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Discussion Starter #21 (Edited)
You mean wrap the port with whatever I line the box with? Seems like people are using fibreglass mostly, or in some cases foam sheets. Would something visco-elastic like Dynamat work well?
 

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Discussion Starter #23 (Edited)
Right-o. I've already considered that. Looks like they are closer to 8.375 inches I.D., although it is a little tricky to measure because the tubes are not perfectly round. I may take these Sakrete tubes back and hunt for some genuine Sonotube.

Part of my plan to determine net volume is to weigh each piece of internal bracing and calculate the volume based on its known (measured & verified) density. Unfortunately, I think the bracing is going to eat up a lot of volume, which would require a longer port length at the same tune. Only so much I can do about it right now as I'm already committed to the external dimensions of 26H x 30D x 49W. Net volume may be as low as 17 ft^3 and port length may wind up being around 35" for a 14.5 Hz tune.

Off to assemble the table saw...

Bradley
 

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Discussion Starter #24 (Edited)
Been mucking around with Google Sketchup. Confusing at first, but slowly getting the hang of it. A concern I have now is how close the 8" port tube is to the rear wall and also to the driver. There is just no way to keep it one port diameter away from those other surfaces. Is this anything I should even concern myself with? At the end of the port inside the cabinet there is more than one port diameter "in front" of the port.



 

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Discussion Starter #25
Also, I had a scare last night about the net/working volume. I thought I had horribly underestimated the volume that the bracing will take up. I knew it would be more than what Collo's Boxnotes program estimates but wasn't sure how much more. Finally, I decided to write a VB script to calculate the net box volume, with reasonable approximations for the number and style of braces being. For the sake of this estimation, the style of brace is simply a rectangular panel that matches the internal dimensions of the box with a LARGE circle taken out of it.

If anyone is interested in having a look at the script, you can see it here.
 

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You mean wrap the port with whatever I line the box with? Seems like people are using fibreglass mostly, or in some cases foam sheets. Would something visco-elastic like Dynamat work well?
I'm not to sure about the dynamat, that is mostly for deadning troublesome vibrating panels, you need something like fiberglass, I used some 1" thick "batting" I picked up at an upholstery shop, better than fiberglass and no itching.
 

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Discussion Starter #27
Yeah, I didn't think so. I was just wondering since I have a bunch of dynamat-like material somewhere in my garage, new in the box.
 

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Discussion Starter #28
Totally out of control over here! Not done and not really happy with the braces. But here's what I've got "sketched up" so far:



Again, this is just a rough draft and work in progress. However, if I'm lucky the port will terminate in that third brace. Both ends are going to get the mondo 1.5" radius flare.
 

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Discussion Starter #29
Here's the latest draft.



I think the only thing left to do is add some donut support rings for the port tube. I'm not going to bother trying to model the flares or rounded edges.

The smaller holes, or "swiss cheese", aren't really necessary. I'm just a little worried about coming in at 17.0 cubic feet (or higher) as planned. I'll need to add up the volume of all the bracing and port tube once the "ink" has dried on the design.
 

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Discussion Starter #30
Came up shy on the net volume inside the box, so I've had to make revisions. The side-to-side braces in the middle (just behind the driver) were retarded anyway, so I got rid of them.



By the way, I ditched the VB script in favor of an Excel spreadsheet to calcuate the volume using surface area measures taken from the Google Sketchup drawing. All the files I'm using are on my web server if anyone wants to check them out, including the Sketchup drawing.

http://s16v.com/subwoofer

Bradley
 

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I'd have to believe window braces are the most efficient use of bracing if trying to minimize displaced volume. Braces with holes cut in them aren't efficient uses of space and add a lot of unnecessary work.

Not having at least 8" of clearance from the port opening is definitely a problem you should concern yourself with, as it can result in early onset of compression. How much clearance will you have as it is designed now?
 

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Discussion Starter #34
I'd have to believe window braces are the most efficient use of bracing if trying to minimize displaced volume. Braces with holes cut in them aren't efficient uses of space and add a lot of unnecessary work.
Do you have a link or pic of what you would consider ideal/optimized window bracing? If I understand you correctly, I could draw up another model and compare the two. Either way, the large (and thin) port tube needs to be supported by the braces and that requires a significant amount of material.

Not having at least 8" of clearance from the port opening is definitely a problem you should concern yourself with, as it can result in early onset of compression. How much clearance will you have as it is designed now?
The outer edge of the port tune is 5 inches from the interior rear wall. It's about the same distance from the back of the driver. There is 11.5 inches of space from the end of the port tube to the side wall that it "points" at.





On the whole, there is a lot of space around the end of the port tube. However, if you think I should move it closer to the driver and/or center it in the box (front-to-back), I will consider that.

Thanks.

Bradley
 

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Discussion Starter #35
Steve,

I thought about for a while and finally came to the conclusion... CIRCLES RULE! Here's my latest design. :rofl:

 

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Discussion Starter #36 (Edited)
The brace pictured below is within 1% of the volume of the "rib" style brace used in the current box design. The "border" of the brace is 2 inches wide. Also note the port tube has been moved 2 inches toward the front of the cabinet, closer to center.



If I add 2" wide horizontal supports from the circle to the edges, it would consume about 10% more space than the large-circle "rib" style brace currently employed. That would probably make for a stronger brace but I'm not sure it is worth the space penalty. Perhaps reducing the border or "stick" width to 1.75" would hit the right balance.
 

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I have to agree that when tuning so low, a single 8" port/driver is more than sufficient on real program material. dual 6" ports will only add complexity in the design/construction while there will be no noticeable benefit. I have hit 125 db from a single driver at WOW and did not notice any bad thing from the port...

I would only advise to use the most power you can. I am powering my drivers with dual 2000 W RMS (4.5% THD) amps, and I really feel that these drivers are rated a very conservative 1000 W.

Congratulations for this project in advance...
 

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Discussion Starter #38
Thanks, Blaser.

Having spent many, many hours already on the design with a single 8" port, I really don't think dual six inch ports would be much more difficult. The two smaller ports would actually integrate better into the rear half of the enclosure space, remaining almost one port diameter away from any of the walls and the driver.

My only concern with using more power would be cone excursion, possibly bottoming out the driver, especially with the 14.5 Hz tune. I'm going to start out with 500-600 watts mostly because that's what the amplifiers I already have sitting here will do.

Hoping to finish up the design tonight and start cutting MDF tomorrow! :)
 

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My only concern with using more power would be cone excursion, possibly bottoming out the driver, especially with the 14.5 Hz tune. I'm going to start out with 500-600 watts mostly because that's what the amplifiers I already have sitting here will do.
Be carefull if you're using less power than the drivers are rated to handle. Running a clipped signal into the drivers will burn them up very fast.
 
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