Having the right box for your speaker determines whether your speaker will pump out a clear and powerful sound or just make noise. Besides the construction materials and dimensions of the box, another major factor is the port. Ports are basically round or square tubes fitted on subwoofers to allow the air inside the speaker to get out. A port can be tuned to eliminate the air at a certain frequency, enhancing the volume. You can calculate the port dimensions with an online calculator --- instead of using very complex mathematical formulas --- to determine the right size.
Access the online calculator at CarStereo.com and go to the "Minimum Usable Vent Diameter" section.
Enter the size of your speakers. You can find the size either from the specification sheet that came with the speakers or by manually measuring their diameter.
Enter the number of speakers your subwoofer has in the "Quantity of Drivers" field. Most common subwoofers have one or two speakers, but make sure you fill the space according to your speaker configuration.
Enter the Xmax value of your speakers. You can find this value in the specifications sheet. If you cannot find the value, you can either contact the manufacturer of your speakers or leave the default value in the calculator field, as it is a general value adequate for most common speakers.
Enter the tuning frequency of the port. This will determine which frequencies will be enhanced by the port. You can find the suggested port tuning frequency in the speaker specifications sheet.
Check the appropriate value in the query about whether the enclosure is a bandpass type or not and click on "Calculate Minimum." The calculator will display the proper dimensions for your port's diameter.
Open the online calculator at CarStereo.com and go to the "Calculate Your Vent Length" section.
Select whether you'll be using a round port or a square port and enter the number of ports you plan to use.
Enter the volume of the enclosure.
Enter the tuning frequency of the port and whether the port is a slot-type port or not.
Enter the port's diameter, which you previously calculated, and hit "Calculate Length." The calculator will display the proper length of your port.
When purchasing or building the port, look for strong non-vibrant materials, such as PVC or hard plastic.
When installing the port, make sure to fix it in place properly otherwise it will vibrate, distorting the sound.