PVC or other cylinders for Isobaric couplings - diyAudio
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Old 10th December 2006, 02:56 AM   #1
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Default PVC or other cylinders for Isobaric couplings

Help! I'm trying to mount 3 driver-to-magnet isobaric pairs of 8" woofers into a small cabinet. I've done all the BassBox6Pro measurements and if the measurements are correct, it's gonna be fabulous, but Im stuck on the isobaric chambers themselves.

Ideal would be three 3.5" sections of 1/2" thick, 7.24" int dia PVC pipe. I need to be able to mount the drivers to the cylindrical chamber itself for the best seal and the smallest internal airspace.

The problem is that 8" PVC pip is industrial strength, and the only places I've found it will only sell 100' orders in 2 50' sections for hundreds of dollars. I'm only making one cabinet! And I'm doing it myself to save as much money as possible.

So does anyone have any ideas? I've been all over the internet but I think I'm searching for the wrong items or something, because I've had virtually no luck.

One alternative would be to build square internal cabinets to mount inside the main cab, but that's harder to seal, uses way more airspace inside the chamber, and takes up a LOT of internal cubic volume which forces me to increase the size of the cabinet -- basically, I'd have to start from scratch. And I can't do that because I have a finite area available in my car to haul this thing to gigs (I'm a bass player), so the square internal cabinet isn't really an option. And the driver-to-magnet thing isn't negotiable either, since this will often have to travel in a trailer with other box-shaped pieces of gear.

The other alternative is to build 8-sided chambers out of wood. What a huge amount of trouble, and tubes of sealant, and I'm not even sure how to best bind the 8 pieces together in the first place... I'll probably end up having to do it if I can't find the PVC pipe I'm hoping to find.

A guy at Crutchfield (I know, I know) told me he had a buddy who did a similar project, and he went to his local contruction companies looking for PVC scraps. I'll give that a shot Monday as well, but c'mon, hasn't anyone bothered to manufacture a line of cylinder pieces for isobaric chamber construction??

I guess I could buy some blown Bazooka tubes if I had to. Still couldn't mount the drivers to 'em though.

So does anyone have any ideas?

with crossed fingers and desperation,
Robert Wright
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Old 10th December 2006, 07:05 AM   #2
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Why not buy an 8" Sonotube, cut out some rings of MDF to mount the drivers to, glue and screw them to the sonotube and call it good?
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Old 10th December 2006, 08:02 AM   #3
Collo is offline Collo  Australia
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You can make a composite tube from two concentric layers of PVC pipe glued together with epoxy. By inserting strips of the correct width, you can make whatever size you need.

To get the approximately 8 inch diameter, you can start with 6 inch pipe. You can even use 4inch if you insert two strips per layer.

I've written up my experience on this page:
http://www.users.bigpond.com/bcolliso/vent-rings.htm

Part way down the page is a link to the downloadable calculator. It's main use was for designing concentric ports, but clicking on the label "Click here to use tubemaker calculator" will get you the screen you need.

Happy gluing...
Collo
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Old 10th December 2006, 06:06 PM   #4
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Thanks to each of you for the suggestions!

I checked out the Sonotube -- it's not thick enough, nor do I think it would be durable enough... This cabinet is gonna get moved around a lot, loaded in and out of trailers and venues and manhandled quite a bit, so what's inside has to be secure. (I haven't been able to find any MDF online anyway.)

I did find a few 12" sections of 8" PVC on eBay, at least. If I use the techniques Collo suggests, I can get the pipe down to the size I need by slicing out just a tiny bit, and then I'll thicken and reinforce the edges with 1" pieces from the same pipe, used as collars, with a single cut. The gold here is the advice of using adhesive that doesn't need air to cure. Brilliant! Collo, is there a specific type and/or brand name you would suggest?

More questions: Any advice on mounting the drivers to the tube? Namely, will the epoxy bond resist splitting if I'm screwing wood or sheetrock screws in fairly close to the seam? Another idea is to use these - http://www.partsexpress.com/pe/showd...umber=081-1094 - I can get them to mold into the plastic if I heat them up -- talk about permanent! But small margin of error, too. What exactly would be my best bet?

Thanks again, guys. This is taking my stress level on this thing way down. I wish I had the luxury of it only being a hobby -- it's gonna be a big part of my livelihood, so I gotta get it right the first time!
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Old 10th December 2006, 10:56 PM   #5
Collo is offline Collo  Australia
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Epoxy resin is known as Araldite here in Australia. I'm not sure what the brand name would be in Nashville....

Those barbed inserts look OK,
I think you would need to build up the collar with sufficient layers to be able to drill a hole to accept them, rather than heating them up and trying to force them into the PVC.

You should then be able to epoxy them into place. Fill them with grease to protect the thread from the epoxy.
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Old 11th December 2006, 12:44 AM   #6
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Quote:
Originally posted by Collo
Those barbed inserts look OK,
I think you would need to build up the collar with sufficient layers to be able to drill a hole to accept them, rather than heating them up and trying to force them into the PVC.

You should then be able to epoxy them into place. Fill them with grease to protect the thread from the epoxy.
Why, I think I'll do exactly that.

See? More great ideas I would never have come to on my own without years of mistakes under my belt. Thanks Collo, o wise sage!
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Old 11th December 2006, 01:40 AM   #7
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why exactly are you going isobaric on this project may I ask... I'm guessing some severe space constraint?

