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Message: <blockquote data-quote="newusername" data-source="post: 3510618" data-attributes="member: 562064">Ok, so the first very important distinction to be made.Electrical Resistance - the measure of an objects opposition to Direct Current through said objectFor interests sake:R = (l*p)/AThe resistance of an object ® is given by the product of the length (l) and the resistivity of the material (p) divided by the cross sectional area of an object (A)Impedance - the measure of an objects opposition to Alternating Current through said objectComplex Impedance considers two parts...the resistance and the reactance of the object.  Reactance is what we really want to look at here, and more specifically, the inductive reactance.  A speaker voice coil is, after all, an inductor.As inductance increases, so does the impedance of the object.  This is one of the primary driving forces behind an increase in impedance.Here is a picture that demonstrates how a speaker's impedance may vary with frequency.  Thank you to npdang at <a href="http://www.diymobileaudio.com" target="_blank">http://www.diymobileaudio.com</a> for the image.<img src="http://diymobileaudio.com/pics/830845/impedance.JPG" alt="" class="fr-fic fr-dii fr-draggable " style="" />Take a look at the orange trace.  It shows a large spike at the driver's resonant frequency (Fs), then flattens out, then increases proportionally with frequency.The proportional increase with frequency is due to the rising inductance of the driver.  As frequency increases, so does inductance, and so does impedance.What else can cause impedance to increase?  Well, we know that inductance increases with the amount of current flowing through the conductor (Linkwitz, Klippel, et al), so increased current can cause impedance to increase as well.  And because we've made that crucial link between inductance and impedance, we can assume that impedance will increase in the rearward direction due to Back EMF (Back ElectroMotive Force).Back EMF is the source of opposition to current flow (also sometimes referred to as CEMF, or Counter ElectroMotive Force).  Back EMF is caused by a change in the electromagnetic field.  Back EMF increases when the coil moves in the rearward direction because the magnetic field changes significantly in the rearward direction...the permeability of the free space changes.  So EMF increases and impedance increases in the rearward direction.For a subwoofer specifically, capacitance causes the largest increase in impedance.  Where inductance (and therefore impedance) increases proportionally with frequency, capacitance (and therefore impedance) is inversely proportional to frequency.  At resonance, the speaker network (as described by Olsen, Thiele, Small, et al) has an increased capacitance.  Since this occurs at low frequencies, there is a very large spike in capacitance (and therefore impedance).  This can be seen in the image I linked to above as the large impedance spike (on the orange trace) around resonance (below 30 Hz).Why does a speaker's impedance change from one box to another?  Well, the air inside and outside the enclosure represent an additional....environmental, I suppose....impedance to movement.  So you have impedance rise from the enclosure as well.And lastly, even the amplifier introduces some back EMF, so changing the amplifier can cause changes in impedance from one system to another.There are a lot of different reasons why impedance changes.  Even temperature plays a part.  In moving electromotive designs such as a speaker, predicting changes in impedance in every aspect is not a simple task.Perhaps not the best explanation, but hopefully it helps.Different homework you can read:1. Gaussian Law2. Lenz's Law3. Klippel's paper on driver Non-linearities.All can be found using google.</blockquote>

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