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Old 27th June 2002, 05:28 AM   #1
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Default rectangular port questions

I'm planning to build atleast one woofer box with rectangular ports. And I was wondering if someone here could help me? What's the difference between rectangular ports vs. round or PVC ports? In a ported box, does the volume in the port count all the way to the front baffle count as "Vbox." Or is it just flush to the back inside of the baffle. You know what I mean? There's going to be a 1.5" baffle so it will make a difference. Or does the port volume not count at all.

The other thing I wanted help with is: infinity, for the subs I'm going to use, states that you need a .57 cu. ft. enclosure and a 2" diameter by 8.1" diameter port for the infinty beta I'm going to use. My question is: do you think they accounted for the volume of the plastic used to make the port in their calculations of box volume? So I would have to subtract the volume of the plastic for the port from .57 cu. ft. to adapt it to my rectanguler ports? Of course then I would have to add the wood use to make the rectangular ports. The beta PDF is at http://manuals.harman.com/INF/CAR/Bo...ers/BETA8X.pdf Is this thing with the displacement of the port material even significant?

Also I'm trying to design a vented box for another woofer. But In the specs there's no Xmax spec and therefore no Vd spec? What can I do to create a vented box? I aproxiamte Xmax to be 9 mm and with that I aproximate the Vd, but I get very long port lengths, with Speaker LAB 2.0. I don't think I've tried it with WinISD yet.
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Old 27th June 2002, 06:52 AM   #2
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As another member said here once, for box volume calculations, treat the port like a solid piece of wood.

The length of the port goes all the way to the front face of the box.

Do your best to calculate how much space is taken up by the port,and how much by internal bracing, and deduct from box volume. This is an approximate process.

Finally, relax. Remember-if you take 2 box volumes-one 10% smaller than the other-and run them through a program, you will find very little difference between curves. So you have plenty of leeway.
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Old 28th June 2002, 01:00 AM   #3
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I think I got it all, except if you treat the port like a solid block of wood with an enclosure that has a rectangular port do you do it for the actual length or the effictive length. I learned about effective length at this webpage http://www.jlaudio.com/tutorials/ports/index.html I think you subtract the effective length of the port and then the actual length of wood used for the top of the port, right?
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Old 28th June 2002, 01:10 AM   #4
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Default Round, rectangular and flared

Another good reason to subscribe to AudioXpress -- the current issue had an article which concluded that the flared ports were worth the effort. (But ya know what, one of the kids knocked the PVC port out of a box of reflex speakers I built with KEF B139's about two decades ago, they still sound pretty good.)
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Old 28th June 2002, 04:58 AM   #5
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For the box volume measurement, use the actual length the port is physically going to be. There isn't that much difference in your link between the two anyway. Remember, you have leeway.
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Old 30th June 2002, 09:42 AM   #6
Variac is offline Variac  United States
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Default Flared port length

OK maybe this is too picky, but when you measure the length of a flared port tube do you use the actual length, or do you make it a little longer to account for the larger diameter as it flares?
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Old 30th June 2002, 12:01 PM   #7
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Hi Variac

The volume of the port and the materials it is made from, as well as any bracing or crossover components. all need to be added to the volume of the box as shown in your box size calculations( as Kelticwizard said though, 10% either way ain't going to make much difference)

Al
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Old 30th June 2002, 02:07 PM   #8
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A flared port needs to be 1" longer than a non-flared port to achieve the same tuning.

Pete
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Old 30th June 2002, 09:37 PM   #9
Variac is offline Variac  United States
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Thanks guys, I had been wondering about this for awhile and found an opportunity to float it.

Pete answered the question I thought I was asking, but Al comfirmed my approach to bracing, etc. So I am now confident I'll get this together.
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