Understanding Speaker Impedance

Speaker impedance is often presented as a complex subject and therefore is either ignored or misunderstood. A basic understanding of speaker impedance is not difficult, and is useful when connecting multiple speakers to an amplifier. This article will give you a practical understanding of speaker impedance and how to connect multiple speakers to your HiFi amp.

What is speaker impedance?

Speaker impedance refers to the load a speaker places on an amplifier. Well, that is the effect of speaker impedance. Technically, speaker impedance is the “resistance” a speaker offers to the current supplied by an amplifier. Because the output current of an amp is AC (not DC, like from a battery), the resistance is called impedance. To be real technical, impedance is the combination of DC resistance, plus any reactance in a AC circuit. But without getting too technical, just remember speaker impedance affects how much current is drawn from the amplifier.

Impedance (like resistance) is measured in ohms, and uses the Omega symbol (Ω) for shorthand. However, unlike resistance, impedance changes with frequency. And since the signal from an amp is voice or music with lots of different frequencies, the speaker impedance is constantly changing. Rather than state the impedance for every frequency, speaker manufactures state the “nominal” impedance, which is sort of the average of the lowest values of the speaker impedance. It is this figure which we use for calculation purposes.Speaker impedance 8 ohms

Most speakers are rated by the manufacture as nominally 4Ω, 6Ω, 8Ω or 16Ω.

Why does speaker impedance matter?

As stated above, speaker impedance determines the current drawn from the amplifier. Remember impedance impedes (or restricts) the current, so the lower the impedance, the more current can flow. A greater current requires the amp to produce more power.  Another way of looking at it is to say the lower the impedance, the higher the load on the amp (and the harder it has to work).

These general relationships can be summarized by:

Lower the impedance → more current → greater load → increased power

Raise the impedance → less current → smaller load → decreased power

The relationship between impedance (resistance), current, voltage and power is determined by Ohms law. See this article for a fuller explanation.

Looking at the above summary, it appears that the lower the speaker impedance, the greater the power the amp delivers through that speaker. This is true – up to a point. It is true up to the point when the amp can not produce anymore current and power. At this point, either the amp fuse will blow, the amp will die or the protection circuit in the amp will kick in and turn the amp off. Therefore, do not run an amp with a load impedance of less than the stated minimum (normally 4 ohms).

The secret is to make sure the speaker impedance is within the range that the amp is designed for.

Why do I need to know about speaker impedance?

You need to make sure the speaker impedance of any speaker (or speakers) connected to an amp is within the capabilities of the amp.

Most HiFi amps are designed for a speaker load impedance of 4-16 ohms. This means the minimum speaker impedance is 4Ω. Therefore if you have a speaker with a rated impedance of 4Ω, 6Ω, 8Ω or 16Ω, the amp will be happy. The lower the impedance, the greater the current flowing through speaker and the greater the power available. But, don’t use a speaker (or speakers) with an impedance below 4 ohms.

This is a real concern when you connect two or more speakers to one amplifier. For example, four 4Ω speakers connected across an amp gives a total load impedance of only 1Ω – way too low for your amp. In this case you should use a speaker selector with impedance protection or impedance matching.

All this is simply illustrated in the following video I put together. It starts with a simple set-up and develops into what happens with multiple speakers, and how speaker switches help with impedance.

Hopefully that has helped you understand what speaker impedance is all about, and how to use your knowledge to safely connect speakers to your amplifier.

Other articles to help you are:

If you have a question on speaker impedance and what it means to your install, please read the FAQs before submitting your question.

7 COMMENTS

  1. Please help me out. I am having a problem understanding something, and I’ve read it multiple times over different sites, but I’m just not “getting it.”

    If an 8ohm speaker is connected in parallel to another 8ohm speaker, (or even connected in any way, even series), why does that LOWER the impedance?

