Audio Systems: speaker impedance, impedance curve, speaker impedance

Question
Why does a speakers impedance vary with different frequencies,(i hear it does down as the frequency goes down)so why when I looked at one chart the speakers was: [20ohms@75hz] and [50ohms@30hz]?

Answer
The speaker's voice coil has inductance due to it being a closely wound coil embedded in a magnetic field.

The reactance or impedance of an inductor is expressed as:

XsubL = 2 x  pi x f x L = (2)(3.14)x frequency x inductance L.

Therefore, a pure inductor will double its impedance as the frequency rises an octave or frequency doubling.

The loudspeaker voice coil has some resistanc; usually just slightly less than the impedance rating.  A typical 8 ohm speaker, for example, will have a dc resistance of about 6 to 7 ohms.  The impedance curve of this typical speaker will show the impedance rising linearly once the frequency rises enough for the inductive effect to become much greater than the dc resistance, but at low frequencies the impedance will stay close to the resistive value until the speaker approaches its resonant frequency which then rises due to the acoustic properties of the cone under air pressure which cause mechanical loading on the cone system and makes the impedance rise.  See:

http://www.churchsoundcheck.com/imp1.html

Fig 1, for example.

Does that help?

C