Speaker sensitivity vs SQ

We're nearly at the end of this one.

Thanks for your post mapman and a couple of others on the same point.
A large part of the answer to my question seems to be that efficient speakers cost more to build to the same SQ level in part because need to be a lot larger, in part because driver manufacturing tolerances are more critical.  This accounts for the huge growth in low-efficiency small box speakers since the 80s.

Indeed, since the 60s - does anyone else remember the introduction of the Goodmans Maxim?   https://www.ebay.co.uk/itm/133504369128

Just out of school a friend of mine had a pair.  Only 10.5 inches high.  Great for a student room.  Maxim has impressive sound quality for the size and surprising bass if you backed it against a wall.

One might observe that avoiding high amplifier costs with efficient speakers is one side of a coin with high speaker costs on the other side.

No free lunch after all.

Phusis and Duke, I beg to agree and disagree. I have always said the more power the better. This is a relative statement, relative to efficiency and other factors such as the output capability of the speaker. A speaker can only do so loud.
I have been listening to as many horn loaded loudspeakers as I can lately and in general they are very impressive or can be very impressive. It is certainly easier to get dynamic sound because they are so efficient and they go very loud. Now an ESLs volume capability is based on its Xmax which is very small in comparison to dynamic speakers. If you try to run it full range with bass laden material it will run out of Xmax pretty quickly and start clipping or rapping the stators. However if you do two things the situation turns upside down. These are, send everything under 100 Hz to a subwoofer array and design the speaker so that it is a full range line source. What you get is every bit as dynamic as a horn system and I think because there are no crossovers otherwise, an effortless naturalness that makes the speakers disappear. Line sources project power better than point sources. Because ESL almost match the impedance of air their transfer of power is very efficient even if their electrical efficiency is not. So, you have a very dynamic, low efficiency speaker system that goes very loud and has no crossovers above 100 Hz.
Phusis, lets say you have two speakers that both clip at 120 dB at 1 meter. One has an efficiency of 103 dB/1watt/meter and the other 84dB/1watt/1 meter. With a paltry 2 watts the first is blasting at 106 dB and the second only at 87db. To get the second to 106 dB you need 160 watts and this is at 1 meter. At a reasonable listening distance you are easily under 100db, probably down towards 95 dB. We are not even talking about peaks here. The point is that low efficiency speakers require a lot more power to hit dynamic peaks, hundreds of watts. Duke, your friend has more work to do. There are so many factors involved that I doubt you can make a blanket statement that high efficiency speakers are all more dynamic than low efficiency speakers of various types given appropriate power. It is certainly easier to make high efficiency speakers sound dynamic and I would rather have a good sounding high efficiency loud speaker than an equally good sounding low efficiency speaker. The more power on a relative basis the better. Fortunately for me ESLs are more efficient than ever 89-90dB/1watt/meter and there are plenty of amps now that can drive them without farting or blowing up. I should be able to hit 100 dB without leaving Class A operation. 

So what is the term for plain old lack of linearity of a driver?  That it's output at 90 dB doesn't match it's output at 70 dB? Is this considered strictly as mechanical compression?

Also, the best AMT's have amazing lack of such artifacts and incredibly robust power handling. :) One of the reason I'm a big fan.

Well, color me surprised, again, that we are comparing insensitive speakers for sound quality.  I though this would be about efficiency and how high efficiency drivers, heating up less, were often less prone to thermal compression.

I'm not sure how we can correlate efficiency or sensitivity to sound quality if we are confounding the discussion with types of motors.

@audiokinesis

A couple of years ago I read an article on compression in traditional dome tweeters.  I was 99% sure it was from the late, great Dr. Siegfried Linkwitz but now I can't find it.

Anyway, yeah, thermal compression happened supper fast.  First tone burst was normal, then by the second only the first couple of cycles were even close to the original output.