making grills for me - circular with a roundover. For finish, my tentative plan is to just go black with some sort of matte topcoat. Hopefully I will begin next weekend starting with some fill and sealing and sanding the exposed mdf edges. I plan to roll on the first few layers of paint with sanding in between then finish it with spraypaint. I welcome other suggestions because I'm not that experienced in the high-quality finish department.
 

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Others probably have more experience that I do but...If you have many rough edges (it doesn't look like it from the photos), you should probably use wood filler first to get the edges smooth. Some use bondo but I'm not sure I like that idea. After sanding smooth, several coats of paint with a satin lacquer should look pretty nice. I have used a sprayer with an air compressor and had decent results. Just watch how close you spray, as it will go on thick enough to run if you get too close.

Mike
 

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I just saw your other thread about the box shake you are experiencing. Once you figure it out, I may decide to build something similar. I am curious how you chose the AE PRs. For instance, why did you choose those over the Exodus EX-PR18? Would you mind explaining your decision on that?

Thanks in advance.

Mike
 

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Discussion Starter #8
I just saw your other thread about the box shake you are experiencing. Once you figure it out, I may decide to build something similar. I am curious how you chose the AE PRs. For instance, why did you choose those over the Exodus EX-PR18? Would you mind explaining your decision on that?
Mike
I chose this particular PR because of the higher weight (2100g). This allowed me to get a lower tuning and squeeze a couple inches off of the box size. Also, they are stated to be able to handle up to 2500g by adding mass. This was another thing I considered when trying to settle on a tuning and box size. I will say that if I had EX-PR18s it would probably look a tad bit more impressive because it would look like 3 drivers.
 

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Discussion Starter #11
Yesterday I began finishing the box and I am hoping to be completely done by the end of next weekend.

I filled in nicks and imperfections with some regular wood filler and I used a couple of coats of shellac to seal the wood in the driver and PR cutouts. As an undercoat I used wallboard primer. I rolled on two coats and sanded in between.



The primer was nice and thick so sanding worked well to get a smooth finish.

Today I started painting but it's going slowly because of high humidity from the rain this morning. For paint I chose a bright red gloss enamel. I am putting it on with a roller and plan on most likely putting 3 coats.

Here is a picture of the first coat (still wet).


I'm still trying to decide if and what to do for a topcoat. The three options I'm weighing are: no topcoat, some sort of spray-can topcoat, or to use some varathane liquid plastic from my last project. I believe the last one may give it a very slight tint but it will give it a virtually indestructible finish.
 

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Wow... bright!!! lol.

I'd give it some kind of topcoat, you know you'll feel safer in the end. I found with my last sub that rolling on a thicker coat looked the best, but I could only do one face at a time, so maybe give the spray-on a shot if you just want to do it quickly?
 

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Discussion Starter #14 (Edited)
thanks for the suggestion

Another update,
Painting is delayed because of rain and humidity over the last 2 days. The 1st coat is still not dry. Good weather is expected Tue-Fri but then rain again after that. I'm going to try to get work done in the evenings and maybe mornings as well, if it dries overnight.

In the meantime, I ordered these spikes from parts express and some grill fabric.

Also, I just received the grills I had made, also by Nik. They are 3 of identical size - 19" ID, 3/4" wide with 1/2" roundover.

The closeup shows them to be made from two stacked pieces of 1/2" MDF. This should give plenty of clearance because the driver and PR surrounds stick out slightly compared to their flush mounted rims. Plus I have to leave room for the Xmax :)
 

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In the meantime, I ordered these spikes from parts express and some grill fabric.
You will like the spikes, I added them a last year to my Paradigm sub and they are of high quality for a reasonable cost.

After emailing back and forth with Kevin H I am thinking about a similar build. I already have the materials cut for a 24" sealed cube (NEOdan's cutsheet) so mine will be a little smaller. Since it is a single 0.75" birch plywood the internal volume may not be to much different. I'm not sure if my wife will let me spend the extra cash, with all the clamps, saws, routers etc this project is getting EXPENSIVE.
 

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Discussion Starter #16
After emailing back and forth with Kevin H I am thinking about a similar build. I already have the materials cut for a 24" sealed cube (NEOdan's cutsheet) so mine will be a little smaller.
Once you hear it you will wonder why you were fretting over $50 here, $100 there, etc. If you would like I can send you the spreadsheet I had made to check the internal clearances. The Exodus audio PRs use the same frame as the ones I used and put in the spreadsheet.
 

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Discussion Starter #18 (Edited)
I would love to get your spreadsheet for clearances. Thanks!!!
Here it is for anyone. It is setup to draw the figure with double thick front and side baffles. If your design is different just pay attention to the internal dimensions.

It currently has my dimensions in it. Playing around with the values it looks like it is possible to fit everything in a 24" cube even with double side baffles. The AE PR18-2100s with weight added to 2500g gives a tuning of about 16Hz.
 

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Discussion Starter #20
Another update,
I have been working on the paint evenings and mornings and just rolled on the 4th coat (I had done a good amount of sanding between the 1st and 2nd, light sanding between others - 400grit then brown paper bag). Depending on how it looks this evening I may decide to do 1 final coat with spraypaint. I'll also have to decide about the topcoat. The t-nuts for the feet are installed and the feet look awesome with the red. I also sealed the grills and received the grill cloth and mounting hardware. I'll be spraying these black.

I have been rushing to get this done because today is the last day with decent weather. The forecast says starting tomorrow afternoon through monday will be cold, humid, and rainy.
 
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