Box Design Steps
Step 3: Converting Gross to Net Volume, after speaker and port displacements.
Net volume is the amount of airspace after subtracting the volume of air occupied by the woofer and port. This is the amount of airspace with which the port is calculated. How is this done without knowing the volume of the port and sub(s)? Is this another, which came first, the chicken or the egg? It is, unlike estimating the timing sequence of evolutionary or whimsical spontaneous creation, we can estimate occupied volumes based on many cases of fixed calculations and come up with a pretty close percentage.
The occupied volume of the port and woofer, for 40Hz tuning and 35Hz tuning, are approximately 18% and 23% respectively. The larger 23% number for 35Hz tuning representing the longer port length for lower tuning, the longer port takes up more of the inner volume.
Multiplying the numbers from our box:
For 40 Hz tuning, 1.73cuft x 0.82 = 1.42 cubic feet of net volume.
For 35 Hz tuning: 1.73cuft x .77 = 1.33 cubic feet of net volume
Port area for a DDBox is 16" of port area, per cubic foot. See "Why Go Ported".
Using the 40 Hz tuning example, 1.42cuft x 16" = 22.72 square inches of port area
Keep in mind that a 10% variation in port area will not be audible so there is a tolerance of plus or minus 2 square inches.