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Changes in f3 and f10: The two plots below show the same plots as before, but nominalized to
compensate for the differences in output due to the impedance change. These delineate the possible
improvement in low end extension due to the change in Qts. -If the increase in impedance is taken into
account. About a 5 Hz extension in both f3 and f10 for the sealed design The vented enclosure models
with a 4 Hz extension in f3, and a 3 Hz extension in f10. Note that the output at 20 Hz is nearly 4 dB
higher than if the increased impedance was not taken into account.
Conclusion: The factors resulting in additional series resistance can cause a significant change in the
box Q, and an equally significant difference in the size of the enclosure to obtain the intended box Q.
This, at least in home speakers, will only be an issue when the driver passband includes the
compliance controlled region, or output is expected at and below fc. In most applications, this will be
limited to woofers, although the loss of sensitivity may be felt by the midrange to some extent as well.
The 8% increase measured in the Stereophile article, was based on the measurement of one high end
speaker, and may represent exceptional driver thermal properties, not necessarily those of the
average driver. Certainly I've seen other articles suggest much higher increases in temperature, but
I've not seen actual data to back up these claims. In any case, it is clear that some compensation for
the thermal effects should be made in the design process to obtain optimum system performance.
Copyright 2006 Curt Campbell
Electrical Factors
affecting system
impedance
-And their consequences on system Q