Speaker sensitivity vs SQ

Quad 57's still sound more like real music than most of today's overpriced offerings! I own two pairs! 

And still a formidable contender in the small box category is the BBC LS3/5A! I have a pair of Rogers 11 ohm's. 

a systems engineering approach should always compare the output with the input, including impulse response. Sure a hyper well executed high efficiency driver using best available materials, engineering, machining, etc are expensive. I think any reputable end speaker company designing own drivers, even semi custom w Scanspeak, are similar $$$$$. Some of those take into consideration that output is not a measure of quality.

I also believe, no designer should ignore ear brain. Many just sell distortion we like. Flavorizers.....

sure, the holy grail is low mass, breakup far from the passband, pistonic, with super high strength motors and high duty cycle....nothing has changed....

also, as an ESL owner, I think about the sonic impact of transformers and edge clamping distortion as well as energy storage in the panel....ain’t no free lunch

glad the conversation has returned to civility....

No free lunches indeed. There are those who'd like to sell the idea that big means much more expensive, but to them boutique, often vintage drivers with field coil or alnico magnets in addition to luxury, hand-build finishes is the only validating entry into the high efficiency arena. Their (affiliated) business, obviously, but NOT representative of what's possible with high efficiency and large size at more reasonable prices that can as well produce great sound.

The real non-free lunch here is size (and the product segment typically associated with that), as we're seeing considerable expense being shelled out already for a speaker package not much larger than a meager shoebox. Sound is physics, meaning displacement and efficiency is irreplaceable as that which accommodates truer dynamic envelope, low distortion and ease - core parameters into what is perceived authentic as a live event, yet they're wildly neglected for above reasons; not price, but size predominantly and the product narrative that follows here. Last but not least: live sound mayn't be that big of a draw after all..

Clearthinker, good questions, great thread!You brought up all the eternal questions in audio - high power vs low power and high vs low efficiency, and of course, cost issues. Looking at the average audio product, 1 in 10 sounds very good, while the rest is mediocre to bad. (Due to flaws, or more frequently, due to equipment mismatching. Even for low efficiency speakers, you have to hear about 20 to find one great sounding. The same is true for high efficiency speakers, and given that it's rare to hear even a single one of those, and you need to hear 20 systems with them to have a comparable basis to low efficiency speakers. Most people do not get to that point, and as there's a minuscule chance that one trips onto a great sounding one right at the first try, the automatic assumption is that H.eff speakers are inferior.
The true difference between low and H.eff speakers is that H.eff speakers couple cone movement to the air much better, hence they translate much more of the audio signal to sound. As a result, you hear much more dynamic resolution, and you also hear much more of any defects the system has. It's not just the speakers issues are magnified with high sensitivity, but also the rest of the audio chain is forced to come out of hiding, and their flaws are exposed. That's why slid state amps sound generally quite bad or at least disappointing with them. Indeed, most tube amps have issues as well, but a good tube amp can be built in a reasonable budget that will sound good, while I have yet to hear a solid state amp that sounds even remotely acceptable once efficiency goes beyond 100dB/Wm.
The issues with H.eff were not just affordability but availability as well. The retailer can stock half a dozen to a dozen plus speakers in the same space that a single pair of H.eff speakers require. When a store owner can stock 5 pairs of speakers total, he's not going to stay in business. Also, while nowdays the price range for H.eff drivers went down, in the 50s-60s when the low efficiency speakers changed the scene the driver costs for H.eff drivers were much much higher than low eff drivers, and the relative cost of even a simple audio system rivaled that of a motor bike or a car, so the only choice really was the low efficiency version. Plus, who wants to give up half the living room space to a stereo system when you can get one that can be shoved onto the shelves? Also, you cannot sell the weak amps and sources with H.eff speakers as any deficiency will be glaring at you, so that's another key factor that limits H.eff and favors low eff speakers.
Class D for the low end is a very enticing solution, and is a fantastic compromise if you want H.eff midrange and top end. Yet, the stark difference in sensitivity between the two parts of the spectrum will come and bite our donkeys (or, asses). Your ears will come to realize after a while that there is a huge disconnect, and you will loose interest in the sound. To me, one of the biggest advantages of high efficiency is the efficient bass. It sounds first as if there was lower bass extension, because you are not getting that "pressurized" feeling that your head is about to explode with the sound pressure. However, you will notice that the sound is not a porcelain muppet freakshow anymore with angry goblins kicking your seat, but you are hearing much more natural presentation - base will sound breathing and alive, and when you turn the volume down the soundstage does not collapse.Amplifier power: for 100dB/Wm efficiency you need 60 milliwatts for a VERY LOUD volume, and you still have tremendous headroom even when using a 500milliW (half a W) amplifier. The quiet passages will play at microwatt levels - that is a few millionth of a watt! So, if you have an amp that excels at 6000W that;s not necessarily going to be a virtue here, as does it also excel at a millionth of a watt?
I have a youtube channel dedicated to audiophile education, (Real World Audio), and most of my videos are about these subjects, distilling my 20+ years of experience building & designing speakers and amplifiers.