Well, my answer is a little bit different - so I guess you get to take your pick!
Unfortunately the relevant terms get used inconsistently or imprecisely, leading many people to think that efficiency and sensitivity are the same thing.
Here is the convention that I follow. Some manufacturers also follow this, but not all.
As commonly used in consumer audio, "efficiency" is the 1 meter on-axis broadband anechoic SPL that a speaker delivers with a 1 watt input. Some manufacturers use a non-anechoic measurement (including room contribution), and some use a peak somewhere in the response curve instead of using a broadband average - so comparing two manufacturer's numbers may not really be apples-to-apples.
"Sensitivity" is the 1 meter SPL the speaker delivers from a 2.83 volt input. Sometimes called "voltage sentitivity" to make that clear. The same opportunities for optimistic specs enter the picture - namely, using non-anechoic conditions and/or taking the measurement at a peak rather than using a broadband average.
So, to recap: Efficiency is related to Watts, and Sensitivity is related to Volts.
(Some manufacturers use these terms interchangeably or backwards from this convention, so it's up to you to read carefully and figure out what they're really saying. Look and see whether they are talking about 1 watt input, or 2.83 volts input.)
Now, here is how the two are related: 2.83 volts into 8 ohms is one watt; 2.83 volts into 4 ohms is 2 watts; and 2.83 volts into 16 ohms is one-half watt.
So if you have a 4-ohm, 91-dB/2.83 volt sensitive speaker, it's an 88-dB/1 watt efficient speaker. If you have a 16-ohm, 85 dB/2.83 volt speaker, it is also an 88 dB/1 watt efficient speaker. I prefer to convert back to watts, because you can usually figure out the amplifier's maximum wattage output into a given load more easily than you can figure out its maximum voltage output (which may or may not vary with load, depending on the amplifier), and because if you are interested in power handling then you want to know the wattage going into the speaker.
Hope this helps.
Duke
Unfortunately the relevant terms get used inconsistently or imprecisely, leading many people to think that efficiency and sensitivity are the same thing.
Here is the convention that I follow. Some manufacturers also follow this, but not all.
As commonly used in consumer audio, "efficiency" is the 1 meter on-axis broadband anechoic SPL that a speaker delivers with a 1 watt input. Some manufacturers use a non-anechoic measurement (including room contribution), and some use a peak somewhere in the response curve instead of using a broadband average - so comparing two manufacturer's numbers may not really be apples-to-apples.
"Sensitivity" is the 1 meter SPL the speaker delivers from a 2.83 volt input. Sometimes called "voltage sentitivity" to make that clear. The same opportunities for optimistic specs enter the picture - namely, using non-anechoic conditions and/or taking the measurement at a peak rather than using a broadband average.
So, to recap: Efficiency is related to Watts, and Sensitivity is related to Volts.
(Some manufacturers use these terms interchangeably or backwards from this convention, so it's up to you to read carefully and figure out what they're really saying. Look and see whether they are talking about 1 watt input, or 2.83 volts input.)
Now, here is how the two are related: 2.83 volts into 8 ohms is one watt; 2.83 volts into 4 ohms is 2 watts; and 2.83 volts into 16 ohms is one-half watt.
So if you have a 4-ohm, 91-dB/2.83 volt sensitive speaker, it's an 88-dB/1 watt efficient speaker. If you have a 16-ohm, 85 dB/2.83 volt speaker, it is also an 88 dB/1 watt efficient speaker. I prefer to convert back to watts, because you can usually figure out the amplifier's maximum wattage output into a given load more easily than you can figure out its maximum voltage output (which may or may not vary with load, depending on the amplifier), and because if you are interested in power handling then you want to know the wattage going into the speaker.
Hope this helps.
Duke