Time coherence - how important and what speakers?

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.

I'll take a stab at Joseph Audio's post: First of all, no one ever claimed that time and phase coherence is the only important parameter in loudspeaker design (at least, no one with any sense). There are many, many factors, all of which are extremely important, and any one of which, if ignored, will detract in a significant way from the end result. THAT is the crux of the matter.

So how do you know that time and phase coherence is critical? So few speakers have these characteristics that many people are fully acclimated to the sound of noncoherent designs, and so will not notice that anything is wrong. All the individual "sounds" are there, and in the right volume proportion, and they all have excellent attack and decay, etc., but the speaker itself does not sound "coherent". I hate to use such a nebulous term, but it truly sounds as though there is not a musical whole, but rather a bunch of separate musical events occurring more or less simultaneously. It is the difference between "natural", which allows you to relax into the music, and "a really great stereo system", which may be fantastically impressive at a hifi show but never actually fools your brain into thinking that you are listening to the real thing. That is my personal opinion, but it is nonetheless exactly what I hear. When it is right, you simply forget that you are listening to speakers. And it occurs with vastly disparate design philosophies, including multiway cone/dome systems, electrostatics, and Walsh systems. Although they are instantly distinguishable from one another due to their vastly different dispersion characteristics, all of them allow your brain to relax and feel as though you are listening to real music. I have never heard a noncoherent system that could do this, not one, in over twenty years of being obsessed with hifi and music. Sure, I've heard plenty that sounded like great hi-fi speakers, but that's an entirely different subject.

That, in a nutshell, is my personal answer. Do not presume that I or others are just making this stuff up for our own intellectual masturbation. It is very real to us, every bit as real as the anechoic-frequency-response or cumulative-spectral-decay graph is to you. And every bit as important, even if it is only one of many things that are every bit as important.