ported box slot vent

This is a tough one and I am getting three different numbers from three different sources on how long my slot port should be.

The subwoofer is a dual 2 ohm Kicker CVR12 getting 380w rms. My box has a volume of 2.63ft^3 including the port. The dimensions are 27.5" x 15.5" x 14" with 3/4" mdf.

The port is 14" wide and 3" tall. I am working on the correct length, but not having any luck.

The box that Kicker recommends is 2.25ft^3 excluding the port. They recommend a port 3" tall, 12.5" wide, and 20.5" long. I'm attaching the specs for the subwoofer.

I tried using WINisd, the JL Audio tutorial, and the carstereo.com site and I got three different lengths(33", 23.5", 19").

If anyone can help me out a little here and let me know how long my port should be to have it tuned to the same fequency as the Kicker box it would be extremely appreciated. Thanks guys.

Port lengths you can choose from:                        

                         17" tuned to 47 Hz
                         19" tuned to 45 Hz
                         21" tuned to 43 Hz
                      22.5" tuned to 42 Hz
                         24 " tuned to 41 Hz

These are based on your slot opening of 3 X 14.  If you looked at the VB graph I showed you in your last thread, at the top right it showed the Fb as 42 Hz.  That's the tuning frequency of the Kicker box that you're trying to duplicate, because I modeled it using the Kicker specs.  With your larger slot opening, the equivalent port length would be 22.5".  The extra port length takes away about .06 cu ft from your box net volume, which is negligible.

Here's a look at the response:  vb_response_kicker_cvr12d2.gif

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Thanks a ton. It is extremely appreciated.

For WINisd do I enter the volume including the port or excluding the port. It doesn't say on there and I think that is where I was messing up.

Keep it at 2.25 in the program.  It is when you actually build the box that you add all the displacements into the total.  The program doesn't account for any displacements at all, but figures on net air volume alone.

When you work up your cut plan, you have to include the total volume of the port which includes not only the air but the walls that are used for it.  You'll also need to add for driver displacement and bracing you use.   Your gross volume should be the 2.63 cu ft you showed above plus the driver, port walls and bracing, which it looks like you didn't figure into it.  When you draw this out on paper it will be easier to calculate total port volume including walls.

Try to keep the volume error  within +/- 5% of the total 2.25 that you're shooting for.  That gives you about +/- 0.1 cu ft margin for error.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

I have a half way built box, but it is gonna have to be scrapped. I made some miscalculations in the inital port volume and didn't account for the driver and bracing. If built it would have about 2ft^3 of volume when I need 2.25ft^3.

I figure I need a 2.886ft^3 box.

.556ft^3 for the kicker spec port
.07ft^3 driver displacement
.01ft^3 construction grade metal corner bracing
2.25ft^3 for the actual box inner volume

Does that look about right?

One wall of your port will be the box wall, so you don't count it as additional.  Just count the inner wall, which should be 3/4" MDF.  The total port displacement should be somewhat higher than what you posted...you might want to double-check that.  The driver is whatever Kicker says it is.  And you might want to add an MDF brace on the opposite side of the box from the port because you'll have an expanse there.

You might also look at modeling that up with the box you've been working on just to see what it would look like.  It would require all calculations over again.  But if you have the wood to work with, might as well start over, eh?

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Thanks again for all you help. This is my first slot ported box and these things are a totally different ball game than sealed.

To find the port displacement I am doing 3.75" x 12.5" x 20.5". The 3.75" is including the .75" inner mdf wall. Am I calculating this wrong?

luckydevil wrote:

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The port is 14" wide and 3" tall. I am working on the correct length, but not having any luck.

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Isn't this your port dimensions and with a 22.5" length? 

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

On this next box I am just going to build it with Kicker's exact port size (3 x 12.5 x 20.5)to make this easier. That is where I got the .556ft^3 from.