This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
When it comes to mids and highs, efficiency (sensitivity) is a fairly
good indicator of output differences at the same power level. When it
comes to subwoofer performance, the driver's sensitivity is irrelevant
unless you are also specifying a box volume.
An efficient sub requires a larger box to achieve equivalent extension
to a less efficient sub. In a small box, the less efficient sub will
actually be LOUDER at low frequencies at the SAME POWER as the more
Linear excursion is a very good indicator of ultimate output capability
(given sufficient power to drive the speaker to that point.) To make
sound you must move air; therefore, the more air you move, the more
sound you make. When comparing two speakers of equal surface area, the
one with greater excursion capability will play louder given sufficient