Links to various ported
calculating port length of a given volume, desired fB and port
This formula is for tubular ports
only such as PVC and
cardboard type tubes.
= Length of vent in Inches
fB = Tuning frequency of box in Hz
Vb = Volume of box in cubic inches
R = Radius of vent in inches
actual tuning frequency of a system, you may use one of the
easiest and most accurate way to measure fB is with a signal
generator and an SPL meter. Set your sub box away from any corners
of your room. Place the meter close to the driver/woofer within 1/2" and sweep
the 10 - 80 Hz frequency region. Adjust the signal generator and look for for minimum
output from your driver, or for a slight dip in the output near fB. This can be hard to find
if fB is very close to the f(-3) point. Yet there still should be
a distinct drop (at fB) then rise (just below fB) then drop in output (way
below fB), even if it is subtle. It should be close to your desired fB as well. So if you get a dip at 43 Hz and you were expecting it
to be at 18 Hz, that's most likely the wrong dip.
A somewhat harder yet still accurate way to measure fB is by
using an impedance analyzer, such the one described in this
web site. Using a signal generator sweep once again between 10 and
80 Hz and locate the two large impedance peaks
which should be above and below fb. Label them fh and fl respectively.
The dip in the exact middle is sometimes referred to as fB, but most times is not very accurate.
To be more precise, seal the opening of the port with a soft rubber ball so as
to make an air tight seal. Now measure the impedance peak
once again. Call this fc. Then fB is calculated by: fB = (fl^2 +
fh^2 - fc^2)^0.5
Here is another simple yet still pretty accurate way to measure fB, when all else
fails. Crank up the sub super loud, so you can see it bouncing back and forth, adjust the generator and watch closely until the
driver's bouncing is reduced to a minimum. This will be fb.
Above and below fb the driver is capable of undergoing maximum excursion,
yet due to the damping characteristics of ported speaker systems, right at
fB the driver's excursion is at a minimum. If the driver appears
to bounce at all frequencies, you could have a leak in the enclosure or your
port diameter or port area is too small. First find the leaks and seal them up and do your tests again until you get consistent
results. If the driver still appears to bounce at all frequencies
and you get considerable port noise, then you may want to consider
adding a second port or increasing the diameter of the port already in
use. Of course this will require a longer port or that both
ports be longer, so calculate this first to see if your sub box will accommodate
the extra length.
For more information on Ported enclosures,
please visit Brian
Steele's DIY 1.1
Thanks to Vance Dickason for his formula on
calculating port length.