Please find the flaws in my sealed shiva sonosub plan before I commit! (1 Viewer)

[Jeff Meininger]

Here's my DIY master-plan. PLEASE find the stupid parts and tell me how I should change them for the better!!
My design is for a sealed Shiva in 16-inch diameter sonotube. I'll be using the crossover at 120 Hz. I prefer a tight/clean bass, but I still like my explosions, so I'm going to shoot for a Q between 0.65 and 0.7. The experts tell me that a tempest is a better match for the size of my room, but not with the high crossover setting. The crossover setting is non-negotiable.
I don't run Windows, so I can't run LspCAD, WinISD, or "SonoSub Kit". I've come up with my numbers using web-based javascript calculators, and I'd very much like someone to double-check them. Especially since what I'm doing is not that different from a "Rava", yet their Q is lower and their enclosure is SMALLER. Seems odd to me.
For a Qtc of 0.665, I'd need 79 liters of internal volume. This means that my tube will be (roughly) 24 inches tall. Would I get a benefit with stuffing the sub with polyfill? How much should I stuff, and how much should I reduce the physical size of the enclosure to accound for the stuffing? If I want to allow myself some adjustability, would it be wise to make the enclosure 60 liters and just use lots of stuffing so that I can later remove stuffing if I change my mind and want to raise the Q? Or is a larger volume a better way to get lower Q than with stuffing?
I plan to use the bass-boosted parts express 250 W plate amp, because that's what has been suggested to me here. The boost at 30Hz will help counteract the sealed box's tendancy to start rolling off above this point? (I have no graph-generating software programs to confirm this for myself.)
I'm supposed to wire the voice coils in parallel, right?
I'm not planning on flush-mounting the driver, because I've read that it isn't necessary except for tweeters. Most people DO flush-mount their driver, though. It's just for show, right?
I plan on using the popular "two layers of .75-inch MDF" endcaps, with one layer fitting inside the tube and one extending out a bit. I plan on making an SVS-looking base-plate with another .75-inch MDF disc, and standing it off from the driver endcap with 4-inch posts. I'll go taller if anyone things it will improve sound quality / output. I plan on putting small speaker spikes on the bottom of the base plate so that the sub has good footing on my padded carpet.
I plan on mounting the driver to the driver endcap with T-nuts, digging into small 1-inch pieces of 1/8" or 1/16" lite-ply glued to the MDF.
I plan on mounting the endcaps to the sonotube with wood glue, and sealing the inside with 100% silicone caulk. I do NOT plan on using the "all-threaded rod" or any other internal bracing between the endcaps because that looks like a lot of extra work. I plan on using a few wood screws through the side of the sonotube and into the MDF instead (first having drilled pilot holes). Is this decision a mistake? I figure the internal pressure in my small, sealed design is probably even HIGHER than the pressure in vented designs, so is my laziness here going to cost me?
I plan on painting the tube black (what kind of paint should I use?) with a brush. No primer. Then I'll cover it with a T-shirt material sock. I'm not sure how I'll make it look pretty where the sock butts up to the MDF.
I'm planning on lightly sanding the exposed MDF discs, and spraypainting them with flat black spraypaint after a coat or two of Kilz primer.

 

[charles_w]

Jeff,

The one thing I would do different is replace the T-nuts with threaded inserts. I've been buying them at the Sears Hardware Store.

Lee

 

[Mark Fitzsimmons]

It is unnecessary to put plywood between the T nuts and the MDF. My T nuts sunk their teeth into my MDF just fine.

Charles, I am unsure what you mean by threaded inserts, but the T nuts I used were threaded.

 

[Mark Hayenga]

HEY you Austinite. Check the Local Home Theater forum meets page. We have a very active DIY membership. I myself have built four Shiva subs and two Tempests, and a bunch of the other guys have similar subs. Swing by one of our meets sometime to shoot the breeze, lots of experienced locals to help walk you through stuff.

 

[Stephen Hopkins]

I'm a lil concerned that just using wood glue for the caps may not be ver stable. If you've not going to use threaded rod, you might atleast want to use some 2" L brackets along w/ the wood glue and inner caulk. I'm hoping to do a sonosub sometime soon, and that was my plan.

 

[PeterV]

Jeff,
I used panel connectors from Rockler.com instead of
T-nuts or threaded inserts for mounting the driver.
They are just the right length for 2 layers of 3/4"
MDF. They are splined on one end so as not to slip
when tightening. The cost is $3.49 for an 8 pack.
The part# is 18350. On the driver side I used external
tooth lockwashers which will keep the connectors from vibrating loose. It has worked great for me since
December of 2001. Hope this helps you!
Best regards
PeterV

 

[Brian Bunge]

Mark F,

The T-nuts can break loose from the MDF much easier than plywood. That's why many people suggest using a layer of ply. If you're not removing the driver often it may not be an issue though.

