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Maelstrom 21" With 4 - 2100 Gram 18" Passive Radiators

57873 Views 180 Replies 23 Participants Last post by  Mike Cason
Here is my build thread for the Maelstrom 21. I'll try to provide enough quality pics during my build for you.

I'm using Kevin's Maelstrom 21"er with the passive radiator design. I normally don't like building a passive radiator sub with a Qts over .4, but it is so close, I won't have a problem.

Here's the deal....23" x 23" x 60"; 13 cubes and 15.5 hz box tuning. With the typical passive radiator design there is more of a roll off but at this already low box tuning, I'll have more than plenty of bass. I needed the height to match my main speaker's height and get the cabinet volume I need. Better WAF here.

I just picked up the pallet of materials from my mill. My construction material is 1.25" mdf. It looked like I had ten bags of concrete in my truck going down the road. In the roll of plastic, I've got two sheets of Pionite black slate laminate to match all of my other speakers.

Cabinet resonance with this thick material; I don't think so. Moving it? Now that's another challenge.

Just got back from Houston with the materials so here is the first build pic:

Read more: So where are the Maelstrom-X 21" Build Threads! - Home Theater Systems - Electronics and Forum - HomeTheaterShack http://www.hometheatershack.com/for...elstrom-x-21-build-threads.html#ixzz0XWvNWnLo


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5 Hz? Whoooooo... I wonder what else I am going to break in my home? There has been a lot of stuff that has been destroyed in the past. You had mentioned wanting to get them coated in a "bedliner" material. I looked into a product called Hurculiner to treat my electrical truck I purchased and restored a couple of years ago, but I went with a commercial company to do the work. You can get a quart to try. If you like it, then a gallon some rollers should do the trick. Use a sample board. Most auto stores around here have it on the shelf. I didn't like it because it wouldn't hold it's shine in the weather. Well congrats on your project Raven!

Crafty way of installing that poly fill! I just threw pillows and a line of acoustic blanket in mine! LOL

As for getting down to 15Hz...well, I'm not sure if my measurements are correct, but I just updated my post. I've got some repositioning to do, but it looks pretty solid down to 5Hz. :bigsmile:
I'm only a half-step ahead of you, and yours is much cooler! I'm looking forward to seeing how yours plays out.
I've got many more steps to go with all of the laminate work and grills. I may ask the Shack's group to all come to my house to help carry it in when it's done!

I am preparing some Bang and Olufsen drivers and passives to be shipped out and I have a bunch of rectangular covered wool packs that they install in their Redline series speakers. I'll probably figure out a way to nicely install a few of them as wool is an excellent fill material and really enhances the speaker's musical quality. A lot of folks don't know that. I purchase pure Romney wool and install it in my Audience A3 builds.

Hey if your's sounds great....who's to say mine is cooler....we are let's just say equals in satisfaction! :sn:

Well I'm ready to seal the coffin. I've decided I've got enough polyfil and the small amount of Sonic Barrier should handle any standing waves or distortion, so I'm sealing it up. Every hole is pre-drilled and ready for the glue and silicone.

I had a driver delivery I had to get to UPS and needed a puter battery to keep my system running, so here is where it's at tonight. After wire transferring funds to China for driver freight tomorrow, I'll get back on it and try to make some progress.

Hey fellows I've got the new F1200TS today and I've got some excellent reports for you and I haven't even used it yet. It is a masterpiece of it's own kind. I've got a lot of input from folks in the pro use and I'll post that info when I get some more time.

I appreciate the support! :T



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Mike, quit teasing with wimpy photos, lets see some nice big build progress!! :wave:
I've got two jobs and by my standards, am coming along fairly fast. I just picked up the raw materials 10 days ago and sold all of my other speakers and amplifiers and shipped them along with some driver orders. All the equipment is here, the back is now on and secured, and when I get back from the bank, I'll start on the bottom and the feet. I get my 18" 2100 gram passives tomorrow. Laminate should start to go on tomorrow if I don't have any service calls.
I know! I'm just funnin' ya!

I should talk, I've been too tired and busy to get real measurements from my system, and still haven't hauled it out to the middle of the living room for more photos.

But I did veg in front of the big screen and finally watched Rambo IV on Blu-Ray. I actually clipped my EP2500 on the mortar fire scene! The impact was impressive! Hell, even TV with the 21" has more impact.
You should get more lows through your TV than a music concert if you have a good receiver putting out a good LFE signal to your sub because the movie directors add the THX and high impact scenes with extra lows and punch, like a war tank sitting next to you, or U571 with the depth charges. Watching a clip from the Haunting, you would think the sheetrock would come off the ceiling with my other two stacked subs it was so earth shaking!! :hsd: I can't even imagine what the Texas monster is going to do?

