The most important word in any discussion about speaker impedance is:
Nominal: 3b: of, being, or relating to a designated or theoretical size that may vary from the actual (Merriam Webster)
Which is to say, nominal also means: sometimes, occasionally, once in a while, or this number’s good enough for you to sort of get an idea what the real number may or may not be.
That’s the thing with loudspeakers: Impedance is not a set value – it changes with frequency. To truly know the impedance of a speaker, you need a graph of the impedance at every single frequency in the speaker’s operating range.
Here’s the way we spec the impedance of our R11:
Impedance: Nominal = 8 Ohm (minimum = 3.2 Ohm)
In a casual conversation we'd say the R11 is an 8 Ohm speaker. This means the rated nominal impedance considers the impedance of the voice coil and the crossover. Or, 8 Ohm is the nominal impedance the speaker presents to the amplifier, and 3.2 Ohm is the minimum impedance seen over the operating frequency range.
The term Ohm is an homage to German physicist Georg Ohm who didn’t discover the properties of resistance (DC) and impedance (AC) but who came up with formulae to calculate them. The Greek symbol for Omega (Ω) is used to designate resistance and impedance simply because omega sounded a bit like Ohm.
Amplifiers don’t have impedance, they use impedance
Without getting into a detailed discussion of voltage, current and impedance, suffice to say the amp is the pump that pushes the signal through the speaker, and impedance is the amount of resistance the speaker gives the amp. But, because we're talking about an AC signal (alternating current–which is what an audio signal is) we consider the combination of resistance, capacitance and inductance which is referred to as impedance.
Think water through a hose: The less you stick your thumb over the end of the hose (lower the impedance) the more water (signal) the pump (amp) sends, conversely, the more you put your thumb over the end of the hose (raise the impedance) the higher the pressure and the lower the amount of water you can spray someone with. The larger the diameter of the hose the lower the impedance and the more powerful a pump (amp) you need to get the same amount of pressure (volume) at the end of the hose (speaker).
To calculate impedance simply add the inductance (L) to the capacitance (C) and multiply the sum by itself. Then multiply the resistance (R) by itself and add that number to the square of the L and C. Then calculate the square root of that sum. Or you can just read the spec and go from there.
In the Real World
An amplifier rated at 4Ω can drive a speaker rated at 6 or 8Ω with no worries, except that it will produce 50% less power driving an 8Ω speaker.
On the flip side, the lower the speaker impedance the harder your amp has to work. Most modern solid-state amps have no problem going below 4Ω on occasion (but don’t take our word for it – check your amp’s specifications!!). Below 4Ω, some mass-market receivers may have trouble in the form of over-loading (or working too hard) to the point that their safety circuit kicks in. Vacuum tube amps present a whole other set of problems pertaining to impedance, so we'll avoid that subject for now.
To simplify: With lower impedance, greater dynamic range and sound pressure is developed but your amp works harder. The higher the impedance the less dynamic range and overall output, but your amp works less (to a certain extent).
Advice: Spend a few extra bucks and buy an amp rated to 4Ω.
While it is true that in the very low frequency ranges speaker impedance typically falls, there is no easy direct correlation between frequency and impedance. Some "8 Ohm" speakers may see impedances as high at 17 or 18 Ohm at certain frequencies and may then remain close to 4 Ohm for wide ranges of frequencies. That’s why speaker manufacturers use nominal ratings: It’s just not feasible to give the rating for each frequency, but it is extremely important to know the minimum impedance of a loudspeaker.
Don’t Over-Power Your Speakers
Know what minimum and maximum wattage your speakers are rated for and your amplifier’s maximum output in Watts. If you send your speakers more power than they're rated to handle, eventually their impedance will become infinite because they will be blown.
An amp with a nominal rating of 4Ω will play nice with a speaker rated at 8Ω – your overall output will be cut in half so you’ll have to raise the volume and make the amp work slightly harder, but that’s about it.
An amp with a nominal rating of 8Ω will not play all that nicely with a speaker rated 4Ω, so you should avoid that match-up.
The spec you really need to be concerned with is the minimum impedance – a speaker rated below 3Ω will need a specially matched amp.
You can’t take a DC Ohm meter and measure a loudspeaker’s impedance. You will get a (constant) but funky number that will only allow you to determine whether the speaker is blown or not.