Speaker Impedance Explained
Nominal impedance is a single number standing in for a curve that varies by a factor of five across the audio band — and the number that actually matters is the one almost nobody publishes.
Impedance is a speaker’s opposition to the alternating current an amplifier pushes into it. It is measured in ohms, and the number printed on the box is called nominal impedance.
The word nominal is doing an enormous amount of work. A speaker’s real impedance is not a number. It is a curve, and it varies wildly across the audio band. A speaker sold as 8 ohms may present 30 ohms at its bass resonance, dip to 3.5 ohms somewhere in the upper bass, climb again through the crossover region and settle somewhere else in the treble. The single figure on the box is a rough average of that curve, arrived at by conventions that differ between manufacturers.
Nominal versus minimum, and why minimum is the one that matters
Your amplifier does not care about the average. It cares about the dip.
Halving impedance doubles the current an amplifier must supply at the same voltage. An amplifier that is comfortable into 8 ohms may run out of current, distort, or trip a protection circuit at 3 ohms — and if that dip happens to sit at a frequency where music has a lot of energy, you will hear it. This is why a nominally 8-ohm speaker can be a harder load than a nominally 6-ohm one.
So the useful specification is minimum impedance. Here is who publishes it across the speakers we cover:
| Speaker | Nominal | Minimum |
|---|---|---|
| Q Acoustics 3030i | 6 Ω | 4 Ω |
| KEF Q3 Meta | 4 Ω | 3.2 Ω |
| ELAC Debut 2.0 B6.2 | 6 Ω | Not published |
| Klipsch R-41M / R-51M / RP-600M II | “8 Ω compatible” | Not published |
| Polk Monitor XT15 / Signature Elite ES15 | “4- and 8-ohm compatible” | Not published |
| Micca RB42 | “4–8 Ω” | Not published |
Two speakers out of the whole set. And note the distance between the two columns in each row: the Q Acoustics is nominally 6 ohms and reaches 4, the KEF nominally 4 and reaches 3.2. Nominal impedance is a label chosen by the maker; the minimum is the moment the amplifier actually has to survive, and it sits roughly a third lower in both cases. Every other speaker here publishes only the label.
The speakers that publish “compatible with” language rather than a measured figure may well dip just as low. You simply have no way to know, and that absence costs them places in our rankings.
Why 2.83 volts is not one watt
This is where impedance quietly contaminates a specification most people read as straightforward.
Sensitivity is normally published at 2.83 V/1 m. That voltage was chosen because, into 8 ohms, it delivers exactly 1 watt: P = V²/R = 2.83²/8 = 1.00 W. Into anything less than 8 ohms it delivers more than a watt — so the same speaker measured at 2.83 V into 6 ohms is being given 1.33 W and posts a higher number than it would at a true 1 W.
P = V² / R · Sensitivity(1 W) = Sensitivity(2.83 V) − 10·log₁₀(P)
Inputs
- ELAC B6.2 — published sensitivity and impedance: 87 dB at 2.83 V / 1 m, 6 Ω nominal (ELAC)
- Q Acoustics 3030i — published sensitivity and impedance: 88 dB at 2.83 V / 1 m, 6 Ω nominal (Q Acoustics)
- Micca RB42 — published sensitivity: 83 dB at 1 W / 1 m (already the 1 W condition) (Micca)
Result
Into 6 Ω: P = 2.83² / 6 = 1.33 W → correction = 10·log₁₀(1.33) = 1.25 dB
ELAC B6.2: 87 − 1.25 = 85.8 dB / 1 W / 1 m
Q Acoustics 3030i: 88 − 1.25 = 86.8 dB / 1 W / 1 m
Micca RB42: 83.0 dB / 1 W / 1 m (no correction needed)
On the same basis the Micca is 2.8 dB behind the ELAC, not the 4 dB the raw headline numbers suggest. The correction does not rescue it, but it does shrink the gap — and it is invisible unless you do the arithmetic.
This correction uses nominal impedance, which is itself an average of a varying curve. It is the best available approximation and it is not a measurement. It also cannot be performed at all on any speaker whose maker publishes no numeric impedance — which, as the table above shows, is most of them.
Can my amplifier drive 4 ohms?
Look for a published continuous power rating into 4 ohms. If there is one, the answer is yes and the number tells you how well. If there is not, you are guessing.
This is a real and current gap, not a hypothetical one:
- Yamaha A-S501 and A-S301publish no continuous 4-ohm rating at all. What circulates as “185 W into 4 ohms” for the A-S501 is Yamaha’s High Dynamic Power figure — a short-burst IHF measurement, not an RMS rating. Treating it as continuous overstates the amplifier by roughly two-fold.
