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Upcoming Seminar:
"Loudspeaker
Measurement"
Lone Star Audio Fest
May 2-4, Dallas TX
Details
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Introduction
Calibrating the Woofer Tester is a simple process that when done properly will yield consistant and accurate results. The
calibration process itself measures the internal and external effects of lead wire resistance and inductance and then
cancels those effects from later measurements. It is therefore important that if you intend to use the woofer tester with
a shorter or longer set of leads that you recalibrate using those leads.
Note:
Calibration will only be as acurate as the calibration source. The supplied calibration resistors are 1% acurate. If you
have a better resistor, you can use it. The only requirement is that it is non-inductive and its resistance is close to the
resistance you want to measure. Keep in mind that many 0.1% resistors are wire-wound and therefore not acceptable. To set
the calibration resistor's default value, first shut down all child windows. Then look for 'Set Calibration Resistor' in
the Options menu.
STEP 1: Set the Current Drive Level
Set the current drive level to the level you want to test at. Generally, this level will be set to maximum but in some cases
you may want to set Idrive lower. An example would be if you want to measure high value resistors or a woofer with a
particularly high impedance. Fully measure the Thiele-Small driver parameters using the Q/Fs test followed by either the
delta mass or delta compliance test.
STEP 2: 10 kHz Shorts Test
Clicking on the 'Calibrate' button begins the calibration process with a high frequency short circuit test. There will be a
message reminding you to short the end of your test leads (at this point clip them together). Wait a moment for the data to
settle down and the message in the lower right should now read 'Pass'. Click 'Accept' or 'OK' to continue. This part of the
calibration process measures residual signals due to resistance or internal signal path leakages. Data taken at this time is
most important when measuring impedances that are close to zero ohms or when the current drive level is set very low.
Short needs to be connected (Check Connections)
Short is Connected (Pass)
STEP 3: 10 kHz Measurement of CalR
The calibration dialog message will now remind you to disconnect the short and connect the calibration resistor to the end of
the cable. This step sets the full-scale gain and phase compensation.
STEP 4: 1 Hz Measurement of CalR
This step in the calibration process compensates for a slight change in the overall loop gain at 1 Hz relative to 10Khz. Test
frequencies below 10 Hz also cause the sample rate to drop from 44 Khz to 11 kHz. Adding to this is the settling time of the
narrow band 1 Hz noise filter. This step in the calibration process takes some time to complete but the results are well worth
the wait.
Testing the Calibration (optional)
Leave the calibration resistor connected and click on ARB1 or ARB2 to collect a data set from SweepLo to SweepHi. Using the axis
control dialog (open this by clicking on the number scale along the edge of the graph), disable the 'automatic' modes and manually
set the range to see the data in greater detail. When configured fro zero length operation flatness is often better than
0.01 ohms and 0.1 degrees from 1 Hz to 20Khz. Flatness is quite good even when long cables are used.
Flatness Example for Zero Length Calibration
Click Image to Enlarge
Zero Length Calibration: Measurement of Cables
Cable and wire effects can be easily measured with the Woofer Tester. The best way to configure the tester for this is to calibrate
without test leads. The cable or wire to be tested is then shorted (at the far end) and plugged directly into the Woofer Tester.
The term zero-length comes from the calibration process, since the calibration resistor and short are plugged directly into the
tester. It is also helpful to have a banana jack fitted with a shorting bar for this calibration.
Another option is to make a combination calibration resistor and switch, as shown. In this case, the switch is SPDT on-off switch,
but other types of switches can be used. This one just happened to fit snugly into the banana jack.
Shorting Switch with Calibration Resistor Modification
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