pulled from sounddomain
the picture didn't come up though. here is the link and explains it real easy
http://forum.sounddomain.com/forum/ultimatebb.cgi?ubb=get_topic&f=11&t=069898
the rest of the board sucks though lol.
The tuning frequency of a vented enclosure is a function of the net internal volume of the enclosure, the cross-sectional area of the vent, and the effective length of the vent. It is determined by using the following formula:
Fb denotes the frequency that the enclosure is tuned at.
Av denotes the cross-sectional area of the vent, measured in square inches. When using a round vent, Av is determined by using the formula {pi}radius^2. When using a square or rectangular shaped vent, Av is determined by the product of the vent's height and the vent's width.
Lv denotes the effective length of the vent, measured in inches. Do not confuse the physical length of the vent with Lv, as they are often times not the same due to end-correction factor. For an explaination on end-correction factor, please refer to this tutorial.
Vb denotes the enclosure's net volume, measured in cubic inches. This is the internal useable airspace within the enclosure; the amount of airspace that the subwoofer can "feel" as useable. Net volume is calculated by determining vent displacement, driver displacement, bracing displacement, end-correction displacement, etc and subtracting these values from the enclosure's internal gross volume. (Remember, the airspace within the vent is NOT airspace that the subwoofer can "use". The vent must be thought of as a solid object, and its entire volume must be subtracted in order to properly calculate net volume.)