- Denon PMA-600NEpublishes 70 W + 70 W into 4 ohms — but at 1 kHz and 0.7% THD, where its 8-ohm figure is rated 20 Hz–20 kHz at 0.07%. That is a much softer test for the harder load.
- Marantz PM6007publishes 60 W × 2 into 4 ohms across the full 20 Hz–20 kHz band with both channels driven. That is the honest way to do it, and it is the exception rather than the rule.
- Cambridge AXA35 publishes 35 W into 8 ohms and nothing for 4 ohms.
So you can end up with a speaker whose 4-ohm dip is published and an amplifier whose 4-ohm behavior is not. Nobody is lying. You simply cannot complete the calculation, and knowing that is better than assuming it worked out.
Practical rules
- Never wire two 8-ohm pairs in parallel onto one set of amplifier terminals unless the amplifier explicitly supports it. Two 8-ohm speakers in parallel present 4 ohms, and their combined dips will go lower.
- A multichannel receiver delivers less per channel than its headline when several channels work at once. Q Acoustics is the only maker here honest enough to publish separate amplifier recommendations for a stereo amp and an AV receiver.
- Impedance is not sound quality. A 4-ohm speaker is not better or worse than an 8-ohm one; it is a different load. What matters is whether your amplifier is happy driving it.
- Speaker cable resistance is not impedance, but it adds to the load. On a 4-ohm speaker, thin cable over a long run is a larger proportional problem than on an 8-ohm one.
Once you know your speaker’s impedance and your amplifier’s rating into it, the power-versus-loudness arithmetic tells you whether the pairing actually goes loud enough. That is the calculation that ends the argument.
Frequently asked questions
What does speaker impedance mean?
It is the speaker's opposition to the alternating current an amplifier supplies, measured in ohms. The figure on the box is nominal impedance — a single number standing in for a curve that varies substantially across the audio band. A speaker sold as 8 ohms might present 30 ohms at bass resonance and dip to 3.5 ohms somewhere in the upper bass.
Is 4 ohm or 8 ohm better for speakers?
Neither is better; they are different loads. A 4-ohm speaker draws twice the current of an 8-ohm one at the same voltage, so it asks more of the amplifier. What matters is whether your amplifier publishes a continuous rating into the impedance you are handing it — and many do not. Yamaha publishes no continuous 4-ohm rating for the A-S501 or A-S301 at all.
What is minimum impedance and why does it matter more than nominal?
Minimum impedance is the lowest point on the impedance curve — the dip where an amplifier actually struggles. It matters more than nominal because halving impedance doubles the current demand. The KEF Q3 Meta is nominally 4 ohms and dips to 3.2; the Q Acoustics 3030i is nominally 6 and dips to 4. In both cases the minimum sits about a third below the nominal figure, which is why an amplifier rated only into 8 ohms tells you nothing useful about either — and why the two makers that print the minimum are doing something the rest are not.
Why is sensitivity quoted at 2.83 volts instead of 1 watt?
Because 2.83 V into 8 ohms is exactly 1 watt, so for an 8-ohm speaker the two conditions are identical. Into a lower impedance, 2.83 V delivers more than a watt — 1.33 W into 6 ohms — so the published figure runs about 1.25 dB higher than a true 1 W measurement would. It is a convention, not a trick, but it means you cannot compare a 2.83 V figure from a 6-ohm speaker to a 1 W figure from another without correcting first.
Can I connect two pairs of speakers to one amplifier?
Not safely onto one set of terminals unless the amplifier explicitly supports it. Two 8-ohm speakers wired in parallel present 4 ohms, and their impedance dips combine to go lower still. Amplifiers with A/B speaker switching usually handle one pair at a time; if you need both at once, check the manual for a stated minimum impedance.
Read next
Will This Amplifier Drive My Speakers?
Two published numbers, one formula, and the answer is almost always less power than you expected.
The Best Bookshelf Speakers Under $300
Five passive pairs under $300, and a ten-decibel sensitivity spread that decides which amplifier each one needs.
The Best Bookshelf Speakers Under $500
Five pairs under $500 — and only one publishes a minimum impedance and a recommended amp range.
Passive vs Powered Speakers
Total cost, upgrade path, and why powered speakers correctly refuse to publish a sensitivity figure.
Speaker Sensitivity Explained
2.83 V/1 m versus 1 W/1 m, what the conversion is worth in decibels, and the twelve-decibel spread that means sixteen times the power.
Sources
Every specification on this page was read from one of these documents. If one of them has changed, or we have made an error, tell us — corrections are logged and dated per our editorial policy.