Speaker Asylum General speaker questions for audio and home theater.

---

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ Speaker Asylum ]

---

[ Alert Moderator ]

---

RE: Clarifications on Time Coherence

Posted by villastrangiato on March 14, 2012 at 05:49:17

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No Roy, you communicated very well - citing an outdated AES paper from 1975 and clearly stating: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 03/14/12

This post is made possible by the generous support of people like you and our sponsors:

Analog Engineering Associates

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---