Resistance vs Impedance:
Until this time, we only discussed resistance. I said that resistance is the opposition to the flow of current. While this is true, it would be more precise to say that resistance is the opposition to the flow of direct current. Impedance is the opposition to the flow of alternating current. AC causes some devices to act differently than they do with DC. This is true with speakers. If you use a voltmeter to measure the resistance of a speaker's voice coil, you may read something like 3.2 ohms (resistance) even if the speaker has a rated impedance of 4 ohms. The impedance of a speaker is not a constant. It's actual impedance changes with frequency and can vary greatly. Manufacturers give you a nominal impedance, which is generally the average impedance that the speaker will present to the amplifier when driven within the part of the audio spectrum that its designed to reproduce.