Why low sensitivity speakers?

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.

First consider radiating area. Typical loudspeakers are only about 1% efficient in an overall sense (that is, for 100 watts of electrical input power, you get an output of 1 watt of acoustical power). The reason for this is that there is a bad impedance mismatch between the surface of the driver and the surrounding air. Think of air as a medium, like a fluid only much lighter and more compressible. The driver only "sees" a very small, light load on its diaphragm, and as such is unable to impart much force against it, because the air moves out of the way so easily. The driver can exert much more force electromechanically than the air can accept acoustically, thus the "impedance mismatch". The reason that horns have such fantastic efficiency is that they gradually expand, allowing the driver to "see" a much larger surface area of air. That is, the driver ends up loaded by the area of the horn opening rather than its own diaphragm area. This makes the air appear much "stiffer" to the driver and results in a much better impedance match. The other way to achieve better impedance matching is to use a lot of direct-radiator surface area. Doubling the radiating area gives you 3dB of efficiency all by itself, just because of the improved coupling to the air. So a 12" driver is inherently four times more efficient at coupling to the air than a 6" driver is, and generally will require four times the enclosure volume as well. (The inside of the box "sees" four times as much air being pushed into it, so for the same compliance, needs four times the volume. It's the same as if there were four 6" drivers in the box.) The reason that 12" drivers aren't vastly more efficient than 6" drivers is that they have a much higher moving mass, see below...

For every additional octave of bass extension, you have to move four times the volume of air. This leads immediately to the necessity for large drivers with large excursions, which in turn requires large enclosures to support them. There is a tradeoff that can be made, though, if you can live with a lower output level capability.

Driver efficiency is primarily a function of magnet force and moving mass. (Think back to F=ma. Sound is nothing but the acceleration of air molecules back and forth. The higher the acceleration, the louder the sound.) So if you increase the mass, you get a lower efficiency but also a lower resonant frequency (better bass extension). Thus you can take a 6" driver which would normally have a resonant frequency of 60Hz, and by doubling the mass and the suspension compliance, get the resonant frequency (and thus the extension) down one octave to 30 Hz. You lose 6dB of efficiency and 12dB of output capability in the process! (Remember the four times air volume principle? This is where it comes back to bite you.) But in many cases, a tradeoff like this is made in order to get good bass at limited output levels out of a small driver.

Hope this helps.