What creates the impedance peaks in ported cabinet?

[teemuk]

A ported cabinet has two peaks in it's impedance plot:
http://www.audioholics.com/productre...age001_000.gif

As the lumped model of a ported speaker cabinet consists of parallel and series resonant circuits (http://www.audioholics.com/productre...p_image002.gif) I’m a bit clueless how these two peaks are actually created. A parallel resonant circuit creates a peak with “gain” but the peak from the series resonant circuit should have a “negative gain”. Are these two peaks just overlapping or is something else happening here?

[Svante]

Originally posted by teemuk


I’m a bit clueless how these two peaks are actually created.

Lces and Cmes results in the driver free air resonance fs.

Lceb and Cmep results in the helmholtz resonance fh.

At fh, the series circuit has a minimum, and so has the impedance of the entire circuit.

Above fh the series circuit is inductive and adds to Lces, two inductors in parallel resuts in a lower inductance and a resonance frequency higher than fs. This resonance is the higher of the two peaks in the impedance curve.

Below fh the series circuit is capacitive and adds to Cmes, two capacitors parallel resuts in a higher capcitance and a resonance frequency lower than fs. This resonance is the lower of the two peaks in the impedance curve.