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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
andy2 OP
1,555 posts
 
Before posting any real data, simulations, plots and what not ... let's put down some basic definitions of what is time-phase coherence.  

I can see three basic types - from easiest to most difficult.  Let's start with the easy first.

1. First order/No Time-Phase coherence: this speaker will use first electrical order, but there is no time-phase coherence.  It will not be able to produce a proper step response.  It's more or less conventional with the exception that it uses first order filters.

2. First order/Time-Phase Coherence, BUT NO "Time coincidence"
    (which will be explained in #3).
  This speaker will be able to produce a proper step response, BUT and an important BUT.  It may not be able to produce an excess phase of 0 degree from say 50Hz to 20KHz.  This means that the speaker, for example, may have a phase shift of 50 degree or more or could be a full 360 degree at 15KHz, but only 5 deg at 500Hz.  That is its excess phase will vary especially at higher frequencies as the tweeter approaching 20KHz.  John Atkinson would agree this speaker meets his definition of "Time-Phase Coherence" since it could produce a proper step response.  My guess is most speakers that were measured by John Atkinson would fall into this category.  I've seen some measurements done on Vandersteens speakers and I was like ... hmmm... I am not quite sure.   But the most stringent definition is reserved for #3.

3. First order/Time-Phase Coherence AND Time-coincident: this is the most difficult definition for any speaker to meet.  That is it has to be able to produce a proper step response like in #2, BUT it also has to be able to have a 0 degree of excess phase from DC - 20KHz.  In reality, no speaker will have absolutely 0 degree, but the variations should be very small.  I believe Thiel claims that their speakers excess phase shift is only a few degrees (less than ten).  To be honest, I am not sure many speakers in the entire history can meet this definition.

So to summarize, you have three distinct possibilities from easiest to hardest:
1. First order/No Time-Phase coherence
2. First order/Time-Phase coherence BUT NO "Time coincident"
3. First order/Time-Phase coherence AND "Time coincident"

As for terminology, I guess you can call anything you want, but as far as the measurements, those are the three categories. 

With real data, graphs, plots, simulations those three cases can be clearly demonstrated.  Just talking about it making things more confusing.  
audition__audio
1,524 posts
 
You can Google and find photos of this crossover. When I look at this thing I dont know if I should laugh or cry. 
andy2 OP
1,555 posts
 
When I look at this thing I dont know if I should laugh or cry.
Ultimately it all ends up in "cry" like the "Crying Game".  But I give that it helps to laugh every now and then.
andy2 OP
1,555 posts
 
You can Google and find photos of this crossover
OK, just did.  Actually they don't look that bad.  It all depends.  


bdp24's avatar
bdp24
9,931 posts
 

As Danny Richie (GR Research) explains in his series of excellent You Tube videos on the subject of loudspeaker design, cross-over design is a matter of getting the individual drivers to coalesce. He uses the waterfall plot as his number one design tool, as it reveals a LOT about what the drivers are doing, both in amplitude and phase, which are inter-related.

The phase responses of the drivers in a speaker is a major component in that speaker’s frequency response. The summed output of two drivers which are out-of-phase at any given frequency cancel, creating a null, and a resulting depression in the frequency response. Listening to Danny discuss a speaker which uses two widely-spaced tweeters to reproduce the same frequency is pretty amusing. ;-)

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