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.
For a more general overview, check out the home theater speaker 101.
- What Is Resistance?
- What's the Difference Between Resistance and Impedance?
- What About Current and Voltage?
- What Has Impedance Got to Do with Your Speakers?
- Matching the Impedance of Speakers and Amplifiers
- Should You Change the Impedance Setting on Your AV Receiver?
- Can You Connect Two Speakers to One Amplifier Channel?
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 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, the voltage or the current must increase, which puts an increased strain on the amplifier’s power supply.
For example, the resistance goes down if you disconnect your 8-ohm speakers from your amplifier and connect 4-ohm speakers.
Less resistance allows more current flow, 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 surround sound 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 nominal 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 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), 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!
You need to look out for the impedance range in the specification of an amplifier (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, 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.
You can run different ohm speakers on the same amp as long as they all meet the supported range of speaker impedance.
If no impedance range is 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 double-check with the retailer or manufacturer first.
The supported impedance is also commonly written on the back of the amplifier, around the speaker terminals where you connect the wire.
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.
Related article: Speaker power rating: what do RMS and peak power mean?
Should You Change the Impedance Setting on Your AV Receiver?
On most modern AV receivers, a setting in the setup menu tells 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 have.
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: a speaker’s stated impedance is an average nominal value.
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:
Can You Connect Two Speakers to One Amplifier Channel?
In most cases, when you wire your home theater, you should connect one speaker to one channel on the amplifier to avoid problems.
This ensures the best performance and that the amp and speaker perform within their specifications.
However, there are scenarios where wiring multiple speakers to a single amplifier channel can be helpful.
For example, you might have a large room and want several surround or overhead speakers that receive the same output.
If you plan on doing this, you must be aware of the implications before you start, or you might damage your equipment.
The problem is that connecting more than one speaker to an amplifier channel affects the overall impedance seen by the amplifier.
The two most common wiring configurations are series and parallel.
Main takeaway: wiring speakers in series increases the impedance seen by the amplifier.
In a series wiring configuration, the speakers are connected end-to-end, with the positive terminal of the first speaker wired to the amplifier’s positive output and the negative terminal wired to the positive terminal of the next speaker.
This pattern continues for all speakers in the series, and the negative terminal of the last speaker is connected to the amplifier’s negative output.
In a series connection, the total impedance is the sum of the individual speaker impedances.
For example, if you connect two 8-ohm speakers in series, the total impedance would be 16 ohms (8 + 8 = 16).
Or if you connect one 8-ohm, one 4-ohm and one 6-ohm speaker, the total impedance is 18 ohms (8 + 4 + 6 = 18).
The main advantage of series wiring is that it increases the total impedance, making it suitable for amplifiers with higher impedance ratings.
However, one downside is that if one speaker fails or gets disconnected, the entire series is affected, resulting in a loss of sound output.
Plus, this configuration will likely sound worse with two-way speakers that use a crossover to split the sound between one or more drivers, as it can degrade the performance.
Main takeaway: wiring speakers in parallel decreases the impedance seen by the amplifier.
In a parallel wiring configuration, the positive terminals of all the speakers are connected to the amplifier’s positive output, and the negative terminals are connected to the amplifier’s negative output.
In a parallel connection, if the speakers have the same impedance, you simply divide the impedance of one speaker by the number of speakers.
For example, if three 6 ohms speakers are wired in parallel, the total impedance seen by the amplifier is 2 ohms.
6 ohms ÷ 3 speakers = 2 ohms
It’s more complicated if the impedance of each speaker is different.
In this case, the total impedance can be calculated using the following formula:
Total Impedance = 1 ÷ (1 ÷ Speaker 1 Impedance) + (1 ÷ Speaker 2 Impedance)
For example, if you connect a 6-ohm and 8-ohm speaker in parallel, the total impedance seen by the amplifier would be 3.4 ohms:
(1 ÷ 6) + (1 ÷ 8) = 0.166 + 0.125 = 0.291
1 ÷ 0.291 = 3.4
If you have more than two speakers, include the extra speakers in the equation, add them all together, and divide 1 by the result.
The advantage of parallel wiring is that it lowers the total impedance, making it suitable for amplifiers with lower impedance ratings.
It also ensures that if one speaker fails, the others will continue to produce sound.
However, the downside is that the amplifier needs to provide more current, which may strain the amplifier if it’s not designed to handle the lower impedance.
Understanding the implications of series and parallel wiring on impedance ensures optimal performance and prevents potential damage to your amplifier and speakers.
Matching the total impedance with the amplifier’s rated impedance is vital to prevent overheating and maintain sound quality.
The Effect on Power Output
The power output is also affected by the wiring configuration.
In parallel wiring, the power from the amplifier is shared among the connected speakers, so each speaker receives less power than it would if it were connected individually.
In series wiring, the same amount of power flows through each speaker, but the increased impedance limits the total power.
Therefore, ensuring that the speakers’ power handling capabilities are compatible with the amplifier’s power output is critical.
Generally, I would advise against wiring more than one speaker to an amp channel. It can create more problems than it is worth.
Even in relatively large rooms, there should be little need to have more than one speaker/speaker pair reproducing the same audio.
For example, suppose you are installing Dolby Atmos overhead speakers.
In that case, the recommended locations are around the primary listening position, so one stereo pair per location should be sufficient for a few rows of seating. One pair just in front and one behind.
Before installing several matching speakers, think twice about the speaker placement and see if you can locate the primary speakers better.
However, if you still feel you need extra speakers to spread the sound better, double-check the advice above and work out if your amplifier is up to the job.
After going through this physics lesson for dummies, you can see there isn’t much to know.
But it is something that you need to be aware of.
These days, most modern amplifiers/receivers will happily drive most sets of speakers, 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.
However, if you plan on wiring more than one speaker to your amplifier channels, you must understand that this will increase or decrease the impedance depending on how you wire it.
And this can cause problems if you don’t understand what you are doing.
OK, schools out – off you go to enjoy yourself!
About The Author
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.