The Subwoofer DIY Page
Ported Systems - Design Notes
last updated: 13 April, 2011
The Subwoofer DIY Page

Choosing an alignment
The alignment (combination box size and tuning) given on the design equations is one possible alignment for your driver. This alignment will produce the flattest response within the passband for a ported system using the driver. However, this may not be the best alignment for you. Examine carefully the cutoff frequency and box size to determine whether or not this alignment lives up to your expectations.

Driver with low Qts
If the driver has a very low Qts value, try using a larger box size and/or lower tuning. Use the frequency and power response equations (or the supplied spreadsheet) to examine the results. It may be possible to build a design that has a lower cutoff point, the tradeoff being a slight ripple in the passband. A ripple of less than 1 dB in the passband will probably be unnoticeable.

High power applications
If you're designing your system for high-power applications, use a slightly lower tuning frequency. This should produce better results at higher volumes. You may also want to use an infrasonic ("rumble") filter to prevent the driver's cone from flapping around at very low frequencies.

Car Audio applications
If you're designing a system for car-audio use, remember that the interior of the car is going to boost the bass by about 12dB/octave below 60~80 Hz. A ported box that sounds flat in open air may sound boomy and flabby in a vehicle. Try using a lower tuning frequency than that predicted by the equations - this will lower the cutoff point and reduce any boominess that might occur.

Check that driver!
Make sure that the driver is suited for the enclosure that you have in mind! For example, just because software or equations predict that the driver may perform great in ported enclosure doesn't mean that it WILL perform well. Read the literature included with the driver (or talk to the manufacturer) to determine what's the best enclosure for the driver. When the design equations predict very big enclosures with very low tuning for your driver, that's usually a sign that the driver really wants to live in a sealed system.

Where should I put the ports?
Ports should be placed at least one diameter away from any adjacent walls. If this is not possible to do this, the tuning frequency for a given port length will be lower than that predicted by the equations, and this may adversely affect the results.

Lining the enclosure
One layer of lining on every wall for each ported alignment will generally give better results. Ensure that no lining obstructs the ports. Fiberglass will work here, but make sure that none's located near the port entrance, as air turbulence can rip chunks of it off the walls and eject it through the port.

Port size
Use the largest port possible for the ported designs. This will reduce power compression effects and port noise caused by turbulence.