Speaker Impedance Rating Explained
That speaker impedance rating on your towers and bookshelf speakers is held up as a mystery—something strange and mystic, yet dangerous. “You’ll blow up your receiver!” says one “expert”. “Bah! You can run anything you want, just set that switch in the back to 4-ohms” says another. The bottom line is that people are confused about speaker impedance. The truth is, it’s not that big of a deal if you follow a few basic rules. We promise that, if you do, you won’t hear anything go “pop!”
But first, what is speaker impedance?
Speaker Impedance Defined
I think the easiest way to define speaker impedance is to say that it is the resistance any speaker gives to the current and voltage being applied to it. In a nutshell, a loudspeaker is a big resister—a really cool one. In fact, speaker impedance can often be referred to as a speaker’s resistance.
The tricky part is to realize that this isn’t a set thing. Speaker impedance changes based on the frequency of the signal fed into it. Since music has a lot of simultaneous frequencies, what you end up with is something called “nominal” impedance. That’s more or less the lowest amount the speaker will dip down to in resistance to the load applied at any given frequency within its operating range.
But specs lie…and so you have most speakers being configured to “more or less” fall into the 4-6 ohm range. While we think that you don’t typically have any issues with that reality, there are some steps you can take to avoid any potential pitfalls.
4-ohm Speakers Should Typically Be Mated with Higher Power Amplifiers
Most bookshelf and tower speakers are rated either 6-ohms or 8-ohms. Any speaker impedance rating that is 4-ohms is typically going to be a high-end, audiophile product that wants an amplifier that can really put out some power. This isn’t a “snobby” thing. The loudspeaker manufacturer likely developed a 4-ohm speaker because he knows what kind of amplifier would be required (or typically mated with it) to get the desired sound. With a lower impedance range, it also opens up different design choices and opportunities.
For most people, a 6-ohm or 8-ohm speaker is going to be the norm. Those types of speakers are particularly well-suited to match with the designs of most AV receivers. This isn’t to be taken as a blanket statement of course—a lot more goes into a loudspeaker than its impedance rating. Still, there are indicators you should be aware of when trying to understand impedance ratings and amplifiers.
Room Size Matters
The problem most people run into when mismatching speakers and amplifiers is that they try to get a pair of speakers to fill a larger room that they really can’t support. The amplifier ends up getting turned way up, and the end result is that the amplifier can’t handle the load. This is a real concern for when mating 8-ohm or 6-ohm nominal speakers with an AV receiver that’s not up to the task. In addition to matching the impedance of the speaker with the capability of the receiver or amp, you want to match the overall system to the room as well.
That Impedance Switch on Your Receiver is a Lie!
OK, it’s not really a lie, but it limits the output of your AV receiver or amplifier. Some AV receivers have an impedance selector switch on the back (Yamaha is famous for this) that opts between 4-ohm and 8-ohm or 4/8-ohm and 6/12-ohm. The idea is that it is safer to match the impedance of the amplifier to the speaker to avoid overpowering it or creating a difficult load on the amplifiers.
The problem is two-fold. One, loudspeakers don’t have “flat” impedance curves. They dip down and swoop up. That means that a 4-ohm speaker may spend a lot of its time in the 8-ohm or greater range. It also means that if you’re using a subwoofer, you may never get down to its lowest impedance dip areas. Second, the only way to really make a system “safer” is by limiting the power output of the amplifier or AV receiver. This is exactly what the setting does. As a result, that little switch cripples, ever so slightly, the output of that brand new AV receiver or amplifier you just purchased. Our recommendation is to leave it on the highest setting possible. That will give you the most power output regardless of the speaker impedance rating of your bookshelf or tower speakers. The nice thing is that making this change (or not making it) doesn’t require any fancy hand tools or other accessories.
Don’t Run Two Speakers Off a Single Amplifier Channel (without an impedance-matching selector switch)
I can’t tell you how many times I’ve walked into a living room to see an entire 5.1 home theater like the SVS Prime 5.0 speaker system parked right next to the TV. Or better yet—two completely unrelated speaker pairs side-by-side connected to the same stereo amplifier. And they wonder why the amplifier keeps shutting off. It just doesn’t make sense, they cry!
It makes perfect sense if you understand the math.
The formula for calculating electrical impedance is found through Ohm’s Law:
V = I Z , or
Voltage = Current x Impedance
We’re not talking about that. We’re going to leave that to another discussion. What we want to understand is what happens when you connect speakers of various impedances in series or parallel.
Because that’s exactly what you’re doing when you start to double-up on speakers in your home. Think you’ll never run into this issue? Perhaps not, but run a bunch of speakers overhead in a whole-home audio system and you might get your feet wet very quickly (figuratively speaking—I don’t recommend you run wires with your feet wet).
Calculating Speaker Impedance
Calculating loudspeaker impedance in series and parallel can be difficult if you don’t know the formulas. Once you know how to do it, however, then it’s just basic math.
Calculating a Speaker Impedance Rating in Series:
To do this is the easiest of all the calculations. You just add. To find the speaker impedance rating of a bunch of speakers in series, follow this formula:
I(All) = I(1) + I(2) + I(3) . . .
So if you have two 4-ohm speakers and an 8-ohm speaker, the total impedance for the system would be:
I(total) = 4 ohms + 4 ohms + 8 ohms
I(total) = 16 ohms
Calculating a Speaker Impedance Rating in Parallel:
Calculating this is a bit more difficult. To find the speaker impedance rating of a bunch of speakers in parallel, we utilize this more complex formula:
1/I(total) = 1/I(1) + 1/ I(2) + 1/I(3) . . .
So if you have the same speakers—two 4-ohm speakers and a 6-ohms speaker—the total impedance for the system would be:
1 / I(total) = 1/4 ohms + 1/4 ohms + 1/6 ohms
1 / I(total) = 0.667 ohms
I(total) = 1.5 ohms
As you can see, there would be a HUGE difference between hooking up some in-ceiling speakers in parallel vs. hooking them up in series. One will result in a leisurely load for your AV receiver or amplifier (series) while the other would likely shut it off once you brought the volume up to any sort of nominal level.
Author’s Note: For more information on Ohms, check out the great Wikipedia writeup here.
What you want to remember is this: match your speakers to your amplifier or receiver so that you aren’t forcing the amp to work overtime just to fill your room with sound. Product managers and engineers take great care in the design and construction of these audio products. Your best bet is to stay within the design parameters. When it comes to 4-ohm speakers and AV receivers, however, only the weakest products should give you trouble. If you have an entry-level receiver you may find that you can’t get all the volume you need (same goes for having a huge room). If, however, you have a mid-fi or high-end amplifier or AV receiver—there’s no need to baby your system. 4-ohm speakers are going to work just fine in most cases.
Above all, have fun and don’t sweat it.
And for those of you who hate math, here is a handy calculator that works well for figuring the impedance load on your amplifier in various loudspeaker configurations. It’s based on car audio, but it’s perfectly usable for home audio. Plus it’s so well done they deserve some credit!