What I have always done is this................

Start off by messing with the measurments until you get the needed amount for your sub(s).

Ex: 34"x13"x18" = 3.56cuft internal.

How you get the cuft is take your measurments subtract 1.5 from each one and mulitply them all and then divide by 1728.

So, 34x13x18 becomes 32.5x11.5x16.5 = 6166.875, then divide by 1728 and you end up with a net volume of 3.56cuft.

Once you have your net volume you then calculate your port displacement.

Ex: Port is 18" tall x 3" wide X 30.5 long and that ends up being about 33.66hz.

What you do to calculate disp. is this...

Take the length of your port (30.5) and subtract .75 from it = 29.75"

Then multiply that by your height. (18)

Then take the width of your port (3) and add .75 = 3.75 then multiply that.

You end up with 1874.25 and once you have that, you then divide by 1728 to get your cuft.

Which is 1.08cuft.

Now once you have all of that, you then take your net volume (3.56cuft) and add your port disp. (1.08cuft) and then add your sub disp. and whatever that eqauls is how much cuft your box needs (gross volume) to end up with 3.56cuft after you add your port and the sub.

So then you take your original dimensions and adjust them to fit your gross volume.

Try not to adjust the height tho, cause then you will tamper with your tuning of the port.

But after all that you should have your measurments and everything.

Hope this helped!

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