Brian

 

[Jeff Meininger]

Thanks for the replies! I'll look into the threaded inserts and panel connectors you guys mentioned.

I'll DEFINITELY come to an Austin meet when I can. I checked out the "local meets" board, and it looks like the Austin community is busy indeed!

About the L brackets... that sounds like an interesting idea. I'm assuming that I should make these out of wood (MDF? pine? other?) so that the surface touching the sonotube can be sanded to a curve for a good glue joint. I could also use them as a means to go deeper than the MDF for the leg mounts.

Should I sand the waxy surface of the sonotube to make sure the glue sticks well? I've not seen this mentioned on any of the sites I've found.

Once again, thanks for all of the replies!

 

[Jeff Meininger]

Anyone have anything to say about my enclosure size and stuffing amount? I'm still confused about how the Adire RAVA has less internal volume and a lower Q. Did they just stuff the hell out of it or something?

 

[Mark gas]

I think the L brackets he is talking about are the small metal one's you can get at home depot.

 

[Mark_E_Smith]

I ran your number through Bass Box Pro and you seem to be right on. as for the stuffing light stuffing seems to end up with a Q about .7 heavy, 50% fill would drop it to .65 of so. I think people start will a lower fill and add it to get the sound they like.

 

[Clay Autery]

I'm a big believer in E-Z LOK "knife-edged" threaded inserts for use in MDF... For instance, here is how I am using them right now:

On my flexy, I am attaching ball-bearing transfers to wood "feet" (actually center drilled standoffs to "get over" the all-thread/nut/washer combo on the bottom)as a sort of compromise between spikes and casters... The transfers attach with #6 machine screws.... or #6 wood screws. Since I will likely want to remove the transfers occasionally to re-configure the flexy, I choose to use machine screws... maybe I'll never remove them, but I don't want to "paint myself into a corner".

So.... I'm using E-Z LOK Part# 400-006 (#6 x 3/8" brass threaded inserts whose OD are coarse, "knife-edged" threads designed to cut there way into a pilot hole. The pilot hole for this particular part is 1/4"...

So, place the transfer centered on the "foot block" with the holes oriented toward the corners to offer the most "meat" to grab into. Use a centering bit to mark the center of each of the 4 holes. Then drill a 1/4" vertical (I use a drill press) pilot hole just shy of 3/4" deep (double thickness here, so it doesn't even break the first layer). Then use a slotted screw driver (as tight fitting as possible) to drive the insert into the pilot hole. (BTW, I personally like to coat the insert's outer threads with wood glue as an extra precaution... just make sure to keep the inner threads clear, or you will need a #6-32 (or appropriate to your app) BOTTOMING tap to clear the threads of dried glue. I drive mine to "just" flush as the threads don't begin inside the insert for about 1/16" or so.

Repeat as many times as necessary...

That's it... Works great, lasts a long time... Use Blue Threadlocker on the machine screws.

 

[Jeff Meininger]

I put in an email to Adire, and tech support kindly got back to me and suggested that I go up to 85 liters with 3 lbs of stuffing. They also suggested that I do _not_ use an amp with too much bass boost. They said their own AVA250 or the no-bass-boost parts express 250 would be a better match than the ~5 dB at 30 Hz boosted parts express amp.

I'm thinking of waiting for a sale on the AVA250, since I prefer the idea of 1 dB @ 25 Hz bass boost to no boost at all. I will not have a lot of room gain in my HT/living room. Too many large openings to other rooms, and too high of a ceiling.

It looks like there are quite a few options for driver mounting! Thanks for the suggestions, everyone.

 

[Alexis]

I made a similar design sealed sub, with dimensions 12x48" using an original SVS. I stuffed it with about 2.5lbs of fill, since 3 seemed too restrictive for it.

I put wood screws through the sonotube into the particle board with good success. I used liquid nails instead of silicone to seal it, but either would work well. Most people don't get ringing of the sonotube, but mine does ring at certain frequencies, probably due to it being sealed and its length. wrapping the sonotube with an asphalt type material should deaden any ringing if you have any. My driver is upward firing, like HSU.

Biggest thing with a sealed sub is to make sure it is air tight

 

[Jack Gilvey]

I'm thinking of waiting for a sale on the AVA250, since I prefer the idea of 1 dB @ 25 Hz bass boost to no boost at all.

I've never seen a sale on them, although I'd venture that the minute difference in their boosts would make no detectable difference at all in-room. And don't worry too much about the exact Vb/Qtc for sealed subs. Again, it'll make no difference in-room. Get close, be happy.