Can anyone hook me up with some "Special K" (correction factor)? - diyAudio
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Old 5th October 2012, 11:36 PM   #1
dstmbgh is offline dstmbgh  United States
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Join Date: Sep 2011
Default Can anyone hook me up with some "Special K" (correction factor)?

I’m in the process of modifying a tested, recently introduced design, Paul Carmody’s Hitmakers, changing them from a round rear port to a front port that utilizes the cabinet’s floor. While I've built bass reflex speakers before, with round PVC ports and also passive radiators, none the less I'm a slot-port-newbie. I have to rely on math/calculations alone, as I don't have my own test equipment enabling me to do it the best way -- prototype, test and fine tune. (If someone in the Indianapolis area would like to help me with testing, by all means, please contact me.) Therefore, I'm looking for the most precise mathematical variables – a “special K” -- that will enable me to hit my target tuning, not with a leap of faith, but with a leap of math. Faith in math?

I came across this: http://www.ctc-dr-weber.de/speaker/PortCorrection.jpg -- instructions (in German) to use a significantly different k (correction factor) depending on how many of the enclosure's walls are being used by the slot port. Many sites make no mention of this, but instead give seemingly across the board directions to make a length correction based on one-half of the slot's height (or smallest dimension) irrespective of how many cabinet walls are used.

Bjorno, a poster here, has previously asserted that the k values above, at least for the shelf slot port, lower right-hand diagram, is for a speaker sitting on the floor and thus includes adjustment for such.

The "special k" that I'm presently trying to determine is for:
- a triangular port
- using two cabinet walls (inside the enclosure, two port walls continue and one port wall ends), i.e., the port is located in one of the lower corners
- with an intended placement of the enclosure sitting on a stand, not sitting on the floor
- with only modest flaring (3/8" roundover) of the port's outside exit. This is probably only of cosmetic value, I'm guessing. Internally, the diagonal "shelf" used to create the port can also receive this same roundover if useful to lessen turbulence (obviously, no cosmetic value here...well, for Superman, I guess).

Do I use the k value of the lower center example from the illustration above (1.728), or does it include effects from the outside floor. And further, once I obtain the correct k value for my application and calculate vent length, do I then make a further length adjustment based on one-half the vent's height (and, if so, what would that be with a triangle port)?

All help is welcome.

David
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Old 6th October 2012, 01:24 AM   #2
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Join Date: Mar 2007
Location: California
Ah, this is where a good measurement tool is useful! Er, is that "a useful measurement tool is good"...

If you are dead set against measurement tools, perhaps because of some fatal allergy to them, you can design a port as if it is facing free space if the distance between the port and any nearby surface (e.g. the floor in your case) is more than 4 times the effective port diameter. For a slot port, calculate the cross sectional area (open HxW) of the slot port and then calculate the effective diameter of an equivalent circular port by Area = pi*diameter.

OTOH, this might be a good time to pick up a woofer tester - $99 at Parts-Express.com. If you can measure the impedance of the speaker you can determine the box tuning!

-Charlie
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Old 6th October 2012, 02:54 AM   #3
dstmbgh is offline dstmbgh  United States
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Join Date: Sep 2011
Quote:
Originally Posted by CharlieLaub View Post
If you are dead set against measurement tools...
Not against measurement tools at all, just don't have any. Perhaps someone with some basic tools will come forward to help me run an impedance sweep. That'd be nice so I could know for sure if I hit my tuning target (already determined).

Yeah, I know all the basics regarding converting from a round port, equivalent area, etc., but thanks for your help. Having come across some information regarding varying k values for specific conditions, I'm most interested to learn what k value best fits my circumstances so that I might more easily hit my tuning target with a correct port length.

David
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