Quote:
Originally Posted by geofy
The Ohmic value of 4 or 8 is for DC resistance of the speech coil, the AC impedance of the speaker varies with frequency and can be several hundred Ohms....
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The impedance of a speaker will vary considerably with frequency. A single speaker will have a DC resistance which is constant and an inductive reactance which rises with frequency. This can be substantially modified by mechanical effects, notably the self resonance of the speaker where the impedance will usually rise to a peak. Capacitative effects can be largely ignored. A multi-unit speaker with crossovers will have an irregular looking impedance curve. A few speakers, notably Quad electrostatics present a very capacitative load to the amplifier. Not all amps are happy about that. Usually speakers are inductive and many amps have a Zobel network (R and C in series) across their output to fool the amp into thinking the total load is nearer to pure resistive.
Typically the rated impedance of a speaker is a pretty arbitrary figure, possibly an average across a frequency range which may or may not be specified. The DC resistance of the coil will always be lower than the quoted impedance.