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:

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3 COMMENTS

  1. 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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