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Using WT3 to Evaluate a Closed Box Loudspeaker
A closed box type of loudspeaker system is characterized by a resonance frequency F(sc), an electrical
Q value Q(ec), a mechanical Q value Q(mc) and a total Q value Q(tc). Just as you can use WT3 to
measure a driver's parameters you can also use WT3 to measure the corresponding parameters of a
closed box loudspeaker system. The frequency response of a closed box speaker is the same as that of
a purely electrical second order high-pass filter with the same electrical resonance frequency and Q as
the loudspeaker's electro-acoustic resonance and Q. While the resonance frequency determines the low
frequency bandwidth of the system, the Q(tc) determines the shape of the high-pass response in the
corner region with Q = .707 corresponding to a maximally flat Butterworth response.
The impedance response of a closed box loudspeaker has a single well defined peak at the system's
primary resonance frequency. Compared to the same driver in free air, the resonance frequency will
always be seen to increase when a driver is mounted on a closed box enclosure. The larger the
enclosure the less the resonance frequency is increased. The same is also true for the driver's Q(ts)
parameter versus the closed box system's Q(tc). The Q(tc) for the driver in an infinitely large enclosure
would equal the Q(ts) of the driver. As the enclosure size is reduced the Q(tc) of the closed box system
increases. So a given closed box speaker will always have F(sc) and Q(tc) values somewhat higher
than the driver's corresponding free air parameters F(s) and Q(ts).
Before starting, make sure the WT3 unit has been calibrated and has stabilized for at least 90 seconds.
Measure the system impedance as follows:
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Launch the WT3 software and make sure the volume is at maximum.
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Make sure that the speaker system is not connected to any other equipment and then connect
the test leads of the WT3 unit to the speaker under test.
•
Click the "Measure Free Air Parameters" button at the left side of the WT3 screen.
•
You should hear the sweep from the speaker, the impedance is plotted and the parameters are
displayed.
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The system's closed box resonance frequency, (F(sc) is displayed in the F(s) data field.
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The system's closed box total Q, Q(tc) is displayed in the Q(ts) field.
•
The system's closed box mechanical Q, Q(mc) is displayed in the Q(ms) field.
•
The system's closed box electrical Q, Q(ec) is displayed in the Q(es) field.
•
The DC resistance of the system 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.