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Discussion Starter #1 (Edited)
Howdy folks, :wave:

Long time, no post. I have taken on a somewhat interesting / challenging sub build and wanted to show it off to you guys. I have been working lately on some upgrades to media programs at historic facilities. The main focus of the project to update the equipment to off the shelf / sustainable technology and address serious deficiencies in meeting ADA [American Disabilities Act for those that don't know]. This includes closed / open captioning, assisted listening devices, etc.

In this location, I am installing a smaller scale 5.1 surround sound system. For the sub, I basically had very limiting options - build into a wall cabinet or into a bench. With input from others, I decided to go with the bench option.

The challenge: turn this bench into the sub for a small alcove with seating for an average of 6-8 people for a 15 minute A/V presentation. The system will run a DVD top menu about 8hrs per day, everyday - with the program being shown an average of a dozen or so times per day in a public / museum type setting.

 

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Discussion Starter #2 (Edited)
My initial assessment was that I would be looking at something around 4 cubic feet in total volume. There are a lot of considerations for a sub in a public place. There is simply too much risk of front firing anything, so down fired is the obvious choice.

For this build I wanted to do something different. It's already hard and limiting, so let's make it harder, right?...:coocoo: I wanted to do a dual isobaric with inexpensive drivers. There is very little sub 30hz content in the media program, however, I did want to try and get the F3 as close to 30hz as possible.

For drivers, I ended up going with four of the 10" MCM 55-2190's.



They barely make the cut with Fs < 40hz and Xmax > 4mm. In staying with the making it harder than it has to be, it is also an experiment. I hope an think they will sound pretty good. Sealed did not model all that great with F3 in the 50's. They did however look great with an LT circuit. For power, I'll be using the MCM 200 watt amp - figuring I don't need a whole lot of power. One thing nice about these woofers are they are 8ohm coils. Each woofer will be wired to 16 ohms to get 4 to the amp.

Modeling: I used an older program called Box Port Design. Obviously, I am having to settle for a less than optimum Vb due to the confines of the bench. The final result is a System Q = 0.682, Vb = 2.27 cf, Tuned @ 28.5hz, 2 - 3" x 19" ports, Vent Mach < 0.04. It will be just a little peaky from 40-80, but not bad at all...:bigsmile:
 

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Discussion Starter #3 (Edited)
Okay, here comes the fun part!...building it and pics.

Since this is work related, I get to use the shop. It's heated and medium large. We are between renovations are are getting ready to add a dust collection system. Basically, I have pretty much what I need to do the job properly. It's also nice to not being chained to a desk and computer for a couple of weeks...:T

I read all the pros and cons of isobarics and the different types: tunnel loading back to cone, clamshell cone to cone, back to back and planar. I am really interested in planar and would love to do one someday. For this, I chose to do back to back - two woofers down firing in phase and two firing into the box out of phase. I could not put the magnets back to back touching because of the vented pole piece. However I did do magnet bracing to add support and to eat air volume to more closely couple the drivers.

Here is the iso box under construction:



Doing the driver cutouts in the iso box:



Here is the completed iso box:





Adding the amp side end cap brace to the iso box:



Here are the completed main port brace, intermediate port brace and port side end cap:



End view of the main port brace:

 

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Discussion Starter #4 (Edited)
Okay, here is the bottom of the bench, showing how it is put together. I will have to break it down into its component parts; where the sub box will become all the supporting structure. I think the biggest challenge will be to figure out how to mount the cushion so that it is removable and will give access to a screwed on top plate that will provide access to the in firing subs.



This is a mock up of all the internals and how they will lay out. The ports are going to come thru the left side vertical leg of the bench. The legs are just over 2 inches thick. I will have a square MDF plate on the outside of the leg where the ports come thru - this gets me more room between the port inlet and the iso box.



Here you can partially see the iso box side brace added today in looking at the mock up from the other end.



And finally, today I only worked half a day, but got the iso box undercoated after pre-wiring. I used Rustoleum Rubberized Spray Undercoat. Let me tell you something, it doesn't go very far. I used two whole cans and still didn't have enough to do the bottom.



So that's where I am to date...more as I progress.
 

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Well this is different than just about anything else here, which is interesting. I'll be following this build out of curiosity as much as anything, let us know how it turns out.
 

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What a fantastic build challenge!

My hats off to you for the creative solutions and quality of the build thus far!

I'm looking forward to seeing it's completion!
 

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Nice work. Why dual iso loading? And why those drivers? With an enclosure built that well loading it with sub-par drivers seems, well, kind of sad. ;)
 

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I'm guessing he went with Isobarik because he was confined to such a small enclosure and I haven't checked but I'm guessing he used those drivers because 4 of them was still cheaper than 1 higher quality subwoofer.
 

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Elite Shackster
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There is also the point that you can hit your target frequency response with a cabinet of half the volume, so he can hit a lower target response than would normally be achievable with the volume he is restricted to, which I think is what evilskit was getting at. Iso can actually be the design of choice where space is the dominating factor of your build.
 

