Speaker Efficiency, Sensitivity and Impedance

 This comes up a lot so worth trying to distill closely related specifications together. 

Speaker efficiency and sensitivity are often treated as the same, but they are different.  Both measure output at 1 meter in decibels, the issue is the input.

Sensitivity:  Output at 2.83V at 1m.  In other words, output vs. VOLTAGE.

Efficiency:  Output at 1W at 1m.  In other words, output vs. POWER.

If you have an 8 ohm speaker, sensitivity and efficiency are equivalent, because 2.83V into 8 Ohms = 1 Watt. 

Here’s a common example: Many mid woofers come in 8 ohm and 4 ohm versions. Nearly identical in all other aspects besides the impedance curves. The 8 Ohm version produces 87 dB at 2.83 V. That is, 87 db @ 1 Watt. The 4 ohm speaker produces 90 dB @ 2.83V, but since it has 2x the current it is 90 dB @ 2 Watts.

We care much more about this difference when dealing with tube or modest output amplifiers. 

If you care about whether a speaker is difficult to drive though, consider the minimum impedance.  Anything below 3.5 Ohms will start to get difficult for most receivers.