Time coherence - how important and what speakers?

Time coherence is difficult to achieve because the acoustical center for a moving coil driver is not a fixed physical point, but actually varies somewhat with frequency. Source - The Loudspeaker Design Cookbook, sixth edition, page 113.

Phase coherence in a multi-driver system requires first order acoustical slopes for both the low-pass and high-pass driver. The acoustical slope is the sum of the inherent driver response and the crossover response. Note that time and/or phase coherence can only occur in a single plane with a multi-driver system.

Human hearing characteristics aren't linear, but are full of thresholds and masking effects. For example, after .68 milliseconds the directional cues from a repetition of the original signal (an echo) are suppressed, and this suppression (masking effect) lasts for about 40 milliseconds. Now, if we knew precisely the thresholds that apply to time and phase coherence, we could take them into account in loudspeaker design. Alas, the published research is inconsistent on the audibility thresholds of phase and timing errors.

There may be more than one way to skin a cat. John Bau's classic Spica TC-50 claimed time and phase coherence via this arrangement: slanted baffle with cossover: high-pass slope, approximately first-order, 6 dB/octave; low pass slope, fourth-order 24 dB/octave; both drivers connected with same polarity.

Unsound---they lied! It HAS to be a first order filter period. The phase shift created by a 24 db slope would be tuff to fix. Read Richard Hardesty's issue #3 of the Audioperfectionist. You might not like his recommendations but his science is correct.

This topic comes up every few months. Designing for phase coherence may be intellectually gratifying, but here are my questions for the Phase Coherence Uber Alles crowd.

1. What does time coherence actually sound like? How could you tell a time coherent design from a non-coherent one without
looking at the measurements? Why does almost every online list of "coherent" models include some that are not?

2. If all time coherent speakers are correct, then why do they sound so different from one another?

3. If this characteristic is so important, then why do only a few companies embrace it? and why aren't those companies dominating the marketplace?

4. What about OTHER factors such as distortion, wave interference, off-axis lobing, compression etc.!? Wouldn't these have much more influence over the quality of sound?

5. If this approach is the One True Way to superior sound, then why do people vote us Best Sound At Show?

Josephaud, to answer your questions:
(1). Time and phase coherent speakers sound as though they are "cut from one cloth" with less noticible diferrentiating sound of different drivers and cross-overs. They consistently demostrate superior sound staging and imaging.
(2). Because they uses different drivers, boxes etc. Would you ask the same question of different manufactures that share similar design priorities e.g. Acustat or Audiostatic?
(3). Perhaps it takes more talent, effort and expense in design equipment.
(4). Perhaps? Perhaps not?
(5). With all due respect not all people vote you Best Sound at Show. You have never been in the running when I vote.