Herman, there certainly has been much debate over the merits of time coherency.
Just to clarify one point first. The way they describe it, time "coherent" demands time "alignment". But the reverse is not true.
You're point about phase is certainly correct. But I think there is also a distinction with the "when" of phase. For instance, it's either the 2nd order or 4th order filter (I can't remember which) that is phase coherent in the crossover region. Which is to say, all the peaks and valley's line up. The difference is that some frequencies started before others. The delayed ones are a full cycle behind. They are still phase coherent but not time coherent.
The easiest way I found to grasp it was to note that TIME coherent was at the pinnacle of the hierarchy. Indeed, most people, when refereing to this type of design, say "Time AND phase coherent". But it's redundant to say that since time coherent demands phase coherent. It demands both time alignment and phase alignment. These other two could be achieved individually by means of physical placement or filter makeup. But both, on their own, was only part of the story. Anyway, thats the best way I know to describe it.
Also, I think phase alignment and coherent ARE the same thing. But I'll have to think about that a little more to be sure. No, I think its true. anyway..........
First order filters do have phase shifting but the filter circuit as a whole compensates. The current lag in an inductor is the same degree (hopefully) as the voltage lag in a capacitor. But, I'm going to have to think about that some more before I try to go any further. I want to make sure I don't mistate anything.
The one measurement that is the arbiter of all this is the acoustical step resonse. Two speakers that easily show this is Wilson and Dunlavy. Both are time "aligned".
Wilson: http://www.stereophile.com/floorloudspeakers/920/index6.html
Dunlavy: http://www.stereophile.com/floorloudspeakers/162/index10.html
Looking at the step responses. Though, be careful when reading what JA has to say. I've noticed that over the years he throws all the descriptions around without much measure of consistency.
The short of it is this. The only speaker design that shows a step response (and therefor the most accuracy in the time domain) that mimics the input is a "time coherent" design. Ala Dunlavy, Vandersteen, Meadowlark, etc etc. They are the only ones that approach the right triangle form.
The merits of this? Well, like I said, Hotly debated.
I gotta run. More later.
Cheers!
