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Speaker Impedance Matching - Ohms, Speakers and Impedance Explained

My word, we'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 (me included).

It's also one of the favourite subjects that the nerds on the forums like to 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 quite simple really.

So let's try and understand the concept of matching impedance/resistance in speakers and amplifiers - in as few words as possible... and then we can go and do something more interesting instead.

Geeky man

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.

The simple answer (for us simpletons who aren't too concerned about the finer details), is that resistance and impedance are essentially the same thing 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) - impedance is the term used when a circuit uses alternating current (AC).

Sine wave

An amplifier sends an AC current to a speaker - so we should use the term impedance when dealing with speakers.

The main difference is that impedance will vary according to the frequency (which is why you may see some speakers quoted with an impedance range - as the impedance will change depending on the frequency of the audio signal).

Resistance and impedance are both measured in ohms - and can be written by 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 electrical current (AC) - this current is measured in amps. The current is 'pushed' to the speaker by a voltage.

Ohms Law states that: current = voltage ÷ resistance.

Therefore if the resistance goes down, then either the voltage or the current must increase (usually the current). Either way, this 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, then the resistance goes down. Less resistance lets more current flow and so the amplifier will have deliver more power to the speakers (current or voltage) - which it may not be designed to do.

What Exactly Has Impedance Got to Do with My Speakers?

All speakers have an impedance.

This impedance will vary depending on the size and design of the speaker - and so it is very important to remember that one set of speakers may have a different impedance than another set of speakers.

Speakers designed for use in the home are usually rated at either 4, 6 or 8 ohms, although more specialised models may fall outside this range. If you are buying new home theater speakers, you should check their rated impedance.

Impedance will usually be indicated as a nominal value. This means that it is an average figure, and the actual impedance over time will be higher or lower than this average value.

Nominal Impedance value

Four ohm speakers have a lower average resistance than 8 ohm speakers - and therefore draw more current.

The 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 - which can be good if you have an under-powered amplifier - but not so good if you turn it up too loud.

In the picture above, the impedance is listed with a minimum amount - this refers to the lower limit of impedance that the speaker will fall to depending on the frequency of the audio signal.

Your amplifier needs to be able to handle that lower impedance.

Matching the Impedance of Speakers and Amplifiers

As stated earlier, the amplifier doesn't have an output impedance.

It is the speaker that has the impedance.

Therefore, the impedance you may see listed for an amplifier refers to the optimum speaker impedance that it is designed to drive.

You can connect speakers of any impedance to an amplifier and they will work. However, if the amplifier isn't designed to drive speakers with a lower impedance (4 ohms for example), then the amplifier may overheat if you turn the volume up very loud - because it will draw more current than the power supply is designed to deliver.

Therefore, if possible, you are looking to match the impedance of the speakers you have to the impedance the amplifier is designed to drive - then you won't have any problems.

The thing you need to look out for on the specification of an amplifier is the impedance range (if there is one).

Speaker Impedance Range

If it states 4-8 ohms then it means the amplifier has been designed to handle speakers of 4, 6 or 8 ohms.

If it says 6-8 ohms, then 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 - and the danger is your speakers may be damaged before the amplifier shuts itself down.

The important thing to remember is, if you want low resistance speakers (4 ohms), then you need to make sure your amplifier/receiver will be able to drive these easily.

If there is no impedance range listed for the amplifier/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, then it should also handle 6 ohm speakers fine.

Speaker measurements at a different impedance

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 4 ohm speaker, as long as you don't turn it up too loud.

If you are planning to use speaker impedances below 6 ohms then you may want to double-check to make sure.

Also, on many modern amplifiers and receivers, there is often a setting in the setup menu for you to tell the receiver the impedance of the speakers you have connected to it.

If you set this, then the amplifier will run in its optimum state for that impedance.

If you have the option of setting your AV receiver to 4 ohm or 8 ohm, and your speakers are 6 ohm, then it would be safest to set the receiver at 4 ohm. This is because the actual speakers impedance will be higher than it is expecting, and we want to avoid having speakers connected with a lower impedance than the receiver is expecting. In the guide to the best AV receivers in 2018, I have some specification tables which highlight the supported speaker impedance of the latest AV receivers.


So as we can see, after going through our physics lesson for dummies, once we look into the details of matching speaker and amplifier impedance we can see there isn't a great deal to know - but it is something that we need to be aware of.

These days, most modern amplifiers/receivers will quite happily drive pretty much any set of speakers - so in most cases you can ignore this issue completely.

However, if you are buying speakers with a low impedance (4 ohms) - or specialised models with an unusual impedance - then you may wish to double-check that your amplifier will handle these ok.

However, even then it may only be an issue if you are going to be running the amplifier at high volumes for long periods.

Ok, schools out - off you go to enjoy yourself!