Frequency response, flatness, dispersion, transient response, etc. What really costs in loudspeakers and separates the great from the also-rans is dynamic range. Simply put, that's the difference between the threshhold of noise and the threshhold of distortion. You can measure it in db. Close miked recorded music has lots of it (much more than music heard live), but most speakers don't.
I thought it was interesting that when asked what worlds were left to conquer in speaker design, the folks at Paradigm answered "dynamic range". Perfectly reasonable, since their conventional driver speakers do just about everything else well. Both horns and charged panel speakers can hardly achieve range and dispersion, but for regular old woofers and tweeters, dynamic range is what's left.
When a conventional driver is asked to play too loudly, its voice coil begins to travel outside the magnetic field of its magnet, or it reaches the limit of its mechanical suspension, and diaphragm movement fails to increase. The driver simply can't play louder. Audio peaks above that level are eclipsed, erased, not heard. It may not sound as if anything is wrong, but the sound lacks impact and definition. Its called compression, and is analagous to a photograph with poor contrast.
And while one doesn't think of loudspeakers as making their own noise, they indeed do. Most of it results from resonance--sound that lingers longer than it should. Its source is the energy imparted from the inward movement of the driver diaphragms which ideally should be fully absorbed, but in reality never is. So this "hangover" sound bouncing around inside the enclosure, though brief, covers up the short periods of silence that define delicate sounds. Again, it's analagous to a video picture that has no black--only dark grey. The shadow detail is lost.
Uncompressing power peaks is usually solved by adding additional drivers, although multiple driver radiation patterns can interfere with each other, so you usually see the drivers all in a vertical row so that the right to left image is free from interference patterns.
But what about getting rid of the noise? Completely absorbing the backwave energy requires huge enclosure volumes, and a design that prevents the backwave energy from being reflected back to the diaphragms. Smaller driver diaphragms help. They present less surface area that can be reexcited by backwave energy bouncing around inside the enclosure. The most elegant solution is perhaps the "transmission line"--a sort of tapering cone that is folded inside the enclosure that prevents backwave energy from ever being reflected back toward the driver diaphragm. When all this background sound is successfully suppressed, minute musical details are revealed against a background of complete silence.
So, how do your speakers do in the realm of dynamic range? Try this real world test. Use a superb pair of headphones such as the legendary Sony MDR-V6, and choose a CD where you hear minute detail resulting from close-miked recording. For example, in an orchestral recording you might hear the squeak of a music stand, the conductor breathing, etc. Those are sounds you would never hear live because your ears were not up close like the microphones. After you locate a track with this kind of detail, listen to it again on your speakers. Is it there? In my experience very very few dynamic speakers accomplish this.
On those that do, it sounds like the audio has been run through an expander--a device that increases the contrast between loud and soft. Not surprisingly, this adds the missing definition you don't hear on lesser speakers. I've never heard a pair of bookshelf speakers that could do this. By the time the level was loud enough to hear detail above the ambient noise in the listening room, the drivers were already compressing the audio peaks.
Speakers that truly perform well will both have high peak handling ability and more importantly, the ability to damp out all backwave energy. The latter just can't be done in a small enclosure, and it's tricky to accomplish it in a large enclosure. Thankfully, the days of the large woofer in a small box--like to old JBL 100s that captured so many dollars in the 70's--are gone. Primary drivers are becoming smaller relative to their enclosures, and we're getting better sound. Subwoofers can use servo control to help counteract backwave energy in their enclosures, but so far this hasn't been done effectively in the higher ranges.
So when you're out listening to speakers and wondering "why does this speaker cost so much?", try listening to a recording with dramatic loud to soft variations. That should allow you to hear if that expensive speaker is worth the price.
My trusted golden ear friends say there are a variety of speakers that do this. The only ones I own that can are an old pair of TDL Reference Standard Concert Grands. They have 7 drivers each and sarcophagus sized enlosures with separate transmission line termination for both the woofers and midranges. Even with twin bridged NAD 218THXs (780 watts each) cranked up, they put out an eerie amount of silence between transient musical peaks. And yes, you do hear the music stands squeaking and the conductor breathing.
There are certainly immensely enjoyable smaller systems out there, but if you're wondering why an expensive speaker can justify its price, I think the folks at Paradigm had the right idea.
