Application
note # 16 
Sample
Champion

VERY IMPORTANT NOTE:
To measure Impedance with the best accuracy,
use
Sample Champion Vr. 3.2 or higher !

Impedance
measurement and ThieleSmall parameters computation
[See
also :Application note #17, Impedance
measurement: pratical construction of the connector]
This application note describes the use of the Sample Champion
Impedance Plugin. This module allows measuring Loudspeaker
impedance by means of a single 2 channel impulse response measurement
and the computation of ThieleSmall parameters.
Impedance
plugin main window
This
measurement requires a minimal external equipment: just two
resistors! One must have a value around 1 KOhm, the other one
must have a low value (around 8 Ohm). The resistance of the
low value resistor must be known exactly because it is used
for calibration.
The
plugin accepts two different configurations for the connection
of the device under test (loudspeaker).
Allowed
configurations
Basically,
one channel output is connected directly to one channel input
and the other one measures the impedance by means of a simple
partitive network. The following examples refer to the first
configuration.
The
calibration resistor or the loudspeaker must be connected to
the Probe terminals, as shown in the figure above.
The
main program must be set for performing a 2 channel measurement;
just a single cycle measure is usually sufficient.
For
example the following settings could be used:
Impedance
settings
The
Impedance measuring procedure is now explained in detail. After
the first time, you'll be able to perform an impedance mesurement
in a few seconds...
Step
by step procedure for the Impedance measurement:

Select, in the main program, an appropriate setting for Impulse
Response measurement. The MLS length can be also lower than
16K. The input mode must be Stereo, the output mode can be mono
or stereo.

Open the Impulse Response window in the main program

Connect the calibration resistor to the probe terminals,
following the scheme in the figure above

Make a test measurement (Syncro Start/Stop MLS and Sampling)
and set appropriate input and output levels with the internal
(or external) mixer, in order to obtain a clean and undistorted
2 channels impulse response. Using configuration 1, during the
measure, the peak level meter should show something similar
to:
If
the upper meter (the left channel) has values higher than 12%,
check the contacts between the calibration resistor and the
probe points. Also 0% indicates good measure (even better) because
here very low levels are involved. Check in the Impulse Response
window the 2 measured responses zooming the amplitude axis.
The result should resemble the following:
The
2 channel measurement (amplitude zoom 128x)
The
same figure, where the right channel plot (red)
has been disabled. Note that the left channel (green) has a
peak value much lower than the right one

Now open the Impedance plugin and press F5 to transfer the measured
data from the main program to the plugin. The 2 impulses and
their frequency spectrum can be plotted also in the plugin (with
the selector at the right of the display) for checking the measurement
correctness.

Start a new stereo impulse response measurement in the main
program (keeping open the impedance plugin) and press F5 to
get the data. Now set the view mode to Impedance (M, Ph)
and, if all has gone in the right way, a raw flat green line
(the impedance magnitude) will be plotted. Also a raw red curve
(the impedance phase) will be plotted; if the phase is almost
flat, you have an high quality soundcard. Anyway all soundcard
defects can be compensated by pressing the Calibrate Now
button in the plugin. This action will normalize the impedance
magnitude to the calibration resistor value (for example 8.5
ohm) and will zero all phase deviations.
READ A VERY IMPORTANT
NOTE ABOUT THIS PROCEDURE

Now replace the calibration resistor with the loudspeaker and
perform a new measurement. The following is an example:
The
impedance magnitude is plotted in green and the phase in red
If
the measure is not satisfactory, check the spectra to locate
the problem. The following is an example of good spectra:
Frequency
spectrum check
(red = right channel, green = left channel)

The measured Impedance can be computed at different frequency
resolutions (FFT from 4K to 64K points), data windowing and
smooth factors. In most cases 16K points FFT can be sufficient;
it can be also lower if the sampling frequency is low. If the
plotted resonance, usually placed around 50100 Hz, is not smooth,
the FFT size in the plugin must be increased or the sampling
frequency in the main program must be decreased. Normally the
smooth factor setting can be left at zero.

The computed Impedance can be saved. Note that the data are
saved at the current frequency resolution. Also the calibration
can be saved but it is recommended to repeat the procedure every
time before loudspeaker measurement because changes in the audio
device level settings and other factors can invalidate the previous
calibration.