Off to get the passives at UPS and the laminate will be started today. There's lots of sanding and tweaking to get to this point. The additional birch bottom is installed too.

Did you really mean 5 hz? How did you measure that low of a frequency? Typo?
I did mean 5hz, but as I said, my measurements are suspect till I figure out why they look so odd.

Again, 15Hz is no problem, even in my open floorplan.
You also have room gain to add to that as well, so that will help. 16hz is the lowest note from an organ.

I just got in with the passives and $75 more in hardware to finish the subwoofer and add more mass to the passives. I have to add 500 grams to each of them to get my 2100 grams. My supplier only had the 18" 1600s partially built, so I told him to just finish them and I'll add the extra mass to the suspension to get my 2100s.

Can't play in the forums and work on the sub at the same time so it's time to get back at it.
Ok, I can play in the forum a minute or so.... :wave:

I've got the bottom painted, the routed and sanded Red Oak runner feet installed, and to tease and taunt ya a bit, I temporarily dry fitted the sub and couple of passives. It looks so small in these pics!

I've still got to fill in the wood around the bottom of the cabinet and sand it down square to get it ready for the laminate. We may get snow tomorrow and the coldest temps of the season, so that's not a good time to fire up my blast furnace and spread contact cement. I may get postponed on the laminate tomorrow. I'll get the final mass loaded in the passive radiators tonight and have them sit overnight in the house with the Devcon two part epoxy. That will be just another step out of the way.

Well Lonley Ravin, here are more teaser pics for ya. Just wait till the black slate Pionite is on! They will really look sharp then. Look at that massive terminal. They will accept bananna plugs, ring terminals, or up to a #4 supply wire! :heehee:

Without the drivers in place, I was able to stand it up in the upright position. I don't have a clue how I'm going to weigh it. I might get two scales, balance it on a 2 x 6, and sum the totals of the scales. But in reality, who cares! :flex:


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Looking good, Mike!

I've finally gotten my act together and updated my thread with photos and measurements, and even a cheesy video for the excursion fanatics.

I'm betting that your box, even though smaller, and even without the weighted passives, is heavier then my completed box! Though I don't *see* any resonation in my larger side panels, I can physically see the box rock a few mm forward and back when the Maelstrom is going full tilt. It's a trade off for a box I could actually move on my own. I swear this box is lighter then my dual folded horn which was MDF with a pair of JL Audio 10"

I don't envy you're having to move those beasts, but I'm betting once in place they will be stunningly good!
You did good with the video. How much power were you giving the driver? You did a good job with the thread and your build pics too. I'm glad you are done and am enjoying it.

BTW.....hate to burst your bubble, but the first one with a finished 21" Maelstrom subwoofer takes the first slot in product completion. That includes the finishing work and grills if used. You better get to that bedliner shop quick!! My laminate goes on today! :devil:

You are right on the weight. I think my sub probably has double the weight of yours. It took everything I had to pick up one end of the cabinet to stand it upright, without the drivers. I wanted to make sure I had no resonance and as usual, I overkill everything I do.

I wouldn't even have considered using the 1.25 mdf unless it was a sub....I'm regretting it now, but by the end of the weekend, I'll be happy with it, I hope. It is snowing and I'm higher than the sky it's falling out of. I really enjoy this laminate work now!!! :whistling: Anybody got some more contact cement? It's a trip......I've also learned how to route thumbs now. It aint fun or pretty......two sides are done now and only two left to go and then the top. I'm staying with it till it's done tonight if I've still got all of my fingers left.....

Sorry, you can't hide behind the ladie's skirt.....the sub has to be finished........:clap:

Payback time here buddy.....
Hahaha! Good one, Mike.

Careful with the sharp objects there!

I think this weekend is going to be family stuff and working on sound treatment. Enjoy your subs!
I'm kind of new to this stuff, but does "working on sound treatment" mean telling everyone to shut up while you're working? :paddle:

Hey dude.....it's Woodstock all over again here.....you can't imagine all of the colors and things I'm seeing again. I don't want to open the door and lose it all.....I'm just about done with the laminate and the drivers go in tomorrow! Weeeee...... I've just got to figure out how to wire these passive radiators!
Well, it's 9:40 p.m. 8 hours of labor, and one gallon of contact cement later. I had to get the laminate on tonight because it's going to be in the 20s in the a.m.

HERE YA GO RAVEN! More not so whimpy build pics! All of the laminate is done and there is about 3 hours of seams to be beveled, cleaned, and treated before the drivers can go in tomorrow. Fast enough fer ya now?

Kevin, I normally don't do this, but I've been making love all day and I'm going to share my lady's pictures with you. Actually you will see her from 3 angles.

