Phase Coherence or Time Alignment: Which More Imp?


This thread is really a follow on from a prior one that I let lapse. Thanks to everyone who contributed and helped me to better understand the importance of crossover design in building a loudspeaker. What I gathered from the last thread that there are opposing camps with different philosophies in crossover design. Leaving aside for a moment those that champion steep slope designs, my question is for those who have experience with speakers that are time aligned and/or phase coherent (using 1st order 6db per octave crossovers). Which is more important, phase coherence or time alignment? In other words, which more strongly influences the sound and performance of a loudspeaker? The reason I ask is because of the four speaker lines currently on my shortlist of floorstanders, three are either phase coherent or time aligned or both. The Wilson Benesch Curve's/ACT's and the Fried Studio 7 use 1st order crossovers but do not time align the drivers through the use of a slanted baffle. The Vandersteen 5's and the Quatro's both time align the drivers and use 1st order crossovers. I guess what I am asking is do you need to do both or is the real benefit in the crossover design? I'd appreciate your views.
BTW the other speaker is the Proac D25 and D38
dodgealum
Bigtee,

I went into the speaker business because I believe we have something special to offer. I think if you look at our track record, "best sound at show" year after, "speaker of the year"
excellent reviews, and a loyal group of Joseph Audio speaker owners who wouldn't give them up for anything else.

I care deeply about musical enjoyment, enough to want to share it with others - enough to bet the farm on others being able to hear it for themselves.
I have no doubt you build a quality product but so do a lot of others who believe in them as much as you. It's good that consumers have choices. If everyone liked the samething then we wouldn't need but one manufacturer! I'm all for driving the economy. But, who is to say what is right and what is wrong when it comes to listening on a personal basis? Nobody can sure decide which slope to use. Seems the best is whoever is doing the talking.
All I can do is relate my experiences in these threads for better or worse. I have been at this over 35 years. Based on that experience, it is wise that people go out and listen for themselves just as I have done(and the fellow who started this thread.) Does this mean I'm an "Authority?" No, it doesn't, but I do know what sounds good to ME and the point of these threads, I thought, was to share our personal experiences, not those who scream loudest get heard.
Most of the population has no clue to what crossover design is all about. I'm not so sure they need to know. Most don't know how a car is designed. They base their choice on looks and performance just as most do with speakers. I think here in the US, we boarder on false advertising way too much. It then makes everyone suspicious of every claim.
All I'm saying is you must satisfy oneself. It's your money and your satisfaction at stake.
Go out at listen for yourself and make a decision based on ones personal wants and needs (ie; does it look pretty or whatever.)
This "Best of the show" stuff is a little free advertising in these threads don't you think? Is that why these threads are here? Or is it for you to justify a point?
Your participation is always welcomed as any manufacturer would be but don't use it as a free format for advertisement. It just proves my point---your design based on your opinion is the one to go with. Hell, it may be but buying based on a show, review or especially advertisement is risky business at best. Tell people to compare your speakers side by side in the same room with the same electronics. Be confident enough to say comparison is welcome and recommended. That's all I'm saying.
How is it possible to build a phase-coherent speaker using 1st order filters?
I learned in electronics that a 1st order filter (ie 6dB slope) shifts the phase between woofer and tweeter by 90deg;a second order (12dB slope) by 180deg; third order (18dB) by 270 and a 4th order (24dB) one by 360deg.
This should mean that it is impossible to create phase-coherent first order systems, in a system using second order filters you'd have only to invert one driver to achieve coherence, a third order system again is impossible whilst a fourth order system is always in phase as it has been shifted by a full 360degrees.

Now, when it comes to to transient response and xover "ringing" ( electric resonance) a first order filter is unbeatable as the transient response is helped by having fewer passive components and the filters total inability to resonate. But for phase-coherency they are, together with 3rd order, the worst possible solution. You can fix 180 and 360 degree shifts, but theres nothing fixes 90 deg shifts, other than active electronics like in better bass-management systems for HT.
Dear Golix,

You are right in several ways. First-order filters shift the phase of each driver by 45 degrees, one driver's output is "leading", the other's is "lagging". Thus, the Phase Difference between them is 90 degrees, as I think you implied. But your concerns about the impossibility of achieving "phase coherency" for "odd-order" crossovers are not warranted:

For a second order filter, the phase shift is 90 degrees per driver at the crossover point. This is where your 180 degrees comes from- as the Phase Difference between them, at the crossover point. That Phase-Difference analysis extends, with your numbers, for the third-, and fourth-, and higher-order filters, at their crossover point.

