Now we start testing with the port. I begin by creating
a sample enclosure called Vifa Box and then execute Calculate Vented.
I chose a C4 alignment which called for a port length of about 4".
For a pipe (for this test) I used thinwalled plumbing pipe (PVC)
spec'ed as 1 1/2" pipe (although it measures 1 3/4" in
diameter for some reason). I cut a length of pipe on the bandsaw
and wrapped the end with duct tape to get a tight fit in the port
I then measured the impedance, which had too low a
resonant point. C4 called for a resonance of about 42Hz and I got
about 35 Hz so I shortened the pipe to 3.45" and did this again.
That produced a resonance of around 40Hz. I stuck with that and
continued testing. The impedance chart for the driver with port
For comparison, the red chart is the calculated impedance
from Speaker Workshop. I think SW still has a level issue but the
shapes and locations are right which is what counts.
We can see the saddle point and zero phase crossing
are right at 40Hz - that is our Fb (box resonance).
The settings I found that matched the measurements
within Speaker Workshop were
I started with a C4 alignment and then tweaked the
port length to match the actual port length and ended up with the
dialog shown. The 0.4 ohm series resistance is probably a good match
with all of the connectors in series with the speaker and it will
be a good match with a typical series inductor (or two) in the crossover.
The box size shown is larger than the one I'm using,
but you can pass that off to the stuffing in the box or slight errors
in compliance calculation. Either way, this is good enough to go
with - especially after we see the frequency response on the next