First Order Crossovers: Pros and Cons

Let me explain my porting question on GMA's C-3's relative to time and phase coherence. I have no idea if there is an audible issue with the C-3's design and based on Roy's published specifications there is nothing to suggest what I'm about ask actually takes place, nevertheless I thought I'd throw it out there to elicit a response regarding the porting theory behind this design.

In GMA's C-3 the bass port is firing in an upward direction, directly below and slightly behind both the midrange and tweeter, with a clearance of perhaps 4-5 inches. I'm wondering if conceptually there could be some type of Doppler effect taking place with the placement of the port relative to the midrange and tweeter that could slightly alter the phasing or timing of these drivers? Although there is no high-frequency whizzer cone riding on top the woofer; as in the Tannoy, in theory as the woofer moves backward and air is pushed out of the port, is it possible that this change in air pressure could somehow modulate the midrange and tweeter response by disturbing their wave lengths? Alternatively, when the woofer pushes forward, does the port suck in enough air to also disturb the wave length of the midrange driver and tweeter, thereby throwing off time and phase coherency?

Just to add to the info on vector diagrams, there's a very good explanation of what's going on at the Rane site, particularly in http://www.rane.com/note119.html

The most telling part of that is "The 1st-order case is ideal when summed. It yields a piece of wire. Since the responses are the exact mirror images of each other, they cancel when summed, thus behaving as if neither was there in the first place. Unfortunately, all optimized higher order versions yield flat voltage/power response, group delay or phase shift, but not all at once. Hence, the existence of different alignments and resultant compromises."

Skrivis, great link. As always, a graph is worth a thousand words.

An even more telling part of the article regarding the group delay of a 8th order L-R crossover.
"Is It Audible?

The conservative answer says it is not audible to the overwhelming majority of audio professionals. Under laboratory conditions, some people hear a difference on non-musical tones (clicks and square waves).

The practical answer says it is not audible to anyone for real sound systems reproducing real audio signals."

It always amuses me when someone makes the claim "It's not audible. Well, not audible to most people most of the time, anyway. Or at least not audible to some people some of the time....."

This is fundamentally no different than the claims that "all amplifiers sound the same" or "lamp cord is perfectly good for speaker wire." Anyone with good ears can only shake his head at such statements. Maybe they hold true in the world of budget-fi, because at some point those differences get swamped by the colorations of the rest of the system. But in a good system, with good music, the differences are plain as day, and time/phase coherence is no exception.

Now, it is not much of a surprise that Rane (and others) would downplay the audibility of time-and-phase coherence, given that they are in the business of selling high-slope crossovers. And in the pro audio world, this is indeed likely the best overall compromise, given that power bandwidth is a serious consideration. I mean that with total sincerity-- if I were designing a pro system with active crossovers, 4th or 8th order L-R would be my first choice, no question. But that doesn't necessarily make it the best for ultra-high-end home audio playback, where other priorities (fidelity to the musical signal in both time and amplitude, for example) take on much higher importance.

The most telling truth is that once someone has lived with a really good time/phase coherent system for some time, he finds it impossible to ever "go back". The lack of coherence in high-order systems, while potentially ignorable if one has never tried anything else, is nonetheless a major step backwards once one has heard the possibilities of an electrostat or a good first-order design. And since the vast majority of systems on the market are still non-coherent, it is quite possible that the majority of audiophiles have never actually lived long-term with a time/phase coherent system, and simply don't know what they're missing.

Luckily, forums such as these allow the minority not only to make our voices heard, but more importantly, to plant seeds of inquiry in the minds of those who may have simply never thought about such subjects before. For to me, there's nothing more satisfying than seeing that light bulb go off, and hearing someone say, "Wow, it sounds like real music!"

Best,
Karl