speaker impedance explained: 4, 6 and 8 ohms

Nominal impedance is a summary, not a measurement, and it moves while music plays. What that asks of your amplifier, and whether you can run 4-ohm speakers.

Four ohms, six ohms, eight ohms. It sits in the specification list looking like a settled property of the speaker, as fixed as the height of the cabinet, and then you start reading around and it comes apart in your hands: your amplifier quotes its power twice at two different loads, its manual asks for something between 4 and 8, and a speaker sold quite openly as an 8-ohm design turns out to dip to 3.1 ohms once you find the full specification. Little wonder impedance ends up filed somewhere between engineering and superstition.

So here is the whole of it, before the explanation. Impedance is the electrical load a passive speaker puts on the amplifier driving it, and the lower that load falls, the more current the amplifier has to find. The figure on the box is nominal, meaning representative rather than true, because the real value swings up and down continuously while music plays. The number worth hunting for is the minimum, and the only check most people ever need to make is that the amplifier's manufacturer permits the load the speakers actually present. Get that right and you've done the ninety per cent of this that matters.

why the number will not stay still

Music is an alternating signal changing in level and frequency thousands of times a second, and the speaker it arrives at is nothing like a simple resistor. Voice coils have inductance, crossover components have capacitance, the wire itself has plain resistance, and every one of those behaves differently as the frequency changes. What the amplifier meets is the combined result, a curve running across the audio band that climbs well above the printed number at one frequency and falls a long way below it at another. Nominal is a summary of that curve. It is the label on the map, not every hill and valley in the landscape.

Wharfedale's Super Denton shows how little the headline tells you, because Wharfedale give it three numbers: six ohms nominal, described as 8 ohm compatible, with a minimum of 3.4 ohms. All three are true, and they answer three different questions. What it broadly looks like, what it is safe to connect it to, and how hard it gets when it is working hardest.

That last one is the figure to chase, because it decides how much current your amplifier has to produce, and it is the one most likely to be missing from a shop listing. Wharfedale's REVA-3 is sold as 8 ohm compatible and has a 3.7 ohm minimum; the Bowers & Wilkins 702 S3 is 8 ohm nominal with a minimum of 3.1. Both figures come from the makers' own documentation. Two speakers that both say 'eight ohms' on the brochure can ask quite different things of the same amplifier, and depth is not the whole story either, because where the dip sits, how long the music spends down there and the electrical phase alongside it all change how hard the load really works. That is why makers state a permitted range rather than a permitted number.

so can you run 4-ohm speakers or not

Usually yes, and the way to find out is to read what your amplifier's maker actually says rather than reasoning it out from first principles.

Most modern integrated amplifiers are specified for a range, and if that range covers the load your speakers present, the basic check has passed. Some come with a condition attached, which is more useful than it sounds. Rega recommend a nominal impedance of 6 to 8 ohms for the Brio MK7 and say it can run speakers as low as 4 ohms, with the caveat that such speakers may cause the thermal protection circuit to engage. That isn't a refusal. It is a manufacturer telling you where the edge of the envelope sits and what happens when you reach it, which is worth knowing on the day you choose the speakers rather than the day the amplifier goes quiet in the middle of something loud.

The other direction deserves more respect than it gets. If the manual asks for 8 ohms or higher, do not assume a 4-ohm speaker will be fine on the grounds that you listen quietly. It may well be, right up until a bass-heavy passage and an optimistic volume setting ask for current the power supply cannot comfortably deliver, and then you get heat, or audible distortion, or a protection circuit shutting the whole thing down mid-track. Amplifiers rarely fail dramatically when overloaded. They just stop being good, and then they stop.

nobody's speakers sound better for being eight ohms

There is no best impedance and no ladder to climb. A 4-ohm speaker is not more powerful, an 8-ohm speaker is not more refined, and a 6-ohm one is not a diplomatic compromise between the two. Designers arrive at the load through the drivers they chose and the crossover they built to suit them, and your job is not to grade the result but to give it an amplifier comfortable with it.

