Why low sensitivity speakers?

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?

I though it was 1)output 2)extension and 3) volume (as it pertains to LF analysis). I don't know why but it is hard to get deep bass and high efficiency out of a subwoofer. But the overall system sensitivity is usually at the mercy of the least efficient driver, and that's usually the LF or Midrange driver. Tweets are usually more efficient and you'll see L-pad. You can hornload a driver to increase its efficiency but thats rarely done. I think once you're in the 90db territory you're getting "warm" for high(er)-efficiency. 100's really freakin great and things above that just get obscene, like 113db 1w/m.

I found an article once, can't find it now. It said a high efficiency speaker CANNOT produce deep bass. Eg: Horns.

Without getting into the physics of speaker design, here's an analogy:

Suppose you have a table with many bottles of different weights covering it. The bottles represent the audio frequency range. The heaviest bottles are the lowest frequencies and the lighter ones are the high frequencies. You attempt to lift all the bottles from the table. The energy you expend is the amplifier power. The height that you lift the bottles above the table is the loudness, or the speaker efficiency. If all bottles are lifted at the same height at the same time, you have a very dynamic speaker.

Now, suppose you want to lift the bottles higher (and still at the same time) with the same amount of energy input, to get a higher efficiency. Physically impossible. The only way to do it is to lift fewer bottles. Since you have to lift as many as possible to get a believable reproduction, you forego the heavier bottles. You can now lift the remaining ones higher. That's the tradoff - a compromise of low frequency extension.

As stated above, you can increase the size of the cabinet to "lift" more bottles, but tradeoffs come into play. Namely, the time domain - which is represented by the speed you lift the bottles off the table, which is how your brain processes the sound. Altering the cabinet and the driver locations has an impact.

All speaker designs are tradeoffs. To "lift the bottles" not only requires that power lift but also lift at the same speed and time, requiring more energy than just amplifying frequency. That's one of many reasons why some of the better speakers will have a relatively low efficiency. It's the price paid - not the design criteria.