Thiel Owners

jon - those ATC measurements are commendable and likely to produce very fine results in the frequency domain. For this Thiel thread, I will add a few comments.

Loudness: 4th order or any higher than 1st order slopes allow the driver to operate in its robust range and attenuate the out of band requirements, so they can play louder cleaner. Things get so much easier in that world; that's why most designers go there. Thiel's 01 used 3rd order and the model 02 used 2nd order slopes. The change for the 03 and after to first order increased the difficulty of the design undertaking by an order of magnitude, at least.

And as I have mentioned in the past, by removing phase and time coherence, the ear-brain gives a free pass to many other anomalies; they no longer are scrutinized as real and therefore can be ignored. Examples include the edge diffraction and soft-dome break-up modes. Diffraction isn't particularly audible with high order slopes because the brain doesn't associate the source with reality, and diaphragm breakup is attenuated to lower loudness and becomes less audible. Note that the literature considers 4th order Linkwitz-Reily filters to have 0 phase shift, but that is because they pretend that 360°, which rotates phase angle a full cycle, is exempt because 360° looks like 0 on a graph. Also, the speakers described do not look time-aligned, so the transient wave-fronts will reach the ear at different times as well as phase delayed relative to the input. Many commentators say that doesn't matter, which is because the ear-brain is so good at reconstructing the probable intended sound which has been scrambled by the speakers. Our work at Thiel demonstrated to our satisfaction how that brain-work of reconstructing the probable-intentioned waveform serves to decouple the listener from the emotional experience of the music. As I have said, most commentators disagree and deem higher order filters to be OK, and first order, phase coherent wavefronts to be unhearable and meaningless.

I suspect that Thiel lovers have identified the "trueness" of phase and time coherence and are willing to put  up with the attendant compromises including less smooth frequency domain performance and higher audibility of many ancillary anomalies. Jim spent a lifetime identifying and reducing those anomalies (sonic baggage) and I am now stretching the envelope to include cleaner electronic performance which original budgets and materials science did not permit.

The ATC actives use some kind of active phase compensation to align the drivers at the crossover frequencies.  I don't think they go to the trouble Thiel did to keep time alignment throughout.  What big active ATCs have that little else can match is massive dynamics.  I think there is a dynamic linearity as well that makes them very revealing in a way other speakers can't match.  This ability lends a different type of realism that would probably be impossible for first order speakers to achieve.  ATC believes in making the drivers as well as possible so the crossover doesn't have to be complex.  They don't image like Thiels and they don't sound near as lively at low volumes.  I consider my Thiel/ATC systems to be quite complementary.

They describe the active crossover as "380Hz and 3.5kHz, 4th order, critically damped with phase compensation".  They discuss phase response in their literature but don't give specifics.  They're clearly in favor of linear phase response and it's a design goal.  "An ideal speaker system should have phase response linear with frequency, which in simple terms means that all frequencies produced by the driver reach the listener’s ear at the same time. "


http://www.transaudiogroup.com/wp-content/uploads/2015/06/ATC_Engineering_Goals_and_Approaches.pdf

Phase coherence does not guarantee time coherence.
Time coherence does guarantee phase coherence.

would I be right in saying that phase coherence means that the drivers are in phase with each other but not necessarily with the source?  So if two drivers are 50 degrees out of phase with the source signal at a particular frequency we say they're phase coherent but not time coherent?

I would say that there are many ways to produce some of the aspects of coherence. Rather than massaging the various aspects, I find the way to "see" that a transducer is keeping all the temporal information straight is to feed it an impulse. If the graph of that impulse rises immediately from zero to a peak and begins a downhill decline (as no more signal is being fed to it) creating a triangle . . . then and only then is the transducer coherent. A single driver such as a headphone acts this way. When using multiple drivers, they overlap and contribute additively and/or destructively in time and frequency and directionality. When they add to act like a single driver, the term of art is "minimum phase response".

A valid test is to overlay (on an oscilloscope screen etc.) the input impulse and the resultant microphone output from the speaker. The speaker will always degrade the signal in some way due to Murphy's Law of Material Physics. If the waves look subtantively the same, then you have preserved the relevant information.

Otherwise, I would find it difficult to wade through the various claims and side-steps and judgements associated with coherence.