Everyone have a great evening cause this ole Texas boy is tired and ready to go in for the nite.

Drumroll.......here she is:


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That is awesome, I don't think I would like to be close when it gets angry.....
Thanks, I've designed her so that she can be very sweet and generous, or as brutal as you want her to be! :heehee:
Good morning Mike...and thank you.

I hope to start the beveling and clean up of all of the corners about mid morning. I would love to go ahead and install the drivers and get to jammin, but I'm doing things right, careful, and want a very nice looking project when I'm done. I appreciate your suggestion about the Maelstrom 21. I didn't even know they were out!

I got a phone call from a very excited person last evening. He was the buyer of my other subs. He said they arrived in Peoria Illinois yesterday afternoon and the truck driver said it was the best and most careful packing job he ever saw. The subs arrived on a pallet in mint condition. I explained the various wiring options he had with the dual terminal cups on each dual driver sub, which he loved. I suggested to him that he reads the Houston DIY forum to see the reviews from the members after we had our auditions. He is buying one of Crown's top of the line amps to drive them.

It's cold here so I'm not so slap happy to get back to work on them, but for $25.00, I can buy some kerosene and fire up my forced furnace and keep at it. Knowing where you live, you are probably sitting there laughing at me complaing about my mid 20 degree weather when it gets 40 degress or more below the -0- degree mark there.....

We'll see how much more gets done today. I would love to have the drivers installed and the grill work started by day's end. Have a fantastic weekend....:sn:
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That looks fantastic, Mike! It looks like you really will be the first one "finished". And in record time too!

Edit: Peoria is only a few hours south of me. Maybe I'll run into your old Maels someday.
Thank you Raven.....my other subs were the dual 2 ohm 15" Soundsplinter RLP-15s; 2 subs with two drivers and 4 - 15" passives each. The Maelstroms weren't out when I built them.

With all of the discounts and special deals I got, I'll have close to the same amount of money in this sub that I sold my other two subs and two amps for. Maybe $200 more, max. That includes the new F1200TS amplifier. Just a of a lot of labor.
Wow! That's quite a deal!
I'm up to $300 more now with the grillwork and more bolting supplies & silicone. I have more actual money in materials and costs with the twin subs, but I've used them for a couple of years, so I've got my money's worth.

With my work and my wife's work almost at a standstill, I may even put this one up for sale. :hissyfit:

The laminate work is completed and the subwoofer is installed. I'll install the passives, binding post, and lay out the grillwork and may get them built too. Either way, the sub will be done today and in the HT late this afternoon with or without the grills finished.
I have been busy lately so not much time to look around the forum...........Glad I caught this!

Great build, can't wait to hear how it compares to your previous build.
I received a delightful email from the buyers from Peoria Illinois of my stacked pair of subs in my avatar, and they simply can't believe the quality. They have done some eq and 15 hz is no problem with them and they hear everything so clearly in the below and above 20 hz frequencies, and at db levels never heard before. They are astounded! Why was I so stupid to get rid of them to build another one? They also have some expensive subs they are gettin rid of now!

Well the Texas Monster is finished as of 5 minutes ago. I'll get the pics to the computer and show you guys what I've built now!

Hey Mr. Raven:

Here are the completed pics. I plan on making nice grills, but I have all week to do that. At least these qaulify for the first completed build of the Maelstrom 21s!! :T They don't have to have grill covers to qualify.

Pardon the spots on the camera lens. I need to take the time to properly clean it to get rid of those dirty circles and spots.

I've got just barely enough room to get a nice grill beside the 21" beast. I've framed it already.

The first pic show just how large that driver is compared to the 18" passives on the sides. They look like 12"ers!!

The next shot shows the passives with my huge binding post for extra large speakers. The binding posts will accept bannana plugs, terminals, or #4 wire.

The third shot is from the side before I installed the passives showing my "basket made out of stretched audio grill cloth" and full of polyfill properly secured to the top of the cabinet. It also shows some polyfill on the bottom and the construction of the rear of the opposing radiators. I have Sonic Barrier on the top and the bottom of the cabinet, just because it was left over stock. It may provide some standing wave reduction, and with this cabinet, I don't think it will have a resonating problem? If so, the Sonic Barrier should handle that.

The fourth picture is a repost that shows the intricate details in construction before the laminating.

I got rave reviews on the Black Slate Pionite laminate from the buyer of my two stacked subs, and with a little application of Armorall once in the house, these should really shine. :wave:

It's late and raining, so I'm not going to attempt to get it inside tonight. Hopefully I'll have it online in my L/R HT and have a report by tomorrow afternoon. I've got a day's of bookwork to do too. I have Linkwitz's tone CD and a bass test cd, so I'll see if the graphs properly represent the 15.5 & 16 hz box tuning shown in the graphs. I really need to run some pink noise through them for a couple of days too, and then re-measure the tuning.

