Thiel Owners

Atkinson and others have hijacked "coherence" to include the smooth phase transitions in non-coincident systems with large phase shift - as long as the phase transitions are smooth, some call it phase coherent

That is certainly what I understand as one class of "phase coherence".

Thiel, Vandersteen and many physicists reserve the term "coherence" to mean that the phase response remains minimum, ie. it does not depart from flat.

I look at some of Vandersteen designs and I am not sure how he could achieve that.  

A good measure is "excess phase" approaching zero.

Yes, in the strictest of the definition, that is true.  But in reality, I am not sure if any speakers can meet that definition but I could be wrong since I have not seen all the designs out there.  The latter Thiel speakers use coax drivers so maybe "time coherence" can be achieved without excess phase on the tweeter as it approaches 20KHz.  In the case of Vandersteens, he uses conventional drivers with the tweeters offset from the midrange so I am not sure how he could do that without some amount of excess phase.  So it's possible he could be sacrificing "smooth phase transition" at the expense of the tweeter excess phase?

I've seen John Atkinson measurement of the CS2.4 step response, and the peak on the step response does suggest that the tweeter response as it approaches 20KHz may not meet that "strictest of definition" as you pointed above.  If a speaker meets the "strictest of definition" of "phase coincidence" , then the step response should be "smooth" on top of the step response where there lies the tweeter phase vs. the rest of the frequency (for example below the tweeter cross over point).

For example, if you take the phase at the cross-over point to be the reference, the tweeter phase will always deviate from that reference as it approaches 20KHz and therefore the speaker does not meet your claim of "phase coincidence" and therefore by definition, the tweeter response will not be "phase coincident" with the rest of the frequencies.  And of course, some speakers are the worse offenders than others with this respect.

While I commend Stereophile for actually measuring some of the gear they review. Some of their published measurements are from less than ideal procedures. In particular their speaker measurements and especially those pertaining to time. Ideally they would use anechoic measurements and do them from a distance consistent with manufacturers recommended listening distances. John Dunlavy took them to task for their measurement protocols.

While I commend Stereophile for actually measuring some of the gear they review.

The discussion is not really about Stereophile measurements but more about the underlying meaning of "time coherent" or "phase coincidence" (Tom terminology). My argument is a perfect "phase coincidence" is not physically realizable in real world environment irrespective of measurements. I think you can have a speaker that is "phase coincident" but only within a certain frequency range, NOT from 0 - 20KHz.

Even with a speaker that only has one driver, it will have different phase at different frequency, and yes it will have a proper step response, but based on Tom strict definition then it is not "phase coincident" at least not at all frequencies.

I am not criticizing Thiel products in anyway, it just seems that Tom was using an argument that is a bit unreasonable.

Tom,

If you look at Stereophile measurement of the Thiel CS3.7 here:
https://www.stereophile.com/content/thiel-cs37-loudspeaker-measurements

You can see there is a rather pronounce peak on top of the step response which means the tweeter phase is not well aligned with the rest of the frequency range, otherwise there would not be such pronounced peak.  Again I don't mean to criticize here but more like trying to understand your definition of the term "phase coincidence".  You definitely threw a curve ball into the conversation :-)  It looks to me the tweeter phase is deviating from the frequency cross over point between the mid and the tweeter.  John Atkinson would call this "phase coherent" and I would too.  But then it appears that from your above post, you would disagree.  If that is the case then it would imply the CS3.7 does not meet your criteria as "time coherent".

Unsound - right on. Dunlavy took them to task and Vandersteen chided in an interview. Thiel chose to not respond as a policy, but discussed it with John. The biggest problem with them is their (usually) 50" mic distance, which does not allow the wavefront to integrate, and then JA reading in to the ragged scan, which was caused by the technique, not the speaker. They published excess phase charts on older models, which are near zero, and even at 20kHz, they drift less than 10°. In our development measurements, all models (in my time, and probably later) fell within that 10° from minimum, except the CS5, which was <5° (plus Jim published a time delay spec which I remember being in the microseconds.)

Andy - I don't remember using "phase coincidence" or "time coherent", since I find both terms confusing.

The separate drivers are placed in 3-D space to sum properly when listening at 35"±~4" and greater than 8', optimized for 3M / 10'. That is a stated constraint, which we believed to be reasonable for real people in real listening situations. A lot of the confusion revolves around magazines/reviewers not being able to measure in those real-world situations.  

Here's the Stereophile measurements of the CS1.5: https://www.stereophile.com/content/thiel-cs15-measurements
Regarding Vandersteen, I plead ignorance of any particulars. Richard didn't go places that were unfriendly. He knew the pitfalls of those measurements and had plenty of sales to side-step that playing field, although he and JA were on the best of terms. From everything I have read, he was pursuing the same expression of minimum phase as Thiel.

Andy - regarding your last paragraph on the 3.7 charts. Those charts would indeed FAIL my definitions of both "time coincidence" and "phase coherence". BUT I don't buy the charts. Note they are taken at 50" and on the tweeter axis, both of which meet Stereophile's MO, but are illegitimate for the system under test. At 100" and 35" ear height, those measurements actually yield clean triangles without those false anomalies.

I am really not the guy to try to explain this stuff. It's pretty deep and complicated by real and imagined factors, and my knowledge is real, but not fluent. Here is what I can say: Jim was an engineer's engineer, and an honest mathematician and physicist. He pursued the problems to their root causes and engineered thorough solutions. I have only conversational knowledge of what he knew to his bones. That said, I was in the lab every day for 20 years and helped devise and implement the test set-ups and cross-checks. For frequencies lower than our chamber could reliably measure (100 Hz±) we had a roof system (100'x 500' flat roof) which we correlated with both straight and ground-plane techniques. It is germane that our test system included impulses for step responses at 1/3 octave intervals from 10Hz to 30kHz. Thiel speakers pass those pulses from bottom to top, including through the crossover regions. I don't know of a more stringent test for coherence, and to my knowledge, all speakers from the 03 in 1977 meet that requirement.