Impedance is the electrical load a passive speaker presents to the amplifier driving it, quoted in ohms. It appears in the spec table for the 76 passive speakers here that publish it, split 31 at 4 ohms, 23 at 6 ohms and 22 at 8 ohms. Only passive speakers carry the figure at all, because a powered or portable speaker has its amplifier built in and matched at the factory.
The short version: 8 ohms is an easy load that almost any amplifier drives comfortably, 4 ohms asks the amplifier for twice the current, and the single number on the box is an average that conceals the parts of the curve an amplifier actually struggles with.
What The Number Actually Describes
A speaker’s impedance is not fixed. It varies continuously with frequency, rising sharply at the driver’s resonance, dipping in the crossover region, and climbing again at the top of the treble. A single speaker might swing between 3 and 30 ohms across its range.
The figure in a spec table is the nominal impedance, a manufacturer’s summary of that curve. There is no strictly enforced standard for how it gets calculated, which is why two speakers with genuinely different loads can both be labelled “8 ohms”.
4, 6 And 8 Ohms Across The Speakers Ranked Here
4 ohms (31 speakers). The most demanding common load. The speaker draws roughly twice the current of an 8 ohm design for the same voltage, so the amplifier works harder and runs hotter. Most competent solid-state amplifiers handle it without complaint. Many budget receivers and most tube amplifiers do not, at least not without using the correct output tap.
6 ohms (23 speakers). A middle ground that has become common in modern bookshelf designs. Comfortable for the large majority of amplifiers, though a nominal 6 ohm speaker can still dip to 3 ohms somewhere in the bass.
8 ohms (22 speakers). The traditional easy load. If an amplifier is rated for anything, it is rated for this. A tube amplifier with an 8 ohm tap is on comfortable ground here.
Why It Matters For Your Amplifier
Halving the impedance doubles the current the amplifier must supply for a given voltage. A well designed solid-state amplifier responds by delivering close to double its rated power into 4 ohms. A weaker one hits its current limit instead, and the result is audible distortion at high volume or a protection circuit shutting the amplifier down.
Tube amplifiers behave differently again, because they drive the speaker through an output transformer with taps for specific loads. Connecting a 4 ohm speaker to an 8 ohm tap is not dangerous, but it does change the frequency response and reduce available power. If you are matching speakers to a tube amplifier or any low-power design, best passive speakers for tube and low-power amplifiers treats impedance and sensitivity together, which is the pairing that actually decides the outcome.
What The Published Figure Hides
Two things matter to an amplifier that the nominal rating does not report.
Minimum impedance. A speaker labelled 6 ohms that drops to 3 ohms in the upper bass is a considerably harder load than the label suggests, and the bass is exactly where the current demand is highest.
Phase angle. Impedance has a phase component as well as a magnitude. A load that is both low in impedance and strongly reactive at the same frequency is far more demanding than the magnitude alone implies. This is why some speakers with unremarkable nominal ratings have reputations as amplifier killers.
Neither figure is published by most manufacturers. Where an independent measurement exists, the review links on a speaker’s own page are the place to find it.
Where To Look Next
Impedance is one half of the amplifier matching question. The other half is sensitivity explained, which sets how much power you need in the first place. For what amplifier power ratings do and do not tell you, see speaker watts explained. For the enclosure side of a passive speaker’s design, see woofer cabinet explained.
Every passive speaker’s published impedance is in its spec table. The full rankings are at large bookshelf speakers, medium bookshelf speakers and large floorstanding speakers.
Frequently Asked Questions
Is a 4 ohm or an 8 ohm speaker better? Neither is better sounding. An 8 ohm speaker is an easier electrical load and works with a wider range of amplifiers, including tube and budget designs. A 4 ohm speaker draws twice the current and needs an amplifier comfortable delivering it. The impedance says nothing about sound quality on its own, only about what it takes to drive the speaker properly.
Can I run 4 ohm speakers on an 8 ohm amplifier? Usually yes with a solid-state amplifier, which will simply work harder and may run hotter. Problems appear at high volume, where an amplifier that cannot supply the current will distort or trigger its protection circuit. With a tube amplifier, use the 4 ohm output tap if one exists, because the transformer tap rather than the amplifier’s headroom is what matches the load.
What does nominal impedance mean? Nominal impedance is a single summary figure for a curve that varies continuously with frequency. A speaker’s real impedance rises at driver resonance, dips near the crossover, and can swing between roughly 3 and 30 ohms across its range. There is no strictly enforced standard for calculating the nominal value, so two speakers labelled 8 ohms can present meaningfully different loads.
Why do powered and portable speakers not list impedance? Because the amplifier is built in and matched to the drivers at the factory, so there is no matching decision left for the owner to make. Impedance appears only on passive speakers, where you supply the amplifier yourself.