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In Reply to: RE: Clarifications on Time Coherence posted by RoyJ on March 13, 2012 at 22:13:09
Roy Johnson said " I failed to communicate properly"No Roy, you communicated very well - citing an outdated AES paper from 1975 and clearly stating:
"The nice aspect about using a first-order crossover circuit is that the phase-shift DIFFERENCE between the low-pass and the high-pass filters is a CONSTANT 90 degrees at ALL frequencies, not varying with frequency as in higher-order circuits, and not just "about a particular crossover point" as you wrote above."
As shown in Rane Corporation's "Crossovers For Dummies" tutorial, the phase shift difference between the drivers is CONSTANT in the first order AND THE HIGHER ORDER CROSSOVERS. You obviously made a major error that unfortunately is common in this industry - even among "experts" but especially among those who are trying to promote "advantages" of 1st order crossover networks.
What is even more alarming though is your refusal to acknowledge that first order networks do exhibit reverse nulls.
Roy Johnson said:
"First, please note that no such cancellation occurs on a first-order type of speaker. "
This last blunder of yours is rather inexcusable for someone who claims to be a professional in the loudspeaker business and I wish for your sake that you'd retract your original statement or you will likely face a lot of future ridicule if you continue to endorse it.
Roy said -"You claimed 30-40dB cancellation, Villa. This is not even remotely close to true on a well-done first-order speaker design. In Figure 13a of that Rane PDF of yours, please note how the addition of 180 degrees to one of its two 0.707-amplitude vectors (which how one is supposed to represent a polarity reversal of one driver), results in two vectors still separated by 90 degrees, not the 180 required for cancellation. Again, perhaps you were misinformed."
To which I say - WHEN THE VECTOR SWITCHES FROM THE UPPER PAIR OF QUADRANTS TO THE LOWER PAIR - THE POSITIVE GOING PORTION OF THE SWITCHED VECTOR IS NOW GOING NEGATIVE. YES THE TWO VECTORS ARE STILL RUNNING IN QUADRATURE - IN A DIFFERENT PART OF THE VECTOR PLOT. BUT NOW ONE HAS A NEGATIVE GOING COMPONENT WITH RESPECT TO THE OTHER VECTOR. THIS CREATES CANCELLATION (NULL).
Added Edit:
[To make it clearer for you and potentially others who might not have a solid background in electrical engineering - discard the vector plot and consider the original input sinusoidal wave. One driver's output is peaking at the 45 degree mark where the other is peaking at the 135 degree mark. They are in true "quadrature" or occurring with a 90 degree phase between them. If you add 180 degrees of phase to one of the outputs, evaluating the amplitude of the original signal at 315 degrees phase produces a negative amplitude (corresponding to acoustic rarefaction instead of acoustic pressure). The two drivers are no longer operating in true "quadrature" with respect to the listening target or original signal and THAT is what counts. Yes, they are 90 degrees apart but now one is negative and the other is positive. This holds true whether you add 180 degrees of phase or subtract it - you still wind up with cancellation. (hope that helps)]
I can't believe someone with industry experience is actually making these statements - particularly when they are shown concrete data from a respected industry figure that disputes what they are trying to say. And no, I never explicitly stated or suggested that a 1st order network was going to produce a 30 or 40 db null - the focus with that statement was with even ordered networks suggesting that reaching a 30-40db null without proper phase integration would be "impossible". Perhaps the next post from you will be a slippery backtrack suggesting that "no such cancellations" meant lesser cancellations are possible. If that's the case, I'll respond here by saying I was only implying the degree of null that is possible with a well balanced crossover - not singling out 1st order networks which appear to be your favorite kind.Please don't suggest that we agree on anything. We clearly don't. You made errant assertions - one of which you're slightly backing away from. But you are still worlds away from the about face that is needed to correct obvious mistakes on your part that someone who builds speakers for a living should NOT be making. First order crossovers are very rare in today's high end speaker market. Anyone who has lived with a first order speaker and the inherent beaming problems off axis can likely attest to why that is so. And those that tout their superiority by citing bogus claims of "time coherence" and "phase coherence" will always attempt to ignore these and other obvious problems with first order designs. I'm only attempting to set the record straight on the ACTUAL differences between the topologies rather than the wished for or "believed".
Edits: 03/14/12 03/14/12Follow Ups:
Hi Villa,
I appreciate that you think I am wrong. but I am not backing away from anything.
Let us look one more time at that same vector example-- in which the upper vector was rotated by 180 degrees, to be pointing at a 225-degree angle (45 + 180), and the other remains at 315 degrees-- just as you also wrote.
