The first phase of driver testing fulfills two purposes.
1. Testing lets you check the drivers to make sure
that they are within specification.
2. Testing lets you design an enclosure for the woofer.
A free air impedance test is the most critical test
here. Perform this by tightly clamping the driver (preferably upright
with no surfaces nearby) and then test the impedance using Speaker
We test the tweeter by simply doing a free air impedance
test. This verifies Q and FS . For the XT tweeter the
free air impedance test looks like
After doing the test, do a Driver / Estimate Parameters
to see what Speaker Workshop considers to be the driver parameters
based on the impedance tests. Don't forget to set the driver's DC
Resistance manually before doing this (Driver / Properties). You
can compare the tested impedance to the .Estimate dataset to see
if they match and to find irregularities in the tweeter impedance
For the tweeter we get an FS of 550Hz and
a QTS of .601. The Vifa spec for the driver is an FS
of 500Hz and a QTS of .55. We are well within spec (and
as usual both FS and QTS are high - perhaps because the driver hasn't
been broken in).
To test the woofer we have to perform two impedance
tests. One in free air and one with added mass or in an known volume
enclosure. These two tests between them capture all the information
needed to verify the woofer is good, and to calculate FS,
QTS, and VAS - the three important Thiele/Small
Here are the two woofer impedance tests I did. Earlier
in the project I used added mass, so here I did a sealed box test.
The box was the bottom part of the two-woofer prototype (sealed
off). These measurements were taken at a sample rate of 48KHz and
a sample size of 256K with 3 samples to give good frequency resolution
and low noise near resonance.
You may note the "hash" around 500Hz in
the sealed box curve. This is caused by reflections from the box
sides or internal box resonances. Here's a close-up of that part
of the chart. Notice how the "hash" is only evident in
the red (Sealed Box) dataset.
Anyway, Speaker Workshop does the Estimate Parameters
using these settings and comes up with values of
As usual, QTS is high and VAS
is low - the typical non-broken-in results. Agreement with factory
spec is actually quite good. The B result is a different, more broken