Why low sensitivity speakers?

What he said, and probably also because not enough attention has been paid to the potential of modified horn-coupling strategies in audiophile speakers (except in the SET fringe), as wide dispersion and flat power response have seemingly gotten priority over wide dynamic range and low distortion, for reasons unknown to me.

Karls,
Can you (or anyone else) explain why good bass, cabinet size and sensitivity are tradeoffs? Thanks.

what do people consider high Efficiency?

Karls is right on the money. The greater the cabinet size the easier it is to move a lot of air, required for bass. It also enables you to use more than one or a larger bass driver, both of which add to the efficiency of moving a lot of air. You can make a comprimise between bass extension and frequency. If you choose a higher roll off frequency, the drivers do not have to use their efficiency to move the massive amount of air required for the lowest frequencies. I'm not a speaker designer but I think it's approximately a squared function, so rolling off can make a huge difference in efficiency.

Thanks Karls, I think I have read this somewhere as well. Rives, with all due respect, I don't think the frequency response of the driver actually has anything to do with its sensitivity.

With regard to efficiency, this is not exactly the same as sensitivity. The sensitivity rating accounts for the impedance of the speaker so it needs to be adjusted accordingly if compared to a speaker with a different impedance rating.

After a little research, I have found that it is true that in order to lower the fundamental resonant frequency of the speaker system, you must either increase the size of the enclosure, or decrease its efficiency.

It's not intuitively obvious why lower efficiency results in a lower fundamental, but I guess this results in more air pressure in the enclosure, which in turn results in a higher amplitude at the resonant frequency of the enclosure?

Can any speaker designers tell us if this makes sense?