Yall have a great evening!


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Re: First report....

I managed to wheel it into the house last night. It didn't take up much more room than the stacked subs, but I don't have any grills made which will make it 4" wider.

Bass??? After discovering my Denon 5308CI had lost all of it's settings, I finally got the Face 1200 to start throwing out some signals.

I was very concerned about a 21" woofer from comments from the unbeleivers. Comments like the driver that size isn't quick enough to give you good bass & so on. You don't see 21" subs in the marketplace either. I think Foxtex has a $2,200 31" and a couple of others may offer something similar, but I've never heard of one finished in retail. If they would be that great, we should see more of them.

Well the Maelstrom breaks the barrier to lower bass, cleaner bass, and lots of both in the 21" driver!

The XBL2 motor design plays a big part in its superior performance.

I wasn't in the position to take some measurements but I did listen to a couple of test THX & Dolby Digital DVD tracks, Santana at Montreaux, and then the ultimate test; John Clayton's stand up bass on Diana Krall's live in Paris DVD. Of all the subs I've built and owned, including Legacy's infamous Point One, Mr. Clayton's bass has always produced a bunch of resonance. This disc is the only one that does that in my collection. Sure, I could probably go out and buy a multi-band equilizer and get rid of the unwanted resonance, but I wanted a sub that would play every note as John played it. I stuck the disc in the OPPO and hit the play button and was probably as nervous as Susan Boyle's debut when they pushed the play button.

Their opening song is probably their best on this album with John Clayton's bass, their lead guitarist, of course Diana's piano playing.

The sub didn't have any distortion, a total lack of resonance, was crystal clear, and with John plucking the bass strings, it sounded like you were there or he was here. The second track had John's bass playing as well and all was good. I was/am very impressed. I'm sure the 4 2100 gram passive radiators played a huge role in the performance as well as the properly built and dampened cabinet. With my new Denon flagship receiver, the picture was upconverted to 1080p, so I watched the concert until the wife went to bed. I haven't watched this DVD with my new receiver and its new HQV upconversion chip. It upconverts everything including Direct TV and component video.

When she got home, she locked the electric gate (we have a chain we use for added security). When she came in the house I told her how great it performed and she said the fence and gate was ratteling and I didn't even have it up to the spl I really like listening concerts at.

I've got a lot of shop, truck cleanup, and paperwork to do, but I'll find the tuning frequency and some spl levels in the next couple of days. I used the Face 1200 and got it to clip a couple of times (on purpose for testing). When I had it cranked up during some of the test cd tracks, the driver was really moving.

I guess by this report you guys know well I like it! It gets an 11 point rating out of a possible 10.
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Sounds GREAT Mike. Your thoughts on it really pick up where mine left off. I can tell you all about movies and impact and dynamics, but your thoughts on upright bass is even more impressive! I couldn't think of anything in my collection that I could test that has bass like that...except maybe some Miles Davis now that I think about it.
On my test DVDs, I had some movie scenes like Ants, Starwars, The Haunting, The Astronaut, and others. The impact scenes on those movie tracks couldn't be any better.

I think for any home theater use, a single 21" Maelstrom is all you need if your cabinet is constructed well and dampened properly. My L/R is 20 x 24 with 9' ceilings, opening up to the kitchen pass thru and wide open to the dining area so we can watch tv at the dining table. I think if you were to properly build two subs with a driver of this size and capability, you would have to turn the gains down to 1/2, thus defeating the use of two 21" subs. Bass is great, but you also need to hear the voices and movie as well. A very large HT may benefit well by having two of them.
Oh come on Mike, everyone needs at least FOUR 21" subwoofers. ;-)

I'm glad it turned out so well for you. Be careful with test signals near the tuning frequency of the PRs. Once the drivers stops moving, it loses most of its ability to shed heat so that is a very easy area to smoke a voice coil with test signals.

Kevin Haskins
Exodus Audio
Oh yea....I forgot....one 21" sub in each corner of the HT will drive out all of the termites, thus saving you thousands of dollars in future repairs!

Thanks for the reminder. Linkwitz's CD only gives me 4 or 5 short bursts and that is why I use it.

Years ago I did happen to blow four of Dan's Adire passive radiators out after I hit the wrong button. It sounded like a train engine coming apart! Weights were being thrown off the backs of the passives and hitting the sides and the passive radiators opposite them. I had the amp cranked up and I was trying to hit a high hz frequency tone signal for some reason, but I hit the wrong button on the controller.

I bet anyone in here that has been building speakers for a while has done something like that once or twice! :ponder:

Thanks for an excellent product. :T

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