Basic question about sensitivity

Thank you to all for the informative responses.

@yoyoyaya  It is a good measurement except :

1. It is not universally available.  We only know about the from what they chose to review
2. It is not easy to calculate from normally published specifications.

Since manufacturers do not publish EPDR we are stuck trying to identify hard to drive speakers without it.  Often we can't even get minimum impedance values either.

@erik_squires - not disagreeing, but at least Stereophile and Hi Fi News are trying to be as informative as possible. But what the EPDR measurements of a range of speakers do tell us is that, in general, speakers are harder to drive than most manufacturers' generally used specifications tell us. So my rule of thumb is to take the nominal impedance and assume that the real impedance is lower than that.

The B&W 800 series are a good example of speakers that are relatively efficient but difficult to drive.

@yoyoyaya , I am running an ancient (going on 30 years old) pair of B&W 805s; I just took a look at the literature that came with them, and it states that the sensitivity is 87 and nominal impedance is 8 ohms. Since I have owned these speakers, I have ran them with three different tube amps, and am currently running them with one that is switchable between ultralinear and 50 wpc triode, and mostly always I have used the triode setting. I had never thought of these speakers as being particularly hard (for any of the amps I have owned) to drive; but with that typed, the largest room I have ever had them in was probably on the small side of medium, and presently, the room that they are in now is just plain SMALL.

@OP. I owned a pair of 805s for a number of years. Very nice loudspeakers and not difficult to drive. The newer 800 series - the floor stand speakers in particular - are a different kettle of fish entirely.