Why is the impedance that I measure with a multimeter different from the systems nominal impedance?
The multimeter looks at resistance, not impedance. Impedance is the opposition to the passage of alternating current and varies with frequency. Resistance is the opposition to the passage of direct current. Resistance is normally lower than nominal impedance. For instance, an 8-ohm nominal impedance cone speaker would typically have a (DC) resistance around 6 ohm. See any DAS spec sheet for an impedance curve. Dedicated impedance meters for audio use exist; they typically use a sinewave generator to measure impedance at 1 kHz, so you need to look at the speaker’s impedance chart to see what value you should expect at that frequency.
