Nominal Loudspeaker Impedance Easily Misunderstood

Plot courtesy of SpinningSpark

In the world of audio speakers, it's easy to think of impedance as a fixed quantity. But as you can see from the above plot of a speaker's impedance versus frequency, this loudspeaker varies in impedance greatly from a low of nearly 4 ohms at extreme lower frequencies to multiple peaks nearing 40 ohms at 50Hz and 20KHz. So what does the "nominal impedance" specification published by all loudspeaker manufacturers tell us?

IEC's Nominal Impedance

The International Electrotechnical Commission (IEC) has kindly defined nominal impedance as the product of 1.25 and the minimum impedance within the speaker's passband (frequencies the speaker can reproduce). This means the minimum impedance of the speaker cannot be less than 80% of this value. Dividing by .8 or multiplying by 1.25 are equivalent mathematically:

z/.8 = z(10/8) = z(5/4) = z(1.25)

So, as an example, if we have a speaker rated at 6 ohms nominal, its frequency minimum cannot be less than 4.8 ohms. A 4 ohm rated speaker may "go as low" as 3.2 ohms.

Minimum Impedance

As the impedance drops, according to ohm's law, the current requirement of the amplifier or receiver driving the speakers goes up. This likely will not present a problem, as most contemporary amplifier designs support a range of impedance values down to 4 ohms. In addition, amplifier protection circuitry will step in under extreme conditions and prevent damage to equipment.

CNET has a wonderful article with ELAC's speaker designer Andrew Jones, describing the challenges from a designer's perspective, in balancing impedance and speaker sensitivity values.

Bottom Line

Nominal impedance is simply an indicator of how low a speaker's resistance will go across its frequency range. Unless the nominal value is very low, say < 6 ohms, most of today's amplifiers will not have a problem driving them, based on impedance alone. However, a loudspeaker's sensitivity is also a factor, and something best addressed in a future article.