Where this genuinely gets muddled is with sensitivity, a different specification answering a different question. Impedance tells you how hard the speaker loads the amplifier; sensitivity tells you how much sound comes out for a stated input at a stated distance, and a speaker can be sensitive and still be a demanding load. There is a trap in the measurement, too: a figure quoted as 2.83 volts at one metre is roughly one watt into 8 ohms but roughly two watts into 4, so two speakers showing the same number are not necessarily equally efficient when their impedances differ. Our amplifier matching guide puts load, sensitivity, room and distance together, because impedance only gives a pairing permission to exist. It says nothing about how loud it will be by the time it reaches your chair.

the two-pair trap

Plenty of stereo amplifiers have A and B speaker terminals, and they look like an open invitation to run two pairs at once. Often those terminals put the pairs in parallel when both are selected, and parallel loads don't add up, they divide. Two nominally 8-ohm pairs present the amplifier with roughly 4 ohms per channel. Two 4-ohm pairs present roughly 2, which is outside what most domestic amplifiers will tolerate.

The makers who have thought about it publish a separate rule, and those rules are specific. NAD state that the C 3050 takes a minimum of 4 ohms for one pair but wants 8 ohms if both are used at once. Arcam are blunter still about the DiVA A85: both pairs must be 8 to 16 ohms if used together, and below that the overload protection engages and the amplifier will not play at all. Separate terminals are not proof of separate amplifiers, so check the permitted load for A alone, B alone and A plus B before you press both buttons.

Four terminals on the back of a speaker are a different matter, and the two get confused constantly. A bi-wireable speaker takes two runs from the same amplifier terminals into the two halves of its own crossover, so the amplifier is still driving one speaker and the load does not change. Our bi-wiring and bi-amping guide covers what the metal links are for. Cable enters the arithmetic only at the margins: too thin or too long wastes amplifier voltage and loosens control over the drivers, a good reason to choose properly as our speaker cable guide sets out, but no gauge of wire turns an unsuitable 4-ohm load into a well-behaved 8-ohm one.

why your multimeter says six when the box says eight

Put a basic multimeter across the terminals of a disconnected speaker and it will very likely read below the nominal rating, often around six on an 8-ohm design. The speaker is not faulty and the manufacturer has not lied to you. A multimeter tests with direct current, so what it reads is the DC resistance of the voice coil and whatever crossover path sits in front of it, and direct current can tell you nothing about behaviour across the audio band. No frequency, no curve to see.

The reading is still worth taking, mind. It will find an open circuit, a broken joint, a disconnected driver or a failed crossover, and for those jobs it is the right tool. It just cannot tell you how demanding the speaker will be while music is playing, and measuring that properly takes a swept signal recording magnitude and phase together, far more than a check two manuals will settle between them.

the two-minute checklist

Read the nominal figure as a summary, then go looking for the minimum, because that is the number your amplifier has to cope with. Check the manual permits that load, and treat any caveat about protection circuits as information rather than small print. Do not rank speakers by the number, and do not confuse impedance with sensitivity, particularly when sensitivity is quoted at 2.83 volts. Running A and B pairs together, find the combined rule, which is almost always the stricter one. Four terminals on a speaker means bi-wiring and changes nothing about the load. And if a multimeter reads six where the box said eight, it is measuring DC resistance and both of them are right.

So what do 4, 6 and 8 ohms actually mean? They are the speaker's summary of how hard it intends to make your amplifier work. Not a grade, not a quality ranking, and not a promise it will stay there once the music starts. Our planner works from the sockets on the back of your boxes and tells you which cable joins them, but the impedance check is the one it cannot make for you, because that number lives in the manual rather than on the panel. Look it up once, before you buy, and then forget about it for as long as you own them. Happy listening.