Ohms?

Ohms are the units which measure resistance or impedance. Resistance or impedance are two terms which define the tendency for a substance or electrical component to attenuate or reduce a voltage when current is passed through it. The relationships between resistance, current and voltage are described by Ohm's Law, a simple mathematical equation. With regard to speakers, generally, the lower the impedance rating the more inefficient the speaker is. That is, speakers rated at 16 ohms tend to be more efficient than 8 ohms which tend to be more efficient than speakers rated at 4 ohms. All speakers have an impedance which varies with respect to the frequency being produced. Most speakers tend to have large impedance dips and increases near their woofer resonance point, depending on the design, as well as at other frequencies throughout their operating range. Since amplifiers have to work harder in order to drive inefficient speakers as well as speakers which have low impedance somewhere within their frequency range, the impedance characteristics of any speaker are most properly expressed on a graph which has frequency response on the X-axis and impedance (Ohms) on the Y-axis. Often two impedance ratings are quoted for a speaker, when a graph is not used to indicate a speaker's impedance behavior. These are the nominal impedance rating and the minimum impedance rating. The nominal impedance rating is meant to express the "average" or "typical" impedance behavior of a speaker while the minimum rating is meant to indicate the minimum impedance at any frequency within the operating range.

bravo, rayhall! yours is an exceptionally literate and erudite answer to a seemingly simple question.

Here's my two cents. In order for electric current to flow it must be drawn by a load, which is like a hungry energy magnet. The resistance in Ohms is a measure of the load/hunger. The higher the resistance in Ohms the less energy flows. Conversely, as the resistance of the load drops, the magnet gets bigger, hungrier for more energy. Good amps can accomodate these load variations and spill more juice into the load. But as the resistance of the load approaches zero the amp goes nuts trying to fill the void and hopefully overheats and shuts down. Any speaker's ability to convert this energy into sound at a given frequency varies, so I wouldn't say offhand that the rating in Ohms will give you much of a clue as to efficiency. The rating does point towards the adequacy of the amp you'll require to feed it. The bottom line is that amp designers and speaker makers have to work from some kind of starting point to get these components to work together, which for some reason has a baseline of 8 ohms. Is it arbitrary? It's probably just a freak of nature stemming from the measurement of early design efforts that later became an agreed upon average design target.