Now, you and I agree they are still in quadrature, which means there is still 90 degrees between them (315 - 225 = 90 degrees).
And let each have an amplitude of 0.707, compared to having a value of One (which would be 'full output'). For the benefit of others, when each vector is drawn .707 units long, that is each driver "being -3dB at this crossover frequency".
When these two vectors are added, the resultant vector points straight down with full magnitude.
And you agree.
However, if 'cancellation' was occurring, the magnitude of that resultant vector would be Zero- it would not point anywhere-- it would not exist on the graph, as it would have a length of Zero, which is a 'point'.
But the magnitude of the resultant vector is ONE, which demonstrates how there will be NO cancellation in output when the tweeter is reversed in polarity from the woofer or vice versa, on a first-order type of speaker. Acoustic tests here confirm this.
Vector addition applets show this:
http://comp.uark.edu/~jgeabana/java/VectorCalc.html
Draw one vector at 225 degrees (into the lower left quadrant).
Draw the other vector at 315 degrees (into the fourth or lower right quadrant). Click 'add'.
On to your other point, to clarify about what happens when the phase difference between woofer and tweeter remains a constant 90 degrees-- this leads to the group delay remaining constant.
With higher-order crossovers, the constant phase difference of 180, 270 or even 360 degrees leads to the group delay NOT remaining constant, which was the point I wanted to originally make. Sorry for any confusion caused by not making that more clear at the beginning.
And to your last point:
Please note that first-order designs do not have beaming or cancellation problems off-axis. The math for that is based on pure sinewave tones measured at one point in space, and yet music is not pure sinewaves, nor do we listen at a single point. Therefore, while the math predicts what you wrote, the result on music is not what you claim.
By the way, that AES paper is 'outdated'? By that, you mean its math is no longer accurate? Wow.
Best regards,
Roy
Man Roy,
You seem to be a nice guy from what I've read. Please, give up while you're ahead!
Roy said - "Let us look one more time at that same vector example-- in which the upper vector was rotated by 180 degrees, to be pointing at a 225-degree angle (45 + 180), and the other remains at 315 degrees-- just as you also wrote."
No Roy, I didn't say that both vectors get advanced when one of the speakers terminals are reversed. I only advanced one of the vectors (from 135 to 315 degrees). Advancing both vectors does nothing and would not be considered the execution of a reverse null in any way, shape or form.
Please, you seem to be regressing with each additional post and I'm beginning to feel embarrassed for you. Let's just agree to disagree, ok? I have cited the documentation by way of published measured data (John Krutke) and the published work of a highly respected corporation (former members of the original Phase Linear Group)to support what I've been saying. The more this is rehashed - the more ridiculous we both look. If anyone has any doubts as to what I've been saying, they don't need to purchase an old AES paper or membership to AES - just review the web links I provided. Have a nice day Roy and go build some boxes!
:)
Hi Villa,
Thanks again for your comments. I did not advance both vectors. I began with one at +45 degrees and the other at -45 degrees (or 315 degrees). Then I added 180 degrees to that first vector, which rotates it to 225 degrees (45 + 180). The resultant vector is as I described: straight down, with full magnitude. There is no cancellation.
I appreciate the information Rane Corp. put on the internet, but as with many of the math relationships underlying time coherence, there are a few mistakes made in their characterizations of what's going on. This comes about, in my experience, because the textbooks for Electrical Engineering used in universities present the math developed for continuous steady-state tones (Frequencies) rather than presenting the underlying ones which show what happens as Time flows along from T=0.
This is done because pure-sinewave math is easier, but mostly because time-domain math (used in transient theory) proves unnecessary to solving most circuit-design problems. What is done to that math to make it simpler is to get rid of Time as a variable, by assuming the sinewave(s) have no beginning or end (unlike music).
One difficulty which then arises is the use of 'phase' to describe the relationship between waves. 'Phase' is a relative term-- used compare what's happening in one wave versus another at ANY point in time (since Time is not a variable).
But when the time-domain is allowed to remain part of the math, then one must consider what happens from Time = 0 onwards, and that leads to Absolute relationships (Relative now only to T = 0) to describe waves, not a Relative one between waves.
And that math is harder to describe, to teach, and for students to grasp (especially when it appears 'they don't need to learn this stuff'). Electrical Engineering has always been a set of tough courses, and this time-domain math I saw discarded in the early 1970's from EE textbooks, to help make room for the emerging digital circuits coursework.
Best regards,
Roy
Roy, peeped your profile, not that I didn't know who you are. I have a Micro Seiki BL91 TT, also, with a Sumiko arm. One of two tables I use. Just sayin. Cheers.
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