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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. 

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 - 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.  This is because it will draw more current than the power supply is designed to deliver.

At that point, the amplifier will shut itself down before it causes too much damage. 

Therefore, if possible, 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. Unless you try really hard with the volume control!

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.

The problem 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.

Should I Change the Impedance Setting on My AV Receiver?

On most modern AV receivers there is a setting in the setup menu for you 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 should change the setting depending on the impedance of the speakers you have connected. 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 will deliver the correct current to your speakers. The default will usually be set to 8-ohms.

Now, you may follow these guidelines if you like. But in most cases 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 are running a powered subwoofer and therefore routing the low-end frequencies away from your main speakers.

This option is there so that the manufacturer can get the unit officially rated for lower impedance speakers. So they can put it in the manual and label it on the outside of the box. 

However, if you change this setting, then all you will be doing is reducing the performance of your amplifier. 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. These clipped signals are the ones that may damage your speakers.

Remember what I said previously. The stated impedance of any speaker is a nominal value. An average. At any given time the actual impedance of a speaker will be higher and lower than it's nominal impedance depending on the frequency it receives.

Just be careful with the volume control on the amplifier, and you will be fine.

This video explains this is 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) simpler explanation should suffice:


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.

Conclusion

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!