Speaker impedance is a confusing subject and based on the letters we get here at CNET (see below), I thought I should try shed a little light on the subject. This letter sums up the typical quandary:
"I have a bit of a dilemma that I hope you can help me solve. I have an entry level Yamaha HTIB system that I purchased a few years back and I would like to upgrade it with better speakers. I've done a lot of research and found that owning a 6 ohm receiver limits the selection of brands that I could look at in store or online."
I'm all for the speaker upgrade, but he went off track with the concern about his "6 ohm" receiver. The writer mistakenly thinks he's limited to buying 6 ohm rated speakers. Wrong! Any 8-ohm rated speakers would work just as well, and since the vast majority of speakers are 8 ohm rated, his choice of speakers is wide open.
Back to the question at hand: speaker impedance. It's never a fixed number, a 6 ohm speaker may be, on average 6 ohms, but its actual impedance varies with the frequency the speaker is reproducing at any given instant. For example, it may be 4 ohms at 50 Hertz, shooting up to 21 ohms at 100 Hz, dropping back to 7 ohms at 1,000 Hz, and up again to 9 ohms at 10,000 Hz. So in other words, it's impossible to "match" a speaker to a receiver. Impedance is a moving target.
The real world impedance concern comes when using 4 ohm speakers with low power A/V receivers, but even then ONLY when the buyer expects to play the system at high volume. You see, low impedance, 4 ohm or less rated speakers, demand more power than 8 ohm speakers when playing LOUDLY, and that's when the receiver's power might come up short. In a typical "mismatch" scenario the receiver's protection circuit would trip, and shut the receiver off.
Which bring us to the hard cold fact that not all 100 watt receivers are not even close to being equally powerful, but I covered that in my "What's up with watts" blog from a few weeks ago.