As you've probably noticed, Punkuk, there's a wide diversity of opinion as to what exactly consitutes a "high efficiency" speaker.
One thing you should keep your eye on is exactly what's being specified by the manufacturer. Typically manufacturers of speakers with an impedance below 8 ohms specify the 2.83 volt sensitivity, as that gives more impressive looking numbers. And typically manufacturers of speakers with an impedance higher than 8 ohms specify the 1 watt efficiency, again because it looks better. Here's how it goes: 2.83 volts into 8 ohms is 1 watt, but 2.83 volts into 4 ohms is 2 watts, and 2.83 volts into 2 ohms is 4 watts. Going the other way, 2.83 volts into 16 ohms is 1/2 watt, and 2.83 volts into 32 ohms is 1/4 watt. When you turn the volume control knob on your preamp, you're changing the voltage output (and the wattage output changes along with it). When you leave the volume control setting the same and switch to lower impedance speakers, the voltage output stays the same but the wattage output increases (assuming the amp doesn't clip).
Let's do an example of converting sensitivity to efficiency. Suppose you have a "93 dB sensitive, 4 ohm" loudspeaker. Sensitivity refers to 2.83 volts input which into a 4 ohm load is 2 watts, so the speaker is really doing that 93 dB with TWO watts input, so the efficiency is only 90 dB with ONE watt.
Unfortunately, some manufacturers use the word "efficiency" when "sensitivity" would be more appropriate, and vice versa. And unfortunately some manufacturers use an "in-room" rather than "anechoic" or "simulated anechoic" measurement, which inflates the specification by 2-3 dB due to the added reverberant energy. Using an "in-room" measurement is not without justification, but does complicate things when you're trying to make an apples-to-apples comparison.
In my opinion efficiency is a more useful specification than sensitivity because wattage input (not voltage input) is what relates directly to voice coil heating and thermal compression. Then if you want to you can take into account how much power the amplifier you have in mind puts out into that particular speaker's impedance. Most solid state amps increase their power output into lower impedance loads, but often there's a sonic trade-off, as many amps distort less into a high impedance load.
I'm not qualified to say where anyone else should draw the line between "high efficiency" and "not high efficiency". Personally, I consider 91-94 dB as moderately high efficiency; 95-98 dB as high efficiency, and 99+ dB (all with a 1 watt input) as very high efficiency. But in my book those are only ballparks, and the lines are blurry.
I've focused on measurements, but your question could be approached from other angles as well (how is a high efficiency driver constructed, what are the techniques commonly used to build a high-efficiency loudspeaker system, what are the challenges involved in building a good one, why would you want one in the first place, etc.) But I'll save the muddying of those waters for another day.
Duke
