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First order/Time Phase-Coherent speakers discussions

"The game is done! I’ve won! I’ve won!"


I would like to use this thread to talk about this subject which I find rather fascinating and somewhat difficult to get my hands on. I went through a course in electromagnetism in college and I have to say this is even more confusing and you won’t find the answer in calculus, physics, Einstein relativity be damned it’s not in there either and definitely not in quantum physics. Listening to the "experts" from Vandersteens and Stereophile but ultimately it all came down to a missing link sort of argument ... something like this:
"Since if a speaker can produce a step response correctly, therefore it is time-phase coherent, and therefore it must be "good".

It’s like saying humans come from chimps since they share 90% genetic content with us, but we can’t find any missing links or evidence. FYI, we share a lot of gene with the corn plants as well. Another argument I’ve heard from John Atkinson that lacks any supporting evidence and he said that if everything else being equal, time-phase coherence tends to produce a more coherent and superior soundstage, but to the best of my knowledge, nobody has been able to produce some semblance of evidence since there is no way to compare apples to apples. Speaker "A" may have better soundstage simply because it’s a BETTER design, and the claim "time-phase coherent" is just a red herring. There’s no way one can say the "goodness" from "time-phase coherence" because you can’t compare apples to apples. Ultimately it’s a subjective quantification.

I’ve been doing some simulation and I will post some of my findings with graphs, plots, actual simulation runs so that we are discussing on subjective personal opinions. Some of my findings actually shows that intentionally making time-phase may result in inferior phase problem and NOT better! (will be discussed more in detail).

Having said all that, I am actually in favor of first order/time-phase coherent if POSSIBLE. I am not in favor of time-phase coherence just for the sake of it. It’s just that there are a lot of mis-information out there that hopefully this will clear those out. Well hopefully ...

Here my preliminary outline:

1. My "subjective" impression of what is "musicality" and how it’s related to first order filters.
2. Interpretation of step-response. I’ve read a lot of online writing with regard to the interpretations but I think a lot of them are wrong. A proper interpretation is presented with graphs and simulations.
3. A simulation of an 1st order and higher order filters with ideal drivers and why time-phase coherence is only possible with 1st order filter. This part will use ideal drivers. The next part will use real world drivers.
4. A simulation with actual drivers and how to design a 1st order/time phase coherent speaker. Discuss pros and cons. And why time-phase coherence may actually have phase issues.
5. Discuss real world examples of time-phase coherence with Thiel’s and Vandersteens speakers (and why I suspect they may not ultimately be time-phase coherent in the strictest sense).
6. I’ll think of something real to say here ... :-)
andy2
atdavid
736 posts
 
You stated that "why time-phase coherence is only possible with a 1st order filter" ... I was filling in the blanks.

3. A simulation of an 1st order and higher order filters with ideal drivers and why time-phase coherence is only possible with 1st order filter. This part will use ideal drivers. The next part will use real world drivers.

andy2 OP
1,538 posts
 
I was filling in the blanks.
OK, I don't think I should be responsible for someone else's words.  

Are you saying time-phase coherent is possible with higher order filter?  I suppose you could come up with your own definition of time-phase coherent but then anything is possible then.
atdavid
736 posts
 
As someone has already done the work, I will simply post that from John Bau. By done the work, someone who has a product on the market and advertises this is what they do. It is not such an advantageous marketing thing these days.
And that is only simple analog domain in the cross-over. With a digital active cross-over, time/phase alignment becomes relatively easy.


John Bau of Spica can show you on his Tecron analyzer that the Bessel 4th-order low-pass filter he uses to cross his tweeter down to the woofer on his TC-50 and Angelus, when used with the particular drivers he chooses, is also phase coherent.
Read more at https://www.stereophile.com/content/thiel-cs12-loudspeaker-phase-coherence#J3V7cQJwU8ILdX7k.99

andy2 OP
1,538 posts
 
With a digital active cross-over, time/phase alignment becomes relatively easy.
OK, you have just changed the rule of the game. Sure with digital processing, you could, but I was clearly talking about pure analog xover filters.

Digital processing has its own set of problems though which is just another can of worms that I am not sure I want to talk about in this thread.


timlub
2,039 posts
 
you really have a lot of homework left.... you can't figure phasing without understanding the drivers being used and how slopes and parts will affect phasing.... it has not been mentioned, but remember there is acoustic phasing as you have been discussing,  but there is also electrical phase. The rare time that I see that mentioned here on Agon is when discussing using tubed amplifiers,  but individual drivers can vary phase when you change crossover points or slopes or even part grades when it comes to coils.  Most of you have probably seen on occasion a tweeter or midrange is wired out of phase or reversed positive and negative.  This is done on purpose when phasing gets too far out of whack to bring it back in line.  On paper only,  at 6db per octave,  a driver goes 90 degrees out of phase,  12 db goes 180 degrees (which is when you reverse terminals), 18 db goes 270 degrees and 24 db come full around 360 degrees.  In real practice most of the time, these numbers are not near accurate.  So 6/6 slopes can get you close in phasing,  but why did Joe Di'Apolitto  use 18 db slopes in his mtm designs.... harder slopes often sound better,  they roll off any bumps in a drivers frequency response and they give the same type (in reverse) phasing as 1st order slopes.  
I have seen a ton of discussion in this forum about why 1st order slopes, but it is a rare few that understand why and how the drivers combined with crossovers affect phase and even time alignment.  
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