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How to tune a vented enclosure.
There is no way to exactly pre-calculate the exact length for tuning purposes.
However I recommend that you download WinISD Pro (Freeware) and do simulations with the Volume of the enclosure and the Port Diameter, then when you get the Port Velocity under 18m/s then take the length given by the program then add about 2 inches to that for your prototype port.
Take the Prototype port and install it in the enclosure and hook it up to your system the way it would be used when finished.
Then make a CD from WinISD using the built in Tone Generator by recording a 5-10 sec passage of a Sine wave of each Tone from about 15Hz to 45Hz, then arrange these tones on a disk with each tone on a separate track.
Now play this CD through the connected Sub with the prototype port, and while holding your finger lightly on the cone feel for the difference in cone movement while switching tones (Tracks). When you get to the corresponding track for the tuning frequency of the box you will notice a great reduction in the cone movement. Hopefully by cutting the port longer than you need the corresponding tone will be lower than you’re target frequency or if your lucky exactly spot on. If the tuning is lower than expected then just cut about 1 inch off per Hz to dial in the exact target frequency.
This method worked great for me, and believe me when you hit the Fb of the box you will know, as the drivers cone is barely moving relative to just 1Hz below or above.
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