Why low sensitivity speakers?

I could probably find this out with a little research, but I'm too lazy. Anybody know what the tradeoff is with a high sensitivity speaker? Why do some manufacturers make such low sensitivity speakers? Is it just so we have to buy huge amps?!
You can pick any two of these three options:
1. good bass
2. small cabinet
3. high sensitivity
This is an immutable law of physics. Since many people want the first two due to decorating and other domestic considerations, the third is the one that gets left behind. And since watts are relatively cheap, it is not a problem. High-sensitivity speakers have their own virtues, but nothing will ever change the above law.
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.
what do people consider high Efficiency?
Can you (or anyone else) explain why good bass, cabinet size and sensitivity are tradeoffs? Thanks.
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.
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.
Gs5556, I guess it's how you look at it. The people designing speakers with a high sensitivity don't have "limited bass response" as a design goal, just like people designing a speaker with good bass response don't have "low sensitivity" as a design goal.

I like your analogy, which makes since, but what physically makes this true is not obvious. I guess that with a given driver, and enclosure, it is through low "acoustic compliance" that lower frequencies response is attained.
Karls, just saw your last comment. Good point. It is the "suspension compliance" along with the "acoustic compliance" that results in the lower resonance of the system. I think we've got it figured out!
Perhaps my explaination was not clear. It is not the driver response, but rather the crossover that makes the efficiency (thus sensitivity) change. Gs556 analogy is really excelent and as to how it applies to speakers, it's back to what I said in the beginning. Lower frequencies require more movement of air to achieve the same SPL, thus they are the "heavier bottles". Great analogy Gs5556.
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.
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.
As usual, I agree with Twl. The problem has to do with two different materials properties: STIFFNESS and DAMPING. These are two entirely different things, but you would be amazed how much confusion there is about them. I have seen plenty of references to how well-damped aluminum is, for example in the Stereophile review of the Krell LAT-1. For anyone who thinks that aluminum is well-damped, do a simple experiment: go to your local musician supply store, and pick up a tuning fork. Knock it against your skull, then keep it next to your ear until you can no longer hear it ringing. Multiply the 10 seconds (or whatever) by the 420 Hz frequency (or whatever), to get the number of cycles it took to die out of audibility. Now ask yourself one simple question: "What's it made of?" Chances are, it's ALUMINUM, one of the most poorly damped materials known to man.

Sorry, but had to get that off my chest. To get back to the issue, different materials have very different combinations of stiffness and damping. Aluminum is very stiff and very poorly damped. Kevlar is still quite stiff, better damped, and lower in density as well. Plastics are generally quite flexible, and usually better damped still, but have a wide range of variability in both stiffness and damping depending on the formulation and the fillers used. Paper is typically stiffer than most plastics, and by itself is not as well damped, but when coated with the correct polymer coating achieves a very good compromise between stiffness and damping.

What you take from this is that there is no perfect material for making cones. What you would like is something with infinite stiffness and infinite internal damping, in addition to zero mass. This combination does not exist in the real world. Aluminum's advantages are very high stiffness, good formability, and relatively low cost. The price you pay is a GIANT resonant peak when the thing finally breaks up. There are two solutions: one, do what Joseph does and use a very high crossover slope to get it out of audibility, or two, do what Thiel does and push the resonant peak high enough in frequency that even a shallow-slope crossover can do a good job of removing it. (Notice the large voice coil on the 1.6? It's there to get the cantilevered length of aluminum down to a smaller value, to drive the resonant frequency up.) Kevlar is probably a better material in that its stiffness is still very good but it has much better internal damping. I believe B&W has a patent on single-layer Kevlar cones, which may be why they are "attached" to that lately. (Like a lot of patents, I'm not sure it would hold up in court, but they have enough lawyers to discourage anyone from trying.) There are many plastic formulations, some of which are quite good, usually using some type of mineral filler to add stiffness, often magnesium based in order to keep it light. The French manufacturers Audax and JMLab/Focal have experimented with all kinds of cone materials, including all kinds of exotic polymers and fiberglass and kevlar sandwich cones, but none of them have impressed me much. Eton made their living on fiberglass/honeycomb sandwich cones, which are very stiff but also have a nasty breakup peak. The only exotic construction I've seen that had some good sense behind it was the Ensemble driver from Switzerland, a sandwich of two thin layers of aluminum separated by a thick layer of EPS foam. Stiff, light, and well damped. Also very expensive to make and even harder to get good quality control due to the variability in EPS foams.

After many years of experience, I personally have come down in the camp of "make it as stiff and light as possible while still keeping very good internal damping". This, believe it or not, means coated paper cones or a very few select plastic-cone drivers.

Cdc, to answer your specific questions: Yes, ribbons eliminate this type of breakup, but they have resonant modes of their own of a different kind. Re cone "breakup", there are two separate issues. You have to distinguish between "piston mode", which is defined as the frequency range in which the cone still behaves as a piston (a flat inflexible surface), and "breakup mode", where the accelerations have gotten so high that the cone itself begins to flex and resonate in response to the drive signal. Even in "piston mode", the sound waves travel outward from the voice coil to the edge of the cone, and then reflect backwards down the cone. Note that sound waves travel MUCH faster in solids than in air, and that they will dissipate as heat if the material has good internal damping. The reflection at the edge of the cone is also highly dependent on the type of surround used; some surrounds do a much better job of absorbing this wave than others. B&W's claim in this area is that since the bidirectional Kevlar cloth they use for their cones is a non-homogeneous material (different stiffnesses in different directions across the cloth), it will do a better job of breaking up this wave. This may or may not be of much significance; I personally would much rather see a cone material with very high internal damping. In "breakup mode", where the cone is literally going crazy with internal resonances, again I would prefer a material with very high internal damping. Because you're right, you can hear cone resonance very easily, and it's not a pretty thing.
Thanks Karls, I will give this some thought. Zaikesman, the PSB were running on a Yamaha receiver/ Sony CDP. This was a store demo. Same music as the B&W.
I have the B&W in a different set-up and to the best I can say, the sound is not as clear as even the cheap PSB. Sound is much cleaner with low distortion, etc. but I don't think as clear.
It does not seem possible to get both sound qualities to their optimum potential at the same time and all the time. Take Revel for example. Some music they are great, other music they aren't. These are all my subjective opinions but B&W seems to be more consistent and livable across the board albeit sometimes sacrificing ultimate resolution.