What Are Ohms? Everything You Need to Know About Speaker Impedance

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If you're shopping for a set of speakers or a receiver, it can be hard to to compare their relative performances, especially when there's only a handful of printed specs to choose from. It seems like the frequency response is always 20Hz-20kHz, everything seems to have 100 watts or thereabouts, and then there's ohms. What's an ohm? Do you need to match a speaker's ohms to a receiver's ohms? Is more, better? Is less somehow dangerous?

Technically speaking, ohms do matter when it comes to matching speakers and receivers. Yet in terms of practicality, not so much. Even still, ohms are a useful indicator to consider. I'm going to explain all this in as simple terms as possible, since the technical answer of "ohm is the unit of electrical resistance" isn't particularly helpful for anyone not currently in high school science.

What's an ohm? 

Ohm is the unit of electrical resistance. Just kidding. Well, it is, but this is how it's of most relevance to amplifying sound. Speakers have at least one, and usually several, drivers. These drivers are typically a magnet attached to a material that moves. When said material moves or vibrates against the air, it creates sound. The electrical field that causes this movement is created by an amplifier. How difficult the driver/drivers are to move is the part of what makes up the speaker's impedance, which is rated in ohms.

The vast majority of speakers on the market today are rated at or around 8 ohms, though you can find some rated as low as 4 ohms. Speakers that are lower than 4 ohms, or higher than 8 ohms, are rare, and usually reserved for the high-end.

I'm not going to judge anyone's setup, but usually it's best to have a chair face the speakers. Andreas Von Einsiedel/Getty Images

Why it doesn't matter (mostly)

Impedance ratings are basically useless. No speaker is the same impedance at all frequencies. If one is rated at "8 ohms," that could be at 1K, 500Hz, 20K or at literally any frequency. There's no regulation that says it has to be rated at a specific frequency. Manufacturers say "8 ohms" because that's what people expect. Is that an average across a range of frequencies? Depending on the design of the speaker, the impedance at any frequency could vary between 2 and 8 ohms, or even go as high as 40 ohms! This rating is almost always an average.

This "nominal" range is also what most receivers/amplifiers are designed to power. This is why most list their power ratings at 8 ohms as well (i.e. "100 watts at 8 ohms"), but most won't have a problem if the speaker is lower than that. If you're trying to run a 4 ohm speaker from a receiver rated at 8 ohms, the most likely thing is it will run a little hotter. It needs to work a little harder to supply the speaker with the power it wants.

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Would a cheap amp have problems powering a speaker with really low impedance? Yes, but generally speaking, the cheap speakers you would normally pair with an inexpensive amp are pretty easy to drive. Why would anyone pair hard-to-drive speakers with cheap amps? Best case, at least for competently designed products, is that if an amp is driven too hard it will simply switch off. This is referred to as "going into protection" and unless it's a particularly bad design, all you need to do is turn it back on again. The reason why this pairing isn't a good idea is due to that worst case scenario: something in the chain could blow and cause horrific damage -- whether it's the amp, speakers, or, even worse, to your hearing. CNET's general rule of thumb is to spend the same on an amplifier as you would do on a pair of stereo speakers.

It's worth noting that many companies who sell speakers that have a lower impedance often explicitly say "Compatible with 4, 6, or 8 ohm rated amplifiers." On the opposite side of the price spectrum, many home-theater-in-a-box systems (remember those?) were very low impedance. These were engineered with specific amplifiers in mind, which came in the box, and the pairing allowed companies to do a bit of creative marketing. As I'll explain at the end, an amp rated at 100 watts into 2 ohms is really only capable of 25 watts into 8 ohms. Which looks better on the side of a box?

OK, I am going to judge this one. The center channel is way too high, and not even angled down. The rear speakers are in the front (absolute blasphemy). There's even what seems like a second center channel on what I can only assume is a wood-paneled laser disc player. Just a lot of weirdness here. Archidea Photo/Getty Images

Exceptions

With the vast majority of speakers, receivers, and amps are all going to work interchangeably. Of course, they will! Manufacturers generally want the widest possible audience, so they're not going to design a product that won't work with a big chunk of it.

There are some speakers on the market, however, that simply need significant amplification. These hard-to-drive speakers typically have low impedance that requires a lot from an amp. The power supply, a vital component in any amplifier, has to provide enough power for the hungry speakers. Just as no one is trying to tow a trailer with a moped, hard-to-drive speakers need a decent amp.

These aren't the kind of speakers you'd find at Best Buy, and if you're in the kind of store selling these higher-end speakers, chances are the salesperson is also going to talk to you about getting an amp to go with them (as they should).

OK, now a little bit of math

If a speaker has low impedance, it's going to require more current at the same voltage. That is, if you halve the number of ohms then you need double the amount of power required. So, if an 8 ohm speaker requires 100 watts for a specific volume, a 4 ohm speaker would require 200 watts to produce the same volume (watts=amps X volts). Can a decent amp do that? Sure. Can a cheap amp do that? Maybe. Interestingly, many high-end amplifier manufacturers will actually boast about this. They'll rate their amps as, for example, "200 watts into 8 ohms and 400 watts into 4 ohms." This is showing that the amp's components are beefy enough to handle whatever a speaker requires of it.

Should you care? Unless your speakers, or ones that you're considering, are particularly hard to drive and you're trying to power them with a cheap or under-powered amp and you're blasting them at high volume, then no. Most mainstream speakers can be powered by pretty much all mainstream receivers and amps. They're all designed to do exactly that.

As well as covering audio and display tech, Geoff does photo tours of cool museums and locations around the world, including nuclear submarines, aircraft carriers, medieval castles, epic 10,000-mile road trips and more.

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