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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

Showing 9 responses by timlub

timlub
2,039 posts
 
@andy2     Not sure that there is a question here.  I've run many simulations with 1st, 2nd, 3rd and 4th order slopes.  I've run many comparisons with butterworth and linkwitz riley in 2nd and 4th order.  

Even putting a single cap on a part does not ensure that you'll end up with 6db per octave.  And really isn't the closest thing that you can get to proper phasing coming all the way around with 4th order slopes?  
1st order are normally at minimum 15 degrees out of phase. 4th order can be very close.  
I'd recommend physically time aligning your drivers then comparing your slopes.  I've had surprisingly good results in soundstage and imaging running odd order butterworth (say 12/18) as well as linkwitz riley 4th order.  Both of these solve issues with driver peaks also.  
again not sure that these comments are what you are after, but I hope they help,  Tim 
timlub
2,039 posts
 
Andy, you have opinions that you don't understand. I'm happy to stick to your original premise, but slamming others for giving accurate information does not work. You never know, a couple of these guys could be speaker designers and worked with most of the quality parts that you have mentioned. If you want my help say so, I'll chime in. If you'd like to continue telling everyone how it is. I'm done, Tim
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.  
timlub
2,039 posts
 
Properly done active should always be better
timlub
2,039 posts
 
Andy, you have shown in your comments that you have a VERY LIMITED KNOWLEDGE. A decent active crossover in knowledgeable hands will crush the performance of a top tier passive crossover. Maybe you should explain what time phase coherence is from your point of view so that we can properly respond. So not to waste your time again. I was talking about time and acoustical phase alignment and how they are effected by the electro magnetic structure of a driver as well as the parts chosen in the crossover and the order of crossover design itself. Active designs make much of this benign and allows correct time and phase alignment with steep slopes. 
timlub
2,039 posts
 
" I am still waiting for someone to tell me who can understand time-phase coherent, including all the experts in the world )-: :-("
That I can help with....
First understand "Time Alignment" 
The speed of sound moves at the same speed at all frequencies, so the first battle would be to make the sound of the tweeter,  mid,  woofer or any other driver to hit your ears at the same time.  The most common way is to align the voice coils, where the sound of the speaker originates from.  When coils are in alignment,  the sound from all drivers hit the ear at the same time.  Could be done by staggering or a slanted front etc. 
Next ... Phasing: 
One reason that a single driver does so many things right is that there are no phase issues.  A single driver does not have to move in unison with any other drivers. 
So what is phasing?  All drivers moving in perfect unison.... when one driver starts moving outward,  all drivers move outward at the same time,  when it comes back,  all drivers move back at the same time.  This helps tremendously to prevent smearing or to improve pinpoint imaging. 
So what did I mean when I said that cross overs change phasing... A single cap or 1st order crossover,  normally will cause a phase shift of 90 degrees,  so to keep the next driver moving in unison,  it must also be moving 90 degrees out of phase to stay in unison with the other driver. 
Again,  24db per octave brings you back around 360 degrees to put drivers back in phase,  but the crossover parts count, the sound of the crossover parts being used and the practice vs theory that you don't truly end of with absolute phase cause most to not use 24db per octave slopes. 
Time/Phase Coherence is just what it sounds:  Time and phase working in conjunction to form a Unified Whole.  

I hope that I have expressed this in a way for everyone of any level to understand, 
Tim 
timlub
2,039 posts
 
By the way,  to be clear: 
Absolute phasing is IMPOSSIBLE. 
You can get phasing done at the crossover point which is the goal.  
a woofer playing bass at 50 cycles will move at a different rate than a tweeter playing at 15000 cycles. 
When we align phase at the crossover points, that is as close as you can hope for. 
I hope this helps,
Tim 
timlub
2,039 posts
 
@cousinbillyl 
You are correct about a lot of your phase conversations.... I spoke with Jim years gone by..... you might remember or go find up the thread where I said something like you can get a 1st order down to around a 15 degree phase shift??? that's what Jim did,  he did remarkable work taking a 90 degree shift down to 15 degrees.  
I won't argue,  but I will say that I've never seen absolute phase alignment between drivers without electronic crossovers. 
timlub
2,039 posts
 
Hi Richard,
     I have read your review on Parts Express several times as I find this speaker that you drastically changed a true bargain in the audiophile world.  I look forward to hearing the results of your constant resistant technology combined with infinite baffle.  
Just curious,  will your constant resistant technology work with any parallel or series crossovers?  Please expand on this once you are comfortable with your patents.  I appreciate you chiming in, Tim 
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