Why low sensitivity speakers?

In response to bass: a stiffer cone (woofer) is required to produce a 'tight' bass. a cone that is less stiff will deform, causing muddy, poorly reproduced bass. the tradeoff here is that the stiffer cone requires more power (is less efficient).

Great explanation, Karls.

Karls, I'm no expert and you seem to have a much better understanding of these things than I do. However, it would appear to me that horns don't see more air in fact I would think they see less air. I'm guessing that the horn compresses the moving air so that there is less lossy disipation and therefore better efficiency.

Pmwoodward - Why would a stiffer cone be less efficient? A more massive cone, yes. But a more flexible cone seems to me as if it would just do a poorer job of moving the air, thus requiring more amp power to produce the same volume. Isn't this why the ultimate theoretical driver surface would be both infinitely stiff and yet infinitely light (impossible in reality)?

To Unsound: to use my analogy above, the point to a horn is that the air is unable to "escape" away from the radiating surface of the driver, that is, to just move to the side and out of the way of the driver, thus unloading it. In a horn, the gradual expansion forces the driver to load a much greater volume of air directly in front of it, and doesn't "release" the driver from this loading until the horn mouth opens into the room, at which point the surface area that the air is pushing against is immense. True horns are so efficient at loading their drivers that they require what are called compression drivers, which are designed to deliver much higher than normal force at much lower than normal excursions.

To Zaikesman and Pmwoodward: a stiff vs. flexible cone mostly affects the fidelity of the signal, not the inherent efficiency of the driver. That, as I said, is mostly dependent on magnet force and moving mass. Of course, enough flex will result in energy being dissipated as heat within the cone itself, which lowers the efficiency, but very few drivers flex enough to do this within their designed passbands. They will do it as they start to break up at the top of their range, but hopefully the crossover has taken over by then.