Slot Port - different ratio's with same area ,,different length's ?? - AVS Forum
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post #1 of 11 Old 03-30-2013, 09:22 PM - Thread Starter
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Im trying to replace a 2" round Port x 8" long by a slot port , the area of a 2"port is 3.14 .

Now i can make a 3.14" x 1" slot ...or a 6.28" x .5" slot port which are both = to a 2" round port(3.14)

....Why when I enter these diff ratio's in a slot port Calculator do i get diff total port lenghts to achieve desired tuning freq ??

Some ratio's like (1.77"x1.77") give me a length of 6.35"length... and (6.28"x .5") give me a 4.11" length

I would have thought they would all be identical length's ?????
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post #2 of 11 Old 03-30-2013, 10:15 PM
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Slot ports with a high aspect ratio tend to slow down the air due to the increase in friction from having more surface area inside the port which the same volume of air must pass over. I can't say for certain how close those calculations are, though it would sure be interesting to see the formula used to determine them.
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post #3 of 11 Old 03-30-2013, 10:54 PM
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"I would have thought they would all be identical length's ?????"

they should be. something is wrong or incomplete if the calculator is giving you materially different lengths.

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post #4 of 11 Old 03-31-2013, 07:36 AM - Thread Starter
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Quote:
Originally Posted by mtg90 View Post

Slot ports with a high aspect ratio tend to slow down the air due to the increase in friction from having more surface area inside the port which the same volume of air must pass over. I can't say for certain how close those calculations are, though it would sure be interesting to see the formula used to determine them.


OK !! now i understand why the thinner slots create shorter Port lengths ,..because of the drag !!! ... I was only looking at the AREA and did'nt realize how much more Perimiter, square or even worse rectangular ports have , even if the Area was the same !!,



.....Now do you know if these shorter rectangular ports BEHAVE the same as the = round port ?
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post #5 of 11 Old 03-31-2013, 07:46 AM
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i have not seen air drag as part of any calculator. that would require some fairly sophisticated fluid dynamics math and would even start to depend on variables such as the 'roughness' of the surface. which calculator are you using?

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post #6 of 11 Old 03-31-2013, 12:26 PM
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Quote:
Originally Posted by LTD02 View Post

i have not seen air drag as part of any calculator. that would require some fairly sophisticated fluid dynamics math and would even start to depend on variables such as the 'roughness' of the surface. which calculator are you using?

Sounds like my kind of calculator!

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post #7 of 11 Old 03-31-2013, 02:14 PM - Thread Starter
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Quote:
Originally Posted by LTD02 View Post

i have not seen air drag as part of any calculator. that would require some fairly sophisticated fluid dynamics math and would even start to depend on variables such as the 'roughness' of the surface. which calculator are you using?

Have u tried a slot port calculator ? the more narrow the slot ,the shorter the length of port.?

Have u ever calculated the perimiter of a very narrow slot compared to it's = area round port ?
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post #8 of 11 Old 04-02-2013, 11:01 AM
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the two areas you proposed would have very similar " drag" effects, but the drag doesn't affect the frequency tuning anyway. sounds like your calculator is wrong, or something else is changing without you knowing.

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post #9 of 11 Old 04-02-2013, 02:21 PM - Thread Starter
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...Found out that when, "Are you using a Slotted Port " is checked on the web calculator, it deducts 1/2 of the Width from the Un -checked calculation !!


Here's the calculator :http://www.mobileinformationlabs.com/HowTo-1Woofer-Box-CAL%20Port%20lenth%201.htm
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post #10 of 11 Old 04-04-2013, 09:35 AM
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Very high aspect ratio (non-square) ports do behave as though they have less effective area than a more square port. I've seen and measured some approximate rules of thumb, but there are multiple variables at play where a slot terminating along a side wall vs. blunt end can have a few inch difference on effective length depending on the design. In short, the further you get from a round port with both ends terminating on a large flat surface, the more adjustment and testing needs to be done. Of course the longer you make a port, the more tolerance you will have to these effects as the end effects and effective area reduction are smaller % changes.

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post #11 of 11 Old 04-04-2013, 07:19 PM - Thread Starter
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Quote:
Originally Posted by Mark Seaton View Post

Very high aspect ratio (non-square) ports do behave as though they have less effective area than a more square port. I've seen and measured some approximate rules of thumb, but there are multiple variables at play where a slot terminating along a side wall vs. blunt end can have a few inch difference on effective length depending on the design. In short, the further you get from a round port with both ends terminating on a large flat surface, the more adjustment and testing needs to be done. Of course the longer you make a port, the more tolerance you will have to these effects as the end effects and effective area reduction are smaller % changes.


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The original design was rectangular box , with a 2" Round x 8" long port at the top of the enclosure. Since i have shortened the box a little and also added an angle, the original port no longer fits.

The Area of a 2" round port is 3.14, so im wondering if the Slot Port in the diagram of (1" x 3.14") x 7.5" long , that is using the top of the Tunnel would be close to original ??

....Air can circulate around tunnel , if not clear in diagram .
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