Given the correct port diameter to increase low end response, there is no measurable or audible advantage to dual port tuning, but it is possible. Both ports have to have the same inside diameter or cross-section area. When two ports are used, they'll be about twice as long as a single port. Once the minimum port cross-section is calculated for a given woofer to prevent whooshing or whistling noises, calculating the port length for two ports may prove to yield a longer port than the cabinet is deep, but no rule says the ports can't bend a few times.
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Things you need
- Driver size in inches
- Driver Xmax
- Desired tuning frequency
- Enclosure volume in cubic feet
- Minimum usable vent diameter
Design and build a subwoofer cabinet around your subwoofer, using Thiele-Small parameters published for you woofer, and a ported-cabinet calculator. Thiele-Small parameters are electromechanical descriptors of any given driver that detail moving mass, suspension stiffness, interaction of the voice coil with the magnet, nominal displacement, and likely reaction to an enclosure.
Visit the referenced vent dimension calculators. Enter the port type and quantity, enclosure volume, desired tuning frequency and port diameter or cross-section area into the vent length calculator. The port diameter or cross-section area should be a commercially-available product with an inside diameter equal to or greater than the figure derived with the minimum vent diameter calculator.
Build your ports so that the inside terminus is far enough from the rear wall to keep from restricting air flow: A good rule of thumb is to make that distance at least equal to the radius of the port. The easiest approach is to use PVC pipe, or thick cardboard mailing or carpet tube. If your ports are longer than the cabinet is deep, PVC affords the opportunity to use elbows or U-traps to "bend" the tube. Measure a centre line through the bends to maintain proper port length.
Tips and warnings
- Minimum usable vent diameter can be calculated using the other parameters in the things needed list.
- Xmax is the maximum distance a driver will travel in or out from standing rest. This usually equates to half the length of the voice coil winding.
- Ports are used to tune a cabinet/woofer acoustic circuit to be more efficient at a narrow low frequency range by reversing the back wave of the woofer at that range.
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