Phase Coherence or Time Alignment: Which More Imp?

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.

Thanks Roy,
indeed these difficulties can be minimized with careful design!
Thats why I use Tannoy LittleReds. It uses a combination of
first and second order filters. The phase response curve shows it to shift the entire audio spectrum uniformly by 30deg compared to amp output with a slight dip and a bump around the crossover frequency (+8deg,-10deg). So it acts as a time delay
(unavoidable) with a maximum phase shift of 18deg but this is limited to about 200hz either side of the x-over frequency ie the shift starts at 1k,
goes through zero (that is the 30deg total shift) at 1.2k rises again and is back at zero (30deg) at 1.4k.
Also, being point sources, they stay in phase no matter where you are in the room, unlike common multi-driver systems.

Personally, I have NEVER heard a speaker to be 'too analytical' or 'too revealing' although I heard speakers being 'too forward' or 'fatiguing' these usually featured metal-dome tweeters. I would have thought that 'analytical' and 'revealing' are essential attributes of any accurate speaker.
Without being 'analytical','revealing' and accurate any critical listening is
obviously impossible. People having problems with these attributes should probably by a Bose waveradio and just be happy with all the money they saved. They are the reason that high quality audio is in the dire state its in.
(Sorry, didn't mean to be ranting but statements like 'too analytical' just get me going. I am also aware that it wasn't you who said it, so don't take it personal.)
In my own experience, based on spending time in recording studios and listening to every speaker I can since I was 15 I have to say that Tannoys are closest to the real thing followed by decent studio monitors. Than come a variety of planars ( fantastic at low volume but lacking in macro dynamics),
full-range drivers ( great dynamics and imaging, bad at low volume, dodgy treble and (if in a horn) lumpy bass response.
After that the majority of dynamic speakers who do everything somewhere between average and badly.

This, of course, is a grotesque over-generalization as there are really bad studio monitors and really good HiFi speakers. I have not come across any
green mountain gear here in Europe so I cannot say anything to your product.
Overall you still have to look at the whole speaker-system and not just the xover, my Tannoys have loworder filters and sound fantastic, most studio monitors use fourth order and sound very good and back in the 80' I came across some 5way, first order filtered Dynaudios which also were excellent!

Cheers Golix
(haven't founded anything but...
can explain gravity without
bending space)

Though my own sensibilities lie with Roy, and the very cogent points he put forth, I want to make it clear that as Jeff Joseph basically said, there's more than one way to skin a cat. Jeff's speakers have established themselves as fine a loudspeaker as one can find in this hobby. I admire and respect both he and his products.

Jeff Joseph seems like a very good guy, he gave a nice presentation in New York last week. Yes, with his speakers' performance he finds himself in good company, but only through a time coherent speaker can one accomplish the acoustic output as a duplicate of its electrical input. The infinite slope cannot maintain the acoustic waveform.

Richard Vandersteen though that I'm more sensitive to time domain shifts than most people. Not that I have golden ears, but it's just where my sensitivies lie. For other listeners, higher order cross-overs may be perfect for them - so yes, there is more than one way to skin a cat.