The port length required to tune a volume of air to a specific frequency can be calculated by using the following equation:
Dv = port diameter (cm)
The value for k, the end correction, can be fine-tuned by using the following values to derive the appropriate end correction figure for each end of the port, then adding them together
Flanged End: 0.425
Normally, k=0.732 is assumed
In practice, it's best to use ports that are slightly longer than predicted by the above equations, then adjust their length until the correct tuning is achieved. It is much easier to shorten a port than to lengthen it!
Minimum Port Diameter
Xmax = maximum linear displacement (mm)
Calculate the minimum port diameter from the following equations:
Sd = pi*(Dia/100)^2/4
Dmin = minimum port diameter (cm)
Note: You CAN use ports that have a smaller diameter than that given by the equation above, especially if the ports are flared at both ends. However, at higher volumes, you may notice some port noise caused by the air rushing through the ports.
Dv' = 2*((W*H)/pi)^0.5
= diameter of equivalent round port
Once you've calculated Dv', you can use it in the equation for Lv above to determine the required length of the slot port to tune the enclosure to the required frequency (Fb).