Higher-order circuits, from 2nd on up, cannot be made time-coherent, because that Phase Difference each one exhibits at the crossover point does not remain a Constant Phase Difference when the tones move away from the crossover point. In other words, the drivers' Relative Phase Difference is always changing- which can be heard in many ways:

- As an image always shifting (depending on the note).
- Complex timbres which are not realistic.
- Dynamic attacks that are slurred.
- Truncation of the depth of the image in that crossover region.
- An audible "disconnect" in the depth of the image heard from the tweeter, compared to depth of the image revealed by the woofer. When an instrument demands some output from each of those two drivers, the same instrument apparently exists in two different rooms.
- There is "height" in many recordings.
- The speakers "don't measure like they sound."

Listen to a tambourine on a high-order speaker (an instrument that requires output from woofer AND tweeter), then listen to it on a decent pair of headphones, which are most always time coherent across that tambourine's tone range. You'll hear most all of the effects listed above from the speakers. Then compare using applause, acoustic guitar, piano, voice, using any wideband signal with transient complexity.

The audible effects of a constantly-changing phase relationship depend entirely on your choice of music- on what frequency range your music occupies, and also on the "frequency content" of each transient.

Different listeners use different music, some that easily reveal phase shift around the crossover point. Most crossovers to tweeters occur around 2 to 3kHz. A constantly-changing phase relationship between the two drivers above and below that crossover frequency has specific audible consequences we have all heard. In my experience, it is the leading cause for someone to say a speaker is "too analytical" or "too revealing", "too forward", or even "too exciting". One often remarks that a recording is "too harsh, too hard to listen to." It is why the preferred audiophile recordings are rather bland.

If the speaker designer physically steps that out-of-synch tweeter back from the woofer, and/or "pulls" the crossover point apart between the two drivers, you often hear "laid back", often with "a little less energy around the crossover point."

So, why use a first-order filter?
That Phase Difference remains Constant for a first-order filter: The output from the two halves of a first-order filter are always 90 degrees apart, on every frequency, not just at their crossover frequency. So their Relative Change in Phase Difference, at every frequency, is zero.

With a first-order circuit, the image does not wander on different notes, transients are preserved right down to knowing when the tip of the tongue left the roof of the mouth, and existing distortions in the recording or in your gear are not re-distorted, by being smeared out in time.

So, after passing through a first-order circuit, when the high- and low-passed signals are recombined, then the original, transient perfect, signal is recreated. Of course for a speaker, those high- and low-passed signals can emerge from drivers that have their own severe, mechanically-caused phase shifts. Perhaps the high and low sounds start off at unequal distances from your ears, or arrive at different times from "duplicate" drivers (like multiple tweeters). They could be time-warped from improperly-designed Zobel networks in the crossover circuit. Then there is a good chance that the high-passed signals will be "hazed-over" by cabinet reflections.

Fortunately, all of these difficulties can be minimized.

I hope this clarifies things. Thank you for your participation in an interesting thread.

Best regards,
Roy Johnson
Founder and Designer
Green Mountain Audio
Roy, I played sax for over 30 years and I became sold on time and phase speakers when listening to this instrument as well as other woodwinds (and other various instruments.) Other speakers with high slopes just destroyed a lot of what should have been there. For some years (in the past), I blamed it on electronics and/or the recordings. With time, I discovered what you already knew. I have found very few speakers that get the attack, decay and harmonics correct for me. It was extremely interesting to read your explanation and examples. It definitely follows what I personally hear.
I have noticed the positive recommendations you are receiving here and other places on your speakers. With the apparent unfortunate situation with Meadowlark, it looks like you, Richard V. and Thiel must carry the load.
It is good to see Green Mountain Audio being successful. I'm certainly going to have to give your product a whirl at some point.
Curently, I am investigating no negative feedback equipment. With time and phase coherent speakers, it has proven to be extremely interesting. There is something to it for sure. It seems to open up the presentation even more and further enhance that "You are there sensation." It is also interesting that time and phase coherent speakers, for all the critcisms they get for being laid back and not transparent, sure show this difference.