subwoofer enclosure, port conversion

This is a reprint of an answer I gave previously to this question:

pi r^2 to find the area of a circle

pi is 3.14

r is radius, or half the diameter of a circle

^2 is squared, or the number times itself

pi r^2 is 3.14 X the result of radius squared

pi r^2 gives you the square inches of a circle

You want to change the shape of the opening but keep it the same square inches.  The length of the port is not a factor and you will leave it exactly the length it already is.

Find the square inches of your circle using pi r^2 (ex. a 6 inch diameter circle yields 28.26 square inches)

Decide on the longest side you want your square or slotted port opening to be

Divide the total square inches of the circle by that long side, in inches (ex. 28.26 / 11.5 = 2.46)

The result is the measurement of the shorter side of the port opening

In the example, the 6" round port equals a slotted port 11.5 X 2.46 inches.  Round measurements to the nearest Imperial or metric value.  2.46 would be  2 7/16 to 2 1/2.

You should keep the ratio of length to width of the slotted opening at no greater than 8:1.

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

If you know the inside net volume of your enclosure and the target port frequency, simply use the port calculator on this site (click on "woofer Calculator") to find the proper port length for whatever diameter you'd like to use.  I suggest a 4" port diameter.

This is simplified, but the reason most subs sound better to the driver if they face backwards is due to the wavelength of subwoofer frequency sounds.  Facing backwards allows the sound energy to reflect off the rear of the car before it gets to the driver's seat, allowing low frequency waves to develop to their max SPL before you hear it.