Ok, guys, another two cents here....

First, Zobels are GREAT at stabilizing the impedance that a speaker presents to a crossover, thus linearizing the voltages fed to the speaker, but why were they brought up in this discussion? If it is to deal with impedances that are too low, note that a Zobel is a network that lowers impedance at particular frequencies, so, basically, huh?


In 1980 I worked in Loudspeaker Engineering Department at Marantz in Southern California, where there was really a fantastic speaker engineer (true to Marantz's policy of ignoring gold in their pocket, very little was done with his product), I realized that Marantz amps NEVER put speakers A and B in series, but lower-priced competition did. I did research on the results.

As Larry points out, two speakers with identical impedance curves will perform the same in series as in parallel, frequency-response wise. I was surprised at this, because I thought resonant peaks in one would mess up the other. But no.

Incidentally, mixing different speaker models in series craps up the response of both speakers, but the response of the better speaker, which usually has the lower bass resonance, is messed up more by the cheap speaker than vice versa.

I still have the Bruel & Kjaer testing equipment impedance and frequency response curves somewhere, the ones that I made doing this research.

As to 4 ohms, 8 ohms, etc: Speaker impedances can indeed go very low, but usually a 4 ohm speaker goes very little below 4 ohms at any frequency. The case is similar with 8 ohms. Don't worry about two eight ohm speakers in parallel being worse for an amp than a four ohm speaker.

How about power, though? An amp with a really beefy power supply will put out twice as much power into 4 ohms as into 8, so two speakers in parallel would be 3 dB louder than one with such an amp. Most amps don't quite do this, so the resulting volume would be less than 3 dB more.

In series, the combination of the two speakers, giving sixteen ohms, will draw half the power from the amp that an eight ohm speaker would. So the series combination WILL be 3 dB down from a single speaker. I say WILL because an amp can easily put out half the current of the rated 8 ohm load.

And don't all of our surround amps have AT LEAST 3 dB of adjustability on the center speaker?

Oh, yeah, when I did this with a pair of Sonance M200s, little speakers, they sounded GREAT and there was no problem with comb filtering, lobing, or beaming. nobody could jump around from left to right fast enough to hear the difference in frequency response.