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RE: Clarifications on Time Coherence

Posted by RoyJ (M) on March 12, 2012 at 20:00:04

In Reply to: RE: Clarifications on Time Coherence posted by villastrangiato on March 11, 2012 at 08:33:52:

Hi Villa,

Thank you for your input and questions.

You wrote " ":
"Non pistonic and non point source behavior relative to cone breakup or beaming problems are usually dealt with by selecting the proper crossover point and driver for the intended function."

And I agree. However, those two problems of cone breakup and beaming remain detrimental to what we hear, because of their need for higher-order crossovers.


"Not sure what you mean by "low frequency tuning" and its impact."

In any dynamic driver, as we proceed down the scale, the driver's acoustic position begins to drift backwards in time-- moving away from us the lower we go. The cabinet-tuning creates the low-frequency resonance of any driver, woofer or tweeter, and different tunings affect both the frequency of the onset of delay and the ultimate amount of the time delay, which occurs down at resonance. In other words, the LF-tuning affects the LF group delay, starting at ~3x higher than the resonance frequency.


"However, as we all know, every driver (especially moving coil drivers) exhibit capacitive and inductive behavior ... result[ing] in what is commonly referred to as a native or minimum phase response. If one is to achieve a well integrated pair of drivers at crossover, this inherent phase behavior must be taken into account - otherwise, achieving a substantial reverse null at crossover (on the order of 30-40 db indicating excellent driver integration) will be impossible."

First, please note that no such cancellation occurs on a first-order type of speaker.

Other readers here may not know the sources of those changes in a raw driver's phase response and in its impedance curve. When those causes are understood, each can be offset or avoided. One result would be a very wide-band raw driver, with a smooth response and low distortion before any crossover is added. Only then can it be crossed-over over with a simple circuit.

"You seem to be implying that steeper roll offs result in "time delays" that can't be compensated for where this doesn't occur with first order crossovers??? If that is what you are trying to say, you're incorrect."

I am correct. Please read on, thanks.

"All passive components in crossovers introduce acoustic phase changes that vary with frequency. The simple delayed discharge of current through a capacitor and resistor in series is ample evidence of that. The fundamental difference between 1st order and higher order crossovers is higher group delay at the crossover point and the level of energy storage contributing to an undamped or overdamped impulse response."

Please know that this is a common mistake made in every electrical-engineering textbook I've seen, because the mathematics behind it was not presented from scratch using Time as the variable, not Frequency. What you'd find is that what appear to be damping issues are actually artifacts created by time delays that vary with frequency. There is an excellent AES paper on exactly this subject at

http://www.aes.org/e-lib/browse.cfm?elib=2618
One does not need a math degree to understand this paper, either. It's the best presentation I've seen on the subject.


"More importantly, contrary to what you're implying, crossover induced "time delays" are a straw man problem. With any properly executed crossover, acoustic phase difference between drivers moving from the lowest frequency portion to the highest about a particular crossover point should be constant (in quadrature with odd ordered networks, and either 180 degrees or in phase with even ordered networks). Group delay and its audibility are another issue not really related to phase and time "coherence". All crossover networks - 1st order through Nth order - suffer from group delay. Widely acknowledged studies have been published discussing the degree of audibility with respect to group delay and frequency. That issue is separate and distinct from the traditional issues of time coherence and phase coherence in speaker design."

I am not quite sure what you meant by your last sentence. I will point out the studies conclude that "We don't hear it." or "Time coherence does not seem to make much difference." That is only an indication that the studies were poorly designed, in my and our customers' and retailers' experience.

Regardless, if you think time coherence is a straw-man problem-- well, anyone is entitled to an opinion of course.

I do know that you have been given wrong information if you think group delay is not intimately related to time coherence. The math is presented quite well in many AES papers, and is unassailable, and is also in that link above.

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.

That word "Constant" is the clue behind the benefits of using first-order networks. Why this one word matters is far too lengthy to present here, but one can see the math for it fully laid out in many AES papers including that link above, so it's definitely not 'Roy's opinion'. And in it, you'd come to see how time-delays that do vary with frequency are then not easily corrected by any means. So, its best to avoid those types of circuits by the design choices made for the rest of the speaker system.

I hope this helps, in a small way at least!

Best regards,
Roy

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Kimber Kable

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