Originally Posted by Snoochers
... 1. What acoustics qualities of speakers are important beyond (1) volume and (2) frequency response? (I suspect there is a lot more, and that I'm simply ignorant. Things like distortion, crossover?).
2. How do these qualities relate to the cost of speakers (i.e., how do they increase price? Is it research? Using expensive materials?)
3. Which of these qualities can be measured objectively? Which can only be measured subjectively? ...
1) For loudspeaker drivers, the order is non-linear distortion (the one you're missing), linear distortion (frequency response), and SPL capability, each over the ~10 octave audible range. At higher frequencies, directivity becomes important.
The drivers then require mounting in a vibrationally inert box (won't buzz, even at high power) that's configured to optimize driver output (porting, sub-chambers), and connected to an electrical circuit (crossover) that makes sure the drivers "play nice."
2) Drivers with low distortion and high SPL capability have lots of exotic parts to gain added excursion, or minimize dome mass. They're also a small market, so economies of scale are limited. The boxes for them are complex, some with intricate bracing to damp vibrations, and finished like fine furniture. That all takes time, and time is money.
And then there's the speaker's interaction with the room, a result of all the things above plus crossover design, selection of crossover frequencies, and the shape of the wave interference pattern(s) when two drivers produce the same frequency. Engineering quality drivers is straightforward. Designing crossovers is part science, part art.
3) lots of this can be measured. What cannot be measured "well" is the human response to a loudspeaker in a room. You'll hear terms like "hear-related transfer function" mentioned in measurements, because our ears don't hear things the way a microphone does. It's an attempt to integrate measurements with human anatomy, get closer to "what you hear." However, your ears are already well equipped to "measure" it.
Set up a pair of speakers in the sweet spots in a room and listen. The best speakers disappear, replaced with a broad expanse of sound in front of, and around you. In Toole, these sensations are called Apparent Source Width (ASW) and Listener Envelopment (LE).
Now, what's your application?
The best stereo speakers are not the best home theater speakers. The ASW/LE stuff is much less important in multichannel applications because there's spatial information in the 5-channel program. You also have 5 speakers (or more) contributing to SPL, not just two, and they don't need the bass extension of a stereo speaker. Subwoofers make more deep bass than any full-range speaker.