How To Figure Box Volume
Cubic feet is measured by multiplying the height X width X depth of the inside dimensions of the box, and dividing that total by the number of cubic inches in a cubic foot (a cubic foot is 12 X 12 X 12= 1728).
For example, a box with outside measurements of 16 inches high, by 34 inches wide, by 14 inches deep would be figured as so; first we subtract the wood thickness. This is usually ¾” thick, times the thickness of both sides of the box= 1 ½ inches. We subtract this from the outside dimensions.
So now we have
14.5H X 32.5W X 12.5D=5890.625.
5890.625 divided by 1728 = 3.4 cubic feet.
You can keep changing dimensions and refiguring the box volume, until you come up with the ideal box size and shape for your application.5890.625 divided by 1728 = 3.4 cubic feet. Calculating Net Volume: The volume occupied by the woofer and port is approximately 18% of the box. We need to deduct this volume from the gross volume of the box to find the net volume. The port will be calculated on the net volume. Now, take your 3.4 cubic feet and multiply by .82 (3.4*.82=2.788) so your Net Volume is 2.788 cubic feet. Calculating Port Area: Our port formula uses 16 square inches of port area per cubic foot of box volume. Now, take your 2.788 cubic feet and multiply it by 16 so get your Total Port Area 2.788*16 = 44.608 sq” of port area. Calculating Rectangular Port Dimension: Port area needs height and width. For a rectangular box with known dimensions, we already have a height, we just need to find the width of the port. Now, take your 44.608 and divide it by the height of your box, so this example we would divide 44.608/16= 2.788 (this is width of your port opening) Round it to the nearest .25”. The “Average” recommended length of the port is 18”, tuning around 40hz. The longer you make the length of the port, the lower the tuning. Those that listen to nothing but hip/hop, rap and chopped & screwed, you can make your port length 22-24” long. **You can keep changing dimensions and refiguring the box volume, until you come up with the ideal box size and shape for your application.