Speaker Impedance: Resistance and Phase Question

Voltage and current are not in perfect syncronization because the load stores energy and then feeds it back to the source on a delayed basis (reactive load). The type of load determines whether current will be leading or lagging behind voltage. An inductive load results in current lagging behind voltage. Speakers present an inductive load that varies with frequency which can make it more difficult to deliver power than a purely resistive load (e.g., power delivery to a light bulb is easy because it does not store electrical energy and feed back that power).

Almarg,

Great expansion on the subject; I tried for a simpler answer, but, it is good to see a complete explanation.

For practical application of such information, the tests you mention that Stereophile publishes are helpful. They plot the imdedance of the speaker against frequency and also plot the phase angle against frequency. The most meaningful use of that data is to look at the low points of impedance and then look to see how far off the phase angle at that same frequency is from zero. A low impedance and high phase angle (positive or negative) represents a difficult load. If that point is also in a frequency range where there is a lot of musical energy, that makes the speaker even more demanding.

A lot of speakers have deceptive nominal ratings; for example, the manufacturer may claim 8 ohms because over much of the frequency range the impedance is close to 8 ohms but at some crucial point (e.g., 80 hz) the impedance drops closer to 2 ohms and the phase angle is more than 30 degrees. That would be a demanding impedance characteristic.

I run tube amplifiers so my experience is mainly with matching speakers with such amps. A lot of people look at efficiency numbers when trying to decide whether a low-powered tube amp will work with a particular speaker. To me, the impedance characteristics is much more telling than nominal efficiency. I have heard a 15 ohm speaker which, I believe, has a 83 db/w efficiency rating that played loudly in a large room with an 8 watt amp. I have heard supposedly 95 db/w speakers that could not play well with even 50 watt amps because of their difficult impedance characteristics.