    This seems logically backwards to me because in my mind each speaker is requiring the amplifier to perform more work with each speaker added — increasing the impedance (not lowering it). I see the speaker as being like a trailer that the amplifier (the truck) has to pull, and the impedance is its weight. But instead I keep reading the opposite.

    Please straighten me out.

    • Hi Benjamin,

      I feel your pain. It can be quite confusing.

      It is all to do with Ohm’s Law. Ohm’s law basically says that for a given voltage (from the amplifier) the current will increase as the resistance (speaker impedance) decreases. (For a better explanation of Ohm’s law see my article on The Dreaded Ohm’s Law.)

      So you are correct in saying that when you add a speaker you are requiring the amp to perform more work (or to supply more power). For the amp to supply more power (at a given voltage) the resistance (impedance) has to decrease. Or to put it the other around, if you decrease the total load impedance, then the current (and power) will increase. Conversely, if you increase the total load impedance, the current (and power) will decrease.

      In fact, if you connect two speakers together in series (say two x 8 ohm speakers), then the total load impedance will be 16 ohms. This is double one speaker (of 8 ohms), and so that load will allow only half the current (and draw half the power).

      However, the tricky bit is when you connect two speakers in parallel. I think this is the crux of your question. If you have one 8 ohm speaker across the amp, it will draw a certain current (let’s say 2.5 amps). Now if you connect another 8 ohm speaker across the amp (ie two speakers in parallel) then each speaker will draw 2.5 amps, or a total of 5 amps. Remember Ohms law says if the current doubles, the resistance (impedance) halves. That is why two 8 ohm speakers in parallel gives a total load impedance of 4 ohms. For a more detailed explanation this maths, see the article in Connecting multiple speakers.

      Does this help some?

      Geoff

  2. Hi Geoff,

    I have an old Fender bass cabinet I need to re-wire.

    The cab has (2) 8ohm 10″ speakers, and (1) 4ohm 18″ speaker.

    What is the best way to rewire these 3 speakers, and what will the resulting OHMS be.

    The amps I will use on this cab will drive either 4ohm or 8ohm loads safely.

    Thanks so much.

    Mike

    • Hi Mike,

      Unfortunately I’m not a speaker designer so I may not be the best person to answer you.

      Simply on an impedance issue, you could parallel the two 8 ohms speakers. This would give you 4 ohms across both of them. Then they could be wired in series with the 4 ohm speaker, giving you a total load impedance of 8 ohms.

      While this neatly solves the impedance issue, it doesn’t take into account any physical effect each speaker might have on each other, and how this might distract from over all performance. If the back of your amp where the speakers are is open, then each speaker will have a less effect on the others. However they are in a sealed box, then the results might be different.

      There is no harm in trying. My limited understanding is the bass response may be affected, but it should work.

      Give it a go,

      Geoff

  3. Could I run 3 sets of small 3-4 inch diameter 4 ohm speakers off a 25w/ch amp that will only be used at very low volume in parallel? The wiring is already in place in separate rooms in the ceiling I would prefer not to have to rewire the current setup.

    • Hi Paul,

      The technical answer is no. 3 lots of 4 ohm speakers in parallel is going to be way too little load impedance (1.3 ohms) for the amp at full volume. So I would advise against it. However it will work if your amp is not running to hard (too loud) as it wont exceed the current(amps) capacity of the amp at low levels. But, you wont know what level is too loud until it is too late – although many modern amps will turn themselves off in an attempt at self protection.

      A much safer way would be to run them through a speaker selector switch which will put a 3-5 ohm series resistor in each speaker line. This will stop the impedance falling below this resistance. You don’t even need to re-wire anything to do this, just insert it between the amp and the first set of speakers, and engage the “protection” switch.

      hope this helps

      Geoff

      • Thanks. I have a selector, too musch trouble to rewire. I wired up the amp this morning, it works OK not overheating after 6 hours, just for background music in my studio, the dial goes to 11, I have it at 1.3 =)

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