You should be able to find everything you need at your local home depot or lowes... I've bought some 8" couplings at lowes before and they were maybe $15 or so...

I'm guessing you are mounting these drivers back to back iso which is IMO the 2nd worse way only to piggyback iso. Clamshell is the best as it minimizes air volume between the subs, but your left with one driver's magnet exposed...(which could be avoided by simply making the mounting wall recessed into the enclosure the depth of the driver and putting some sort of grill over it...

What exactly are you building this for and how do you plan to configure your drivers...?
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Old 11th December 2006, 09:06 PM   #8
y8s is offline y8s  United States
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Default Re: PVC or other cylinders for Isobaric couplings

Quote:
Originally posted by esroberto
[B]Help! I'm trying to mount 3 driver-to-magnet isobaric pairs of 8" woofers into a small cabinet. I've done all the BassBox6Pro measurements and if the measurements are correct, it's gonna be fabulous, but Im stuck on the isobaric chambers themselves.

Ideal would be three 3.5" sections of 1/2" thick, 7.24" int dia PVC pipe. I need to be able to mount the drivers to the cylindrical chamber itself for the best seal and the smallest internal airspace.

The problem is that 8" PVC pip is industrial strength, and the only places I've found it will only sell 100' orders in 2 50' sections for hundreds of dollars. I'm only making one cabinet! And I'm doing it myself to save as much money as possible.

Robert,
I just ordered some SCH40 8" PVC pipe from www.usplastic.com and they sell in 10' lengths. I don't plan to use all 10 feet of it and would be happy to sell some scrap to you to save us both a little cash.

Alas it is only .322" wall and 7.981" ID but undoubtedly a better deal than you'll find elsewhere!

Drop me a line if you're interested.

Matt
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Old 12th December 2006, 02:29 AM   #9
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I am interested, indeed! Thanks for the post, Matt.

Lemme tell you guys about this project real quick. I wasn't gonna include too much boring info in my request for help, but I can see that it restricts the amount of advice you're able to give, so... before I make a big mistake on this, mebbe I'd better throw some details into the forum here before I get started (still waiting on all the parts to get here).

The point is to get the most SPL and power handling out of the smallest enclosure possible, with a specific frequency response tailored to the application, which is going to be bass guitar amplification. Space for transporting (and even set-up) is severely limted or I would go with completely different designs.

I'm building two cabinets. One is basically a sub, a 24"x14"x18" box with two 12" Kicker Solobaric L5's, mounted isobarically, both facing forward (cone to magnet). Being square woofers makes it easy to do a minimal V iso chamber, for obvious reasons. I'd intended to mount them side by side in a ported cabinet, but BassBox6Pro made it clear that the specific requency response I want would be much more pronounced if I mounted them isobarically, owing (I assume) to the drivers thinking they're in a larger cabinet than they really are. (I'll try to attach the BB6P files of both cabs to this post.)

The other cabinet is the beastie, however. It will have 8 drivers in it, 3 iso pairs of 8's, all drivers facing forward. I'd intended to mount them all to the baffle as well, in the shape of two stacked chevrons, but BB6P again told me I'd sound more like what I needed with iso pairs. Realizing I could save space, I reduced the cabinet dimensions to minimal and I could still get what I was looking for, so that cab will be 24"x17"x18", 3 inches taller than the sub cab.

Here's what's really cool. I tried different box types and configurations at great length, having just gotten BB6P and trying to learn it as well as learning all the effects of port lengths and sizes, cabinet dimensions, driver configs, etc. I discovered (through a LOT of trial and error) that the top cabinet can serve two purposes, full-range or high pass. I'm going to make a port (vent? a hole.) in the back of the cab that will give me essentially flat response down to 40Hz with a slight bump around 85Hz - perfect for smaller clubs. I'm going to make that vent air-tight resealable for use with the sub cab in larger venues, as making the cabinet a sealed box changes the response to flat down to 80Hz with an 18dB/oct rolloff below that. That pretty much mirrors the sub cab's response, the sum of which (visually) gives me what I hope I'm correctly interpreting as essentially flat response down to 20Hz with a substantial bump around 45Hz. 5-string bass has a lowest fundamental tone of 31.5 Hz (B natural) with the E-string being tuned to 43.25Hz, so the boost turns out to be right where a bass player would want a little extra oomph. (Does anyone know how to get BB6P to give the summed frequency response of two cabnet designs working together?)

Matt, I won a 12" section of gray 8" PVC on eBay for $17, but I'm gonna need a little more to put 1" collars on the iso chamber sections. Your sections would give me a combined wall thickness of .86", well over the .75" I'd hoped for. I'm planning on using three 3.25" sections which means I need another 4 inches' worth, but for consistency's sake (to soothe my OCD, heh), I'd like to use identical collars in all 6 instances, meaning I'd need 6 inches' worth from you. What would you take for that amount, in any sized scrap so long as I could get 6 solid 1" sections out of it? (Email me off-list at esroberto-at-hotmail-dot-com and we'll work it out!)

My last question now is how to best brace the inside drivers with minimal materials so I don't eat up internal volume. Any ideas??

Thanks for your time and your help, guys. Greatly appreciated.
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Old 12th December 2006, 02:31 AM   #10
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The plot for the 3 designs: red = the sub cabinet, white = the top cabinet, sealed, and yellow = the top cabinet, ported.
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