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Old 27th March 2003, 07:45 AM   #11
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Hello,

first of all, I apologize because I'm a newbe in english forums, so sometimes my technical terms may not be correct (maybe literally translated from the german expressions).

What has been said is in some way true but you always need to have a closer look at the actual application. Is it a Subwoofer, a huge 3 oder 4 way speaker? Maybe 2 ways in a tall narrow cab.

In a subwoofer standing waves (is that correct?) a no problem, because the dimensions of the cab are small compared to the wavelength, so you don't need to worry about them. Just build the cabinet as stiff as possible. Even in a ported box, you don't need stuffing. No mids will escape the port, when they simpley are not reproduced by the driver.

When you have to stuff the cab, IMHO there is no need to fill the hole box up. This overstuffing sounds quite whipy and boring to me. Just put it in the place, where the air velocity haas its max: right in the middle. Trying to damp the max of the pressure is much more ineffective. Build to horizontal dividers and drill lots of big circles (dia>50mm) or do this with a jigsaw and place them up und below the middle of the box. Put lots of polyfill in between.
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Old 27th March 2003, 12:10 PM   #12
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Here are some links for bitumen roofing products that might be cheaper than buying smaller quantities from speaker suppliers.

Johns Manville used to make a product called Horsey-Set that was a co-polymer rubber coating, that was easy to use. I later heard that Karnak had the recipe, and was marketing it under the name Karnak TDI. I would try talking to local wholesale roofing suppliers to find these products, and builder/roofers for extras and scraps. Self-adhesive rolled roofing doesn't seem to have a long shelf life, when it comes to initial adhesion.

Karnak coatings

http://www.certainteed.com/croof/crct02401p.html


http://www.iko.com/products/commerci...al_region_id=6
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Old 27th March 2003, 12:23 PM   #13
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Those bitumized roofing mats are cheap, but won't they keep outgassing in your living room?
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Old 27th March 2003, 03:33 PM   #14
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Won't all of these absorbers change the inside volume of the enclosure? Isn't that a bad thing?

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Old 27th March 2003, 04:10 PM   #15
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Quote:
My understanding is that in ported cabs you want to leave the basic air mass alone but prevent reflections off the walls.
This is exactly correct. The normal T-S calculation make assumptions about box-loss Q. In most tables, it's set to 7. When you stuff a ported cabinet, you change that Q dramatically (downward, and often non-linearly) and your alignment won't work.

Oversizing cabinets a bit will more than make up for the loss of volume from wall dampening or stiffening treatments. One should oversize anyway, since it rarely happens that everything is in spec; when it comes time to do the tuning, box volume may have to be adjusted and it's a lot easier to make it smaller (can you say "brick?") than to make it larger.

You ARE planning to do measurements to verify tuning, aren't you? If not, well, you've got a 1 in 10 chance at best of having the system perform the way you think it will. If I were Dictator of the World, I wouldn't allow anyone to do a ported cabinet for a first DIY project.
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Old 27th March 2003, 05:44 PM   #16
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I'm building the bass reflex cabinet at the bottom right of the PDF in the link below. I'm assuming that the engineers have made the correct calculations. I'm building them out of 1" birch plywood. Before everybody jumps on me with the normal "that speaker won't work in that enclosure!" let me just say that I have heard the speakers I'm building. I went to Dave Dicks house a listened to them being powered by my 5 watt SE EL84 amp and they souded great. The cabinets that he sells are built out of MDF. I chose Birch because I don't want to deal with veneering. Once they are built I will use a router to "bullnose all of the edges with a 3/4" radius curve. They should turn out nice. Besides. The enclosure is desogned by Fostex. Though Dave's enclosures don't use exactly the same dimensions as Fostex's do.

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http://www.solen.ca/pics/fostex/fe206e_enc.pdf
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Old 27th March 2003, 06:03 PM   #17
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Calculations are fine. Translating that into a successful box assumes that the drivers are on spec (not common) and that your box losses are such that QL = 7 (or whatever else the person designing the box assumed).
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Old 27th March 2003, 06:06 PM   #18
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Hi Sy,

Would I tune the cabinets by changing the port length or by changing the internal volume of the cabinets?

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Old 27th March 2003, 06:43 PM   #19
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Depends on what you're trying to tune (alpha or h).
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Old 27th March 2003, 06:57 PM   #20
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I'm not sure. In the design it calls for a port tube that is about 3 1/4" in diameter by 3" long. I can't find a 3 1/4" ID port tube anywhere. So if I use a 3" port tube would I make it slightly longer to make up for the difference in ID?

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