Question about speaker impedance ratings

[Special K]

Speaker impedance ratings are given in ohms, but my understanding is that speakers are not purely resistive devices. So my question is, is the impedance ratings given to speakers really only the resistive component of the speaker, or is it really the impedance, and they just leave off the phase angle?

 

[xxXXDeathXXxx]

Units of impedance are ohms. For reactive circuits like speakers, a given impedance only makes any sense when there is a frequency given with it. This means that the rated speaker impedance is only what is stated while measured at a certain frequency which is likely somewhere in the audio range. The impedance will be different at other frequencies. For example at DC you will probably see less resistance than rated for most audio speakers and the rated impedance will only be seen by some signal in the audio frequency range.

 

[bobsmith1492]

The Laplace transform of an inductor (basically what a voice coil is) gives a resistance of SL where L is the inductance and S is the frequency. So as frequency decreases, impedance decreases as well. Yay for circuit analysis class!

The question is does that act in series with the pure resistance? I would guess it does (all that wire in the winding would make a resistor) so the minimum expected impedance would be what you would measure with an ohm meter (S = 0 so the inductor doesn't matter).

I've noticed when measuring speakers that they are always a bit lower in DC resistance than they are rated, so I would guess they choose some minimum frequency to add onto the DC resistance for the "average" or whatever resistance (that's on the label.)

 

[Howard]

Speaker impedance, as given by the manufacturer, is only a nominal specification at some frequency which includes the effect of inductance (IIRC) on top of the DC resistance.

Take a look at any speaker's impedance chart and you'll understand.