Hi Jim,
Strictly speaking, speaker efficiency would be the ratio of acoustic power out to electrical power in.
However, it is common practice for the term "efficiency" to be used to refer to the sound pressure level (SPL) that would be produced at a distance of 1 meter in response to an input of 1 watt.
It is also common practice for the term "sensitivity" to be used to refer to the SPL that would be produced at a distance of 1 meter in response to an input of 2.83 volts.
2.83 volts into 8 ohms corresponds to 1 watt. (2.83 squared/8 = 1). So for an 8 ohm speaker, that is truly 8 ohms, sensitivity and efficiency are equal.
2.83 volts into 4 ohms, though, corresponds to 2 watts (2.83 squared/4 = 2), which is 3 db more than 1 watt. So the efficiency of a 4 ohm speaker would be 3 db less than its sensitivity, assuming those terms are used in their most usual sense.
Also, keep in mind that as can be seen in measurements provided by John Atkinson in Stereophile, and measurements that are provided in reviews at SoundStage.com and elsewhere, manufacturer sensitivity and efficiency specs are often overstated by a few db. In some cases, I believe, because the manufacturer’s rating may be based on a single frequency, rather than an average across a wide range of frequencies.
Best regards,
-- Al