The plugin can work on loaded Impedances or on Impedances computed
using impulse response data measured in the main program. When
an Impedance is loaded, the selector below becomes yellow:
Note
that the frequency resolution of loaded impedances cannot be
changed. When an Impedance file is loaded, the calibration is
cleared.
The
Impedance can be shown also as a Nyquist Plot:
Impedance
Nyquist Plot
IMPORTANT
NOTE: remember that any loudspeaker acts like a sensible
microphone! Perform the Impedance measure in a very quiet environment
and place it away from any source of vibrations (for example
the table where the computer is placed). Otherwise you will
get a poor measure, affected by spurious oscillations and peaks.
If the background noise cannot be eliminated, an average of
48 measures can help. In extreme cases the smooth factor can
mask small peaks, when they cannot be eliminated in other ways.
Thiele
Small parameters computation:
The
plugin can compute the following parameters:

Fs = Resonance frequency
 Zmax = Impedance at resonance
 Le = Inductance at 1KHz

Qms = Mechanical Q
 Qes = Electrical Q
 Qts = Total Q
 Mmd = Cone mass
 Mms = Dynamic mass
 Cms = Compliance
 Rms = Mechanical resistance
 BL = Force Factor
 no = Acoustic reference efficiency
 Vas = Equivalent volume
 Reference Sensitivity
 Sensitivity
The
computation can be performed on loaded or measured impedances.
In order to compute the Thiele Small parameters, some input
data must be set:

The DC driver resistance Re (usually measured with an
ohmmeter). It is possible also to get this value automatically,
using a low frequency impedance value, but a separate measure
is recommended.

The driver cone surface Sd. The plugin allows also to
enter the cone diameter d and compute automatically the
surface area.
Two
methods are implemented for the computation of the parameters:

The Delta Mass method
 The Delta Compliance method
A
first Impedance measurement must be performed with the driver
standing at free air.
For
the Delta Mass method, a small mass is attached on the
cone and a second Impedance measurement is performed. The weight
of the added mass Ma must be set in the plugin.
For
the Delta Compliance method, the driver is mounted in
a test box and a second Impedance measurement is performed.
The test box volume Vt must be set in the plugin.
If
the Delta Mass method is used, a dummy value for the
test box volume can be set. If the Delta Compliance method
is used, a dummy value for the added mass can be set.
Now
an example of TS parameters computation is shown, using Impedances
saved on files. The same procedure can be followed for measured
impedances.
Download
the Impedance files used in the example:
Download
The
above file contains the following:
 Speak1_free_7.5 liters.txt (free standing
driver 1)
 Speak1_box_7.5 liters.txt (driver 1
mounted in a 7.5 liters box)
 Speak2_free_20 grams.txt (free standing
driver 2)
 Speak2_mass_20 grams.txt (driver 2 with
an added
20 grams mass)
 ImpedConfig.INI (plugin configuration
and Re, Sd, Ma, Vt)
 deltacompliancets.rtf (driver 1, TS
with Delta Compliance method)
 deltamassts.rtf (driver
2, TS with Delta Mass method)
Step
by step procedure for the Thiele Small parameters computation:
Delta
Mass method:

Set Re, Sd, Ma and Vt in the plugin
(or load the configuration file ImpedConfig.INI)

Load the free standing measurement Speak2_free_20
grams.txt and press the Set Curve 1 button in the
plugin

Load the added mass measurement Speak2_mass_20
grams.txt and press the Set Curve 2 button in the
plugin
The
method (Delta Mass or Delta Compliance) is automatically selected
on the basis of the selected curves resonance peaks.
The
window must look like the following:
Delta
Mass configuration
The
2 Impedance curves can be plotted:
Curve
1 (yellow) and Curve 2 (purple)

Press the Compute TS button and that's all! All the
computed Thiele Small parameters will be shown:
Thiele
Small parameters computed by means of Delta Mass method
The
computation by means of the Delta Compliance method is quite
similar:
Delta
Compliance method:

Set Re, Sd, Ma and Vt in the plugin
(or load the configuration file ImpedConfig.INI)

Load the free standing measurement Speak1_free_7.5
liters.txt and press the Set Curve 1 button in the
plugin

Load the mounted in a box measurement Speak1_box_7.5
liters.txt and press the Set Curve 2 button in the
plugin
The
window must look like the following:
Delta
Compliance configuration
The
2 Impedance curves can be plot:
Curve
1 (yellow) and Curve 2 (purple)

Press the Compute TS button and that's all! All the
computed Thiele Small parameters will be shown:
Thiele
Small parameters computed by means of
Delta Compliance method
IMPORTANT
NOTE: the 2 impedance curves used for TS computation
must:
 have the same frequency resolution
 have the same sampling frequency
 have the resonance peaks at different frequencies
 be computed using the same smooth factor (because it can slightly
affect the resonance peak)
The
numerical values can be exported (copied to clipboard).
Parameters
exporting
[See
also :Application note #17, Impedance
measurement: pratical construction of the connector]
To
check if the calibration is correct, simply move the
mouse over the Impedance plot after the calibration and read
the computed resistance values. All values must be equal
to the calibration resistor.
Download
PDF file (1370K) Shiftclick
to download
Back to Application Notes
A
freeware software for unit conversion (m, inches..) can be downloaded
here.
Many
thanks to Johny Grenander for his help in the development of
this plugin.