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Yes, absolutely, Iso loading can be a solution for enclosure size issues, but he ended up with a 2.25 cuft enclosure so there really are other choices available... my question was more along the lines of discovering the designer's intention. Plus I maintain that a system built this well really deserves a better driver. :)
 

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Elite Shackster
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With the right expertise, you can get fantastic result with really very ordinary drivers. Just look at M&K subs. I get what you mean though :T
 

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Discussion Starter #13
Nice work. Why dual iso loading? And why those drivers? With an enclosure built that well loading it with sub-par drivers seems, well, kind of sad. ;)
One of the major cons of isobaric loading is a -3db loss, but as you probably already know, you gain +3bd for each driver added - so dual loading helps to offset that. I am a firm believer in multiples - multiple drivers or especially multiple subs. I probably would not build a single driver anything. In this case, I am using four drivers in a space that would typically be used by one. One the other hand, I lost a bunch of available volume with the back to back loading. Tunnel loading is much more efficient on the use of space, but has two major drawbacks - the exposure of one cone to excessive heat [one motor hot and the other cool] and all drivers in phase [losing some of the iso loading push / pull benefits].

Don't feel sad about the drivers. They are not sub par - just untested. The 8" version of the same driver is rather well regarded. According to Brian Steele [diysubwoofers.org], the minimum requirements for a subwoofer are Fs < 40hz and Xmax > 4.0mm. These meet that, so they will work. Partsexpress list several inexpensive drivers in their subwoofer section that don't.

I could have used just about any affordable drivers within reason. Another solution that would work would have been to use two 10" Dayton Titanics [love these] and a much bigger / more powerful amp...and a whole lot more $ that would have been overkill...:dumbcrazy:
 

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Discussion Starter #14
Hi guys,

The latest is that I was able to get 2-1/2 days further along in the project this past week. Things keep popping up. The plan for this upcoming week will be to get the box lined, final assembly of the removable top, cushion mounting and to start in on finishing. Finish materials for the exposed sides is still undecided - possibly some type painted beadboard / face frame or face frame / carpet panel deal.

All the photos are of the main box going together. The exterior walls are 3/4 ply wrapped in 1/2 MDF and the final layer in 1/8 tempered masonite. All the layering was to get the outside dimension of 17" wide so that when I add the 3/4 trim to the outside, I will be right at 18 - 1/2" - the required width to support the top cushion. The box is very strong and already weighs a ton...:R















As always, more as I continue to make progress...:sn:
 

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Discussion Starter #18
I should have an update for you guys this weekend. I had to burn up two weeks of vacation time over the holidays and I have made very little progress this week due to all my regular duties.
 

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Discussion Starter #19
Hi guys,

Got some more photos for you. Here's the latest; I had a set back on using the existing bench sides and trim. Everything went okay taking it apart until the very last step where the side rails were doweled and glued into the side. On the left side, all of this was glued to the veneer - which suffered some damgage. I decided to just make new sides instead of putzing around trying to repair. In hindsight, it was a blessing [even though I was really mad when it happened...:boxer:]. The good part is that now I can get the box a little higher off the floor and can side mount the amp.

For sound deadening, I carpeted the interior of the box with a glued in place berber style carpet. It made a big difference.





After carpeting. I built the top. There is a fixed section and a removable section. I am using the recesses in the top as the mounting system for the cushion. Basically, I have plates mounted to the bottom of the cushion that sits into the recess.





The next series of shots are of the new sides. I made them out of Red Elm. This is the first time I've worked with this species. My take on it is that it has some beautiful grain, is a bit snaky and not terribly stable - it cuts easy enough, but sanding takes forever! The grain is cross-linked, so maybe that's why. In hindsight, I would of done this out of red oak or honduran mahogany...did I mention it takes forever to sand...:paddle:







Sometime next week, I should have the construction done and ready for the finishing stage. After staining and poly, I am pretty sure I am going to take out to have the non-trim portion of the box shot with bedliner. This will be quick and easy and provide good protection from public use.

More later...:spend:
 

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Discussion Starter #20
Howdy folks,

Been awhile since my last update. I have great news - this build is finished!...:bigsmile:

I just finished running through two consecutive 72hr burn-ins and even though its still in the big ol' shop, it sounds and works great. I can't wait to hear and feel it in its new home. My take on the sound is that it has the tight sound of a sealed sub with significantly more low end output. It has excellent performance with music. I am very happy with how it turned out. I found it to sound best with the xover set at 120hz, but I suspect that will change once it's in a regular size room - probably not much if any room gain in the shop.

In the end I decided against bedliner for the box finish and used spray-on Rustoleum Hammered Black. It took 4-5 cans and at least 10 coats to get the look I was after. The wood is red elm stained with Minwax Gunstock and 3-4 coats of Minwax Polycyrlic.

Here are some of the last build pics through testing. I will post another and final update after it goes to its new home. Hope you enjoy them.













 
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