What do ohms mean for speakers, and is it important? If you've nothing better to do, learn about ohms and speaker impedance matching. You may even enjoy it.
My word, you’re really heading into geek territory here!
Speaker impedance matching, now here’s a subject that can send even the most hardened technophile running for cover.
It’s also one of the favorite subjects that the nerds on the forums use to bully poor unsuspecting newbies.
Well, you won’t get any of that around here – and when you actually look into this subject, it’s pretty simple, really.
This article will help you understand the concept of matching impedance in speakers and amplifiers – in as few words as possible – and then you can go and do something more interesting instead.
What Is Resistance?
The resistance of an electrical circuit refers to how hard it is to send an electrical signal through it.
The more resistance, the harder it is to send the signal.
What’s the Difference Between Resistance and Impedance?
Good question! You’re getting the hang of this geeky thing.
For simpletons like me who aren’t concerned about the finer details, the simple answer is that resistance and impedance are essentially the same things, i.e., they are both a measure of how hard it is to send an electrical signal through a circuit.
Resistance is the term used when a circuit uses direct current (DC), whereas impedance is used when a circuit uses alternating current (AC).
An amplifier sends an AC current to a speaker – so we should use impedance when dealing with speakers.
The main difference is that impedance will vary according to the frequency.
This is why you may see some speakers quoted with an impedance range – because the impedance will change depending on the audio signal’s frequency.
Resistance and impedance are measured in ohms and can be written using a little squiggly omega symbol like this – Ω.
What About Current and Voltage?
Own up – you’re really getting into this now, aren’t you?
An amplifier sends the audio signal to a speaker as an AC electrical current, measured in amps. And, this current is ‘pushed’ to the speaker with a voltage.
Now, Ohms Law states that:
Current = Voltage ÷ Resistance
Therefore if the resistance goes down, then either the voltage or the current must increase, which puts an increased strain on the power supply of the amplifier.
For example, if you disconnect your 8-ohm speakers from your amplifier and connect 4-ohm speakers, the resistance goes down.
Less resistance allows more current flow, and so the amplifier will have to deliver more power to the speakers – which it may not be designed to do.
What Has Impedance Got to Do with Your Speakers?
All speakers have an impedance.
This impedance will vary depending on the size and design of the speaker, and it is essential to remember that one set of speakers may have a different impedance than another.
Speakers designed for use in the home are usually rated at either 4, 6 or 8 ohms, although more specialized models may fall outside this range.
If you are buying new home theater speakers, you should check their rated impedance.
The impedance will usually be indicated as a nominal value – meaning it’s an average figure – so the actual impedance over time can be higher or lower than the average.
Four-ohm speakers have a lower average resistance than 8-ohm speakers – and therefore draw more current.
One benefit of 4-ohm speakers is that the increased current means they can be turned to high volumes more easily.
Or, to put it another way, it takes less voltage to drive a 4-ohm speaker to the same sound pressure level (SPL) as an 8-ohm speaker.
This can be good if you have an underpowered amplifier – but not so good if you turn it up too loud.
In the picture above, the impedance is listed with a minimum amount, and this refers to the lower impedance limit the speaker will reach – depending on the audio signal’s frequency.
Your amplifier needs to be able to handle that lower impedance.
Matching the Impedance of Speakers and Amplifiers
So, in terms of our amplifier, the important thing is the connected impedance load.
It is the speaker that has the impedance.
Therefore, the impedance you see listed for an amplifier refers to the optimum speaker impedance it is designed to drive.
You can connect speakers of any impedance to an amplifier, and they will work.
Your speakers won’t explode as soon as you switch on the amp.
However, if the amplifier isn’t designed to drive speakers with lower impedance (4-ohms, for example), then the amplifier may overheat if you turn the volume up very loud.
This is because it will draw more power than the unit is designed to deliver.
At that point, the amplifier will shut itself down before it causes too much damage.
Therefore, you should try to match the impedance of your speakers to the impedance the amplifier is designed to drive – then you shouldn’t have any problems.
That is unless you try really hard with the volume control!
The thing you need to look out for in the specification of an amplifier is the impedance range (if there is one).
