Driver efficiency vs. overall speaker efficiency

You can't measure the efficiency of a bare driver. It needs to be in some sort of test enclosure. The test enclosure is not the same as the final system enclosure, and may yield a lower efficiency measurement. This is not a problem if your objective is just to compare different drivers (all tested in the same box).

Satch...Efficiency is stated as dB per VOLT (not current). Of course the current will be more with a 4 ohm load, but that's the power amp's problem.

"Efficiency"???...The only thing I see quoted in specs is SPL in units of dB at 1 meter for 2.83 volts. This is 1 watt for an 8 ohm load.

Power = Volts squared divided by ohms.

(2.83 * 2.83) / 8

For 4 ohms it's 2 watts.

Jeffreybehr....My points are:

1. The term "Efficiency" is just confusing. I don't see it quoted by manufacturers.

2. You gave the answer, but did not "do the math" so that others could understand where you are coming from. I am not disagreeing with you. Just making a clarification.

Trelja...What if the "box" includes a passive radiator? Of course the PR draws its power from the driver, but I think it does the electrical-to-acoustic conversion more efficiently (because cone excurion is less) than if the audio all came from the driver cone.

I am talking about SPL. Audio quality is another matter.