Easy Answers to Confusing Specs: Sorting Out "Impedance"
You've seen references to "impedance" and "ohms" in various loudspeaker specifications or in your owner's manual for an AV receiver. But what is it? Do you have to "match" speaker impedance to your AV receiver or amplifier?
Let's first get a couple of things clear: Impedance has nothing—I repeat, nothing—to do with sound quality. It is an electrical measurement of a loudspeaker's resistance; its opposition to the flow of electric current (the audio signals) from your AV receiver or amplifier through your speaker cables to the speaker drivers and the fine wire in the driver voice coils. It's a kind of electrical "friction" to the movement of electrons through the copper wires in your speakers. We measure impedance in "ohms," named after George Ohm, the German physicist.
In a loudspeaker, current does all the work; voltage is the "push" behind the current, kind of similar to the way water pressure (voltage) forces the water (current) through a hose. If you have a narrow hose (a high impedance), not as much water (current) flows. Use a larger diameter hose (lower resistance) and more water (current) flows.
Current has to flow through your loudspeakers, but we certainly don't want them or the speaker cables to heat up and waste all your amplifier power! We want the electric audio signals to drive the speakers to produce great sound.
Loudspeakers have impedances of 8 ohms, 6 ohms or 4 ohms (those are "nominal" or approximate values, because the impedance of a speaker changes all the time with the different frequencies of music). A 4-ohm speaker draws more electric current through your AV receiver's output transistors, and since more current equals greater power, 4-ohm speakers tend to have greater dynamic range and play louder more easily than 8-ohm speakers.
AV receivers also produce more power into 4-ohm speakers than 8-ohm speakers, as much as 50% more. There isn't any way you can lower the impedance of your speakers--that's set by the designer and the voice coil windings and crossover parts, but you can check the impedance of any speaker by looking at the identification plate on the speaker's rear panel, where its impedance will be stated in ohms.
Your AV receiver has essentially zero output impedance (0 ohms) so you do not have to match the impedance of your amplifier to the speakers. The amplifier does not expect to "see" speakers of given impedance and you can connect speakers with different impedances (8 ohms, 6 ohms, 4 ohms) to an AV receiver with no negative effects so long as the impedance of any of your speakers doesn't go below 4 ohms. If speaker impedance is too low, too much current will run through the AV receiver's output transistors, causing the receiver to overheat and shut down. If you get 4-ohm speakers, make sure your AV receiver is able to drive them easily without overheating. Some brands of AV receivers have no problems driving 4-ohm speakers, others cannot.
Finally, you do not want your speaker cables to raise impedance or resistance and waste your AV receiver's power on its way to your speakers. So use 12-gauge speaker cables between your AV receiver and any loudspeakers and you won't have problems of increased resistance. You can run 12-gauge speaker cable to lengths of 50 feet or more without problems.