If it states 4-8 ohms, the amplifier has been designed to handle 4, 6 or 8-ohm speakers, which are the most common impedance ratings for audio speakers in the home.
If it says 6-8 ohms, you can still connect 4-ohm speakers to it, and it will work.
But if you need to turn up the volume control too far to get a good sound in your room, then you are in danger of overloading the power supply, and the amplifier will shut down.
The problem with this is your speakers may be damaged before the amplifier shuts itself down.
The critical thing to remember is that if you want low-resistance speakers (4-ohms), you need to make sure your amplifier/receiver will drive these easily.
If there is no impedance range listed for the amplifier or receiver, then you can get a good idea from the listed power rating.
If the power rating is rated into an impedance of 8 ohms – then you should be confident it will handle 8-ohm speakers.
Similarly, if there is another rating listed into 6-ohms, it should also handle 6-ohm speakers fine.
If you are unsure, then it may be wise to double-check with the retailer or manufacturer first.
The bottom line is most modern amplifiers and receivers will handle 8 ohms – and probably 6-ohm speakers with no problems.
It may also happily run a 4-ohm speaker, as long as you don’t turn it up too loud.
If you plan to use speakers with an impedance below 6 ohms, you may want to double-check to make sure.
If you want a little more detail on this subject, check out my article on matching speakers and amplifiers.
Should You Change the Impedance Setting on Your AV Receiver?
On most modern AV receivers, there is a setting in the setup menu to tell the receiver the impedance of the speakers you have connected to it.
In some cases, it may be a physical switch on the rear of the unit.
If you read the manual, it will suggest that you set it depending on the impedance of the connected speakers.
So, if you have speakers rated at 4 ohms, you should change the setting to 4 ohms. This setting is sometimes called high and low impedance.
The theory is that this will make sure the amplifier delivers the correct current to your speakers – in most cases, the default will be set to 8 ohms.
Now, you may follow these guidelines if you like. But generally, you should leave the impedance setting at the default of 8 ohms – regardless of the impedance of the speakers you are using.
In real-world situations where you are playing movie and music audio, you will have plenty of headroom to play with – especially if you have a powered subwoofer and are routing most of the low frequencies away from your main speakers.
This impedance option is there to get the unit officially rated for lower impedance speakers – so the manufacturer can put it in the manual and label it on the outside of the box.
However, if you change this setting, all you will do is reduce the performance of your amplifier, and it will lower the output voltage of the receiver and consequently reduce the current sent to your speakers.
In simple terms, it will reduce the amount of available power and is more likely to send distorted signals to your speakers – and it is these clipped signals which can damage your speakers.
Remember what I said previously: a speaker’s stated impedance is a nominal value – an average.
So depending on the audio frequency it receives over time, the speaker’s actual impedance will be higher or lower than its nominal impedance.
Just be careful with the volume control on the amplifier, and you will be fine.
This video explains this in a bit more detail. These are clever guys who know what they are talking about.
However, it is a little geeky, so for some of you, my (hopefully) more straightforward explanation should suffice:
In my guide to the best AV receivers, I have some specification tables which highlight the supported speaker impedance of the latest AV receivers.
So as you can see, after going through this physics lesson for dummies, you can see there isn’t a great deal to know.
But it is something that you need to be aware of.
These days, most modern amplifiers/receivers will quite happily drive pretty much any set of speakers, and so in most cases, you can ignore this issue altogether.
However, if you buy speakers with a low impedance – say, 4 ohms – or specialized models with an unusual impedance – you may wish to double-check that your amplifier will handle these OK.
However, even then, it may only be an issue if you will be running the amplifier at high volumes for long periods.
OK, schools out – off you go to enjoy yourself!
About Home Cinema Guide
Paul started the Home Cinema Guide to help less-experienced users get the most out of today's audio-visual technology. He has been a sound, lighting and audio-visual engineer for around 20 years. At home, he has spent more time than is probably healthy installing, configuring, testing, de-rigging, fixing, tweaking, re-installing again (and sometimes using) various pieces of hi-fi and home cinema equipment. You can find out more here.