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Using WT3 to Measure a Tweeter's Parameters
A typical sealed back moving coil tweeter operates as a very small closed box system and therefore
can be characterized by it's closed box resonance and Q, F(sc) and Q(tc). You can use WT3 to measure
both of these parameters for tweeters as well as woofers and midranges. The frequency response of a
tweeter is the same as that of a purely electrical second order high-pass filter with the same electrical
resonance frequency and Q as the tweeter's electro-acoustic resonance and Q. While the resonance
frequency determines the low frequency bandwidth of the tweeter, the Q(tc) determines the shape of
the response in the corner region with Q = .707 corresponding to a maximally flat Butterworth
response.
The impedance response of a tweeter operating as a closed box loudspeaker has a single well defined
peak at the tweeter's primary resonance frequency at the low end of the tweeter's operating range.
Before starting, make sure the WT3 unit has been calibrated and has stabilized for at least 90 seconds.
Measure the tweeter's impedance as follows:
•
Launch the WT3 software and make sure the volume is at maximum.
•
Under the Edit menu select Preferences and set the L(e) Measurement Frequency to 10 kHz.
•
Make sure that the tweeter is not connected to any other equipment and then connect the test
leads of the WT3 unit to the tweeter under test.
•
Click the "Measure Free Air Parameters" button at the left side of the WT3 screen.
•
You should hear the sweep from the tweeter, the impedance is plotted and the parameters are
displayed.
•
The tweeter's equivalent closed box resonance frequency, F(sc) is displayed in the F(s) data
field.
•
The tweeter's equivalent closed box total Q, Q(tc) is displayed in the Q(ts) field.
•
Save the equivalent closed box data to a memory for future reference.
•
The DC resistance of the tweeter is displayed in the R(e) field. Ignore all other parameters.
•
Repeat the measurement to confirm the result. Repeated measurements should be in good
agreement.