I'm working on some cabinets for a pair of Coral Flat 6's. The plans call for a port that is a square.

Would I be better off using a circular port and if so, how do I convert for size? Is the volume of the ports all that matters or am I missing something.

The port size from the plans is 2.6" square x 3.9" long.

Thanks for any help.

2.6" x 2.6" = 6.76 sq. in. area.

Area of a circle is A = pi R squared.

6.76 = pi R squared. Solve for R.

R = sqrt(6.76/3.1415926) = 1.4669

Diameter = 2 x R = 2 x 1.4669 = 2.934"

Length will still be 3.9"

I would use a 3" Inner Diameter port, and make it 4" long. When a port's diameter is increased, it will usually be slightly longer, too.

Thanks Paul.

Any time!

One more question Paul.

I have 2" ID ports here, would it make a big difference if I used that over a 3" port?

You would use a different length with a different ID. I could be wrong, but with a larger ID, you'll have to increase the length.

What is your cabinet volume? Either in cubic feet or cubic inches?

What size woofer?

It's for a full range Coral driver.

The Coral plans call for a 1 cubic foot cabinet for this driver with the described port. I have a beautiful pair of real old school Pioneer cabinets that will fill the bill perfectly once I replace the baffle. Just need to get the porting right.

http://audiokarma.org/forums/showthread.php?p=871250

Go there, learn how to download and use Boxplot.

Adjust the cabinet size (in Boxplot) to be the same as your cabinet size. Do this by going into Box Parameters and adjusting Alpha until Vb equals the volume of your cabinet.

In the Vent Design box, check Rectangular, and enter 2.6" x 2.6".

Go back to Box Parameters and adjust H (Fb) causes the port length in Vent Design to be equal to 3.9"

Now, in Vent Design, check Round, and enter 2" diameter. There it will tell you the port length for a 2" port in that particular cabinet size that will resonate at the proper Fb.

Doing this myself, I get Fb of 55 hz and 2" dia port length of 1.35"

Alternative method. Click on this link and Save the zip file.

http://audiokarma.org/forums/attachment.php?attachmentid=26564&d=1159591082

Inside this zip file is an Excell sheet I made up.

In Port Calculations enter 1 (or whatever the box volume) in the blue Vb blank.

Enter 2.934 in the Vent Diameter blank. That is the round equivilent to your 2.6" x 2.6" port.

In the Desired Fb blank, enter 54.3, which is the port tuning frequency, and will cause the resulting length to be the desired 3.9" (in the pink blank). I arrived at that just fiddling with the numbers in Fb.

So, now you know that Fb is 54.3.

Change Vent Diameter to 2, enter.

Now for a 1 cft box, tuned to 54.3 hz, with a 2" port diameter, you can see that the needed port length is 1.31". So, 1.31" or 1.35", I don't think it will make any audible difference.

I use some different fudge factors, I mean, end corrections, for the sake of simplicity. This causes my port lengths to be very slightly different from Boxplot's. But we are only talking .04". That's close enough to suit me.

What Speaker Design Worksheet will not do is give you the frequency response graph for various box sizes and port tunings. You still need boxplot for that. But this is a quick and dirty calculation sheet.

For you guys in the rest of the world, I have a metric version available. Contact me via IM and send your request and email address.

Thanks again Paul. You are darn close to the Fb that the Coral info states. They claim a 58hz Fb, so it looks like we are right in there.

Downloaded and saved. Thanks again.

If using a flared port do you need to add length to the port? I thought I read that on the Madisound website somewhere.

Thanks again Paul. You are darn close to the Fb that the Coral info states. They claim a 58hz Fb, so it looks like we are right in there.

Downloaded and saved. Thanks again.

If using a flared port do you need to add length to the port? I thought I read that on the Madisound website somewhere.

I'd count half of the flared length.

You had stated 1 cubic foot for Vb. Is that correct?

Did Coral say Fb was 58 hz or F3? Two different things. Fb is the box tuning frequency. F3 is where the response is -3 db, which is considered the lower limit of frequency response. Fb can be above or below F3.

No matter, we are in the ballpark. If you want to raise F3 to 58 hz you can cut a little off the port.

In your case, if the cabinet, that is Vb = 1 cu ft, port diameter is 2", then the port length is 0.96" for 58 hz. Ahhh... just make that 1". Really, it won't make a bit of difference.