Why low sensitivity speakers?

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.

Karls, I learned a lot from your post. Could you explain something I have heard.

Listening to PSB Image 4T with plastic drivers, I was immediately disappointed to hear how much cleaner the sound was than B&W kevlar. But B&W's sales pitch is that Kevlar has benign breakup and plastic / metal etc. doesn't. That is, as the sound wave hits the edge of the driver and ripples back, the kevlar breaks it up while the plastic doesn't. The result is this reflected wave produces sound (which is not musical nor created by the input signal) and mucks up the true signal.

I can verify this as I have heard distortion on the PSB's which I think was due to breakup.

Is there any way to get the clarity without the nasty breakup? I asked B&W about this and they said using an aluminum driver, for example, the designer has to use a smaller driver to push the breakup mode higher and out of the range the driver is designed for. Even still, I have heard nasty ringing on small metal driver speakers.

I don't seem to hear this problem on Thiel speakers even though they have metal drivers and maybe this is because Jim Thiel is so obsessive about his crossover designs.

Would ribbon mid or tweeter drivers solve this problem?
Thanks, this question has bothered me for a while.

Cdc, IMO, there is no way to lay the differences you heard at the doorstep of driver material for sure - there are simply too many concurrent variables at work in any given loudspeaker design to determine which is responsible for what by ear. And BTW, were the two speakers you refer to set up in exactly the same system, and fed the same material?

IMO, of all the available cone materials, paper is still the best.