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Old 1st November 2012, 06:36 PM   #1
dstmbgh is offline dstmbgh  United States
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Default Slot port k (end correction) factor question. Clarification needed.

The speakers I'm building have a triangular port in the lower corner, i.e., the port shares two enclosure walls. However, they are monitors not subwoofers, and as such will be elevated on stands rather than be placed on the floor. My question is what k (end correction) value to use in calculating port length.

I came across this, but can't locate its origin to explore the conditions: http://www.ctc-dr-weber.de/speaker/PortCorrection.jpg

A regular poster here, bjorno, has previously commented that the k value of 2.227 (lower right) includes the influence of the floor plane outside the enclosure. If so, then I would assume this is also the case for all three lower examples.

My triangular port correlates with the lower center example in that two cabinet wall are shared. However, if the illustration assumes the speaker is sitting on the floor then there is a continuing plane at three port exit points, two inside the enclosure and one outside (the room floor).

I understand that the total k value is arrived at by adding up the conditions at each port exit, and have tried to extrapolate from the three lower illustrated k values for a sum for "port wall plane continues beyond port exit." No success. The consistent sum appears to be ~ 0.498 per shared wall.

Can anyone shed the light of knowledge into my conundrum? If in fact k = 1.728 with two cabinet walls shared plus the influence of the room floor, then what is the correct k when placed on a stand?

Thanks for all help,

David

Last edited by dstmbgh; 4th November 2012 at 02:30 PM.
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Old 1st November 2012, 06:53 PM   #2
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David,

For what it is worth, using triangular corner ports and slot ports I have found the difference between being on the floor and on a stand to only change fB by around 1Hz, less than I would have thought.

Since port calculators seldom agree with each other or actual measured response, I find the best solution is make the port a bit longer than predicted, measure and cut down if needed.

You can easily determine Fb by visually measuring excursion using sine wave tones, the excursion minima is at Fb.

If the Fb is lower than predicted, reduce port length.

Art
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Old 1st November 2012, 06:54 PM   #3
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You're unlikely to nail it on the first try anyhow. If you can fit the port that you calculate for 1.23, start with that, temporarily attached, and then cut it down until you hit your target.
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Old 1st November 2012, 11:57 PM   #4
dstmbgh is offline dstmbgh  United States
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Quote:
Originally Posted by weltersys View Post
...using triangular corner ports and slot ports, I have found the difference between being on the floor and on a stand to only change fB by around 1Hz.
That's reassuring to know. I also thought it would make more of a difference.

Since I lack test equipment I'm trying to optimize my math, so to speak.

Quote:
Originally Posted by dumptruck View Post
You're unlikely to nail it on the first try anyhow. If you can fit the port that you calculate for 1.23, start with that, temporarily attached, and then cut it down until you hit your target.
So, dumptuck, you're suggesting to start with the k value indicated for one shared wall (1.23) and then shorten as needed to hit Fb. Regarding fitting the port length, a nice thing about the type of port in question (customized triangular slot port) is that I can vary the area of the port until the corresponding length fits most comfortably in the enclosure's depth (as large as possible provided there's sufficient breathing room at the back).
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Old 2nd November 2012, 12:04 AM   #5
dstmbgh is offline dstmbgh  United States
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Does anyone know the origin of the illustrated k values for slot (or shared-wall) ports?
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Old 2nd November 2012, 12:11 AM   #6
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Quote:
Originally Posted by dstmbgh View Post
That's reassuring to know. I also thought it would make more of a difference.

Since I lack test equipment I'm trying to optimize my math, so to speak.
Getting close is about all you can expect of "math".

The method of finding Fb I mentioned in #2 requires no test equipment other than your eyes and a ruler.
You can find sine wave files on line if you don't have a generator.
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Old 2nd November 2012, 12:18 AM   #7
dstmbgh is offline dstmbgh  United States
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Quote:
Originally Posted by weltersys View Post
You can easily determine Fb by visually measuring excursion using sine wave tones.
Thanks for this suggestion. As stated, I don't have test equipment. Is there freeware that includes a sine wave generator? If so, in addition to the visual check, I believe I could measure Fb with the mere purchase of an inexpensive DMM, could I not?
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Old 2nd November 2012, 12:55 AM   #8
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This works just fine: NaTCH Engineering - SigJenny an Audio Signal Generator for Free

P.S. You're right, of course, that the closer you get the math/simulation, the better decisions you can make from the start. It is what it is .

Last edited by dumptruck; 2nd November 2012 at 12:58 AM.
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Old 2nd November 2012, 01:50 AM   #9
dstmbgh is offline dstmbgh  United States
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Quote:
Originally Posted by dumptruck View Post
Thanks so much. I'll check that out.
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Old 2nd November 2012, 04:21 PM   #10
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Quote:
Originally Posted by dstmbgh View Post
Thanks for this suggestion. As stated, I don't have test equipment. Is there freeware that includes a sine wave generator? If so, in addition to the visual check, I believe I could measure Fb with the mere purchase of an inexpensive DMM, could I not?
A digital multi meter will not directly measure Fb, though it is possible using dummy resistors to measure the speaker's impedance at various frequencies and infer Fb at the impedance minima.
Because meters are generally most accurate at 60 Hz, voltage has to be measured at each frequency to insure accuracy of the results, very time consuming, and lots of math.

I find it far easier to sweep a tone and look (or even feel) for the least cone movement, which is Fb.
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