For almost every speaker you can buy, you'll find a specification for impedance, measured in ohms (or Ω). But they never explain what impedance means.
Fortunately, impedance is kind of like great rock'n'roll. Understanding everything about it is complicated, but you don't need to understand everything about it to "get" it. So for this article, I'll tell you exactly what you need to know about speaker impedance without making you feel like you're taking a graduate-level course at MIT.
It's Like Water
When talking about things like watts and voltage and power, a lot of audio writers use the analogy of water flowing through a pipe. Why? Because it's a great analogy! That's why I'm going to use it here instead of coming up with something more original.
Think of the speaker as a pipe. Think of the audio signal (or, if you prefer, the music) as the water flowing through the pipe. The bigger the pipe, the more easily water can flow through it, and the more water can flow through it. A speaker with a lower impedance is like a bigger pipe. It lets more electrical signal through, and lets it flow more easily.
This is why you'll see an amplifier rated to deliver, say, 100 watts into 8 ohms impedance and maybe 150 or 200 watts into 4 ohms impedance. The lower the impedance, the more easily electricity flows through the speaker.
So does that mean you should buy a speaker with lower impedance? Not at all, because a lot of amplifiers aren't designed to work with 4-ohm speakers. Think back to that pipe carrying the water. You can put a bigger pipe in, but it'll only carry more water if you have a pump powerful enough to provide all that extra water.
Does Low Impedance Mean High Quality?
Take almost any speaker made today, connect it to almost any amplifier made today, and you'll get more than enough volume for your living room. So what's the advantage of, say, a 4-ohm speaker versus an 8-ohm speaker? None, really. Except one: Low impedance sometimes indicates the amount of fine-tuning the engineers did when they designed the speaker.
First, a little background. The impedance of a speaker changes as the sound goes up and down in pitch (or frequency). At, say, 41 Hz (the lowest note on a standard bass guitar), the impedance of your speaker might be 10 ohms. But at 2,000 Hz (getting into the upper range of violin), the impedance might be just 3 ohms. Or it could be reversed. The impedance specification you see on a speaker is just a rough average. You can see in the chart at the top of the article how the impedance of three different speakers changes as the frequency of the sound changes.
Some of the more exacting speaker engineers like to even out the impedance of the speaker for more consistent sound through the whole audio range. Just as when sanding a piece of wood you remove the high ridges of the grain, a speaker engineer might use electrical circuitry to flatten the areas of high impedance. This is why 4-ohm speakers are common in high-end audio, but rare in mass-market audio.
Can Your System Handle It?
If you do choose a 4-ohm speaker, make sure your amplifier or receiver can handle it. How do you know? Sometimes it's not clear. But if the amp/receiver manufacturer publishes power ratings into both 8 and 4 ohms, you're safe. Most separate amplifiers (i.e., without a built-in preamp or tuner) can handle 4-ohm speakers, as can probably any $1,300-and-up A/V receiver.
I'd be hesitant, though, to pair 4-ohm speakers with a $399 A/V receiver or a $150 stereo receiver. It might be OK at low volume, but crank it up and the pump (amplifier) might not have the power to feed that bigger pipe (speaker). Best case, the receiver will shut itself off temporarily. Worst case, you'll be burning up receivers faster than a NASCAR driver wears out engines.
Speaking of cars, one last note: In car audio, 4-ohm speakers are the norm. That's because car audio systems run on a 12 volts DC instead of a 120 volts AC. A 4-ohm impedance allows car audio speakers to pull more power from a low-voltage car audio amp. But don't worry: Car audio amps are designed for use with low-impedance speakers. So crank it up and enjoy! But please, not in my neighborhood.
Image of impedance chart by Brent Butterworth