Ported Systems - Design Notes
last updated: 15 April, 2006
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 thinks 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.