Every speaker should have a sensitivity rating, in dB, its usually measured at 1 meter with one watt. So a speaker with a 90dB sensitivity rating requires twice the power of a speaker with a 93dB rating to get the same output. Typically speakers with higher sensitivity have less power handling vs. a similar sized speaker with low sensitivity. What this means is that higher sensitivity requires less power to move the speaker to its maximum output level. Since power relates to voltage the same principle is applied to a microphone diaphragm. Using a speaker as a mic it really is only reproducing the low end, so using it in applications generally needs something with low end or at least moving air. Bass cabs and kick drums are the typical application for using a speaker as a mic. so what you get with a higher sensitivity speaker is a quicker transient response and faster recovery between transients, where as a lower sensitive speaker doesn't have the same transient response with the same signal because it requires more air to move the cone enough to get the same output as the higher sensitive speaker. if you think about sensitivity for microphones it works the same way. Some mics have a quicker transient response than others. You will also need less preamp gain on a higher sensitive speaker because since it is moving more its voltage output is higher.