First Order Crossovers: Pros and Cons

Viridian. Your explanation is helpful but leaves me wondering--Why would a speaker designer want to allow each driver in, say, a three way design want to "bleed" into the frequency spectrum of the nearby driver(s) rather than use a steep slope (is this sometimes called a third or fourth order crossover? I need help with the terminologies!) to prevent interference between them? What is to be gained from using first order designs that allow the bleeding to take place? Do the advantages of the first order design depend upon drivers of sufficient quality to be able to blend accurately? Which of the designers using first order designs do the best job getting the blending to occur seemlessly? Sorry for all the questions but your post raised more questions for me than answers gave!

I don't have the technical chops to make a great contribution to this discussion, but my understanding is this: first-order crossovers confer benefits in terms of maintaining phase accuracy (coherency?) across the drivers. The price for doing this is as Vridian stated, the need to have all the drivers operate well beyond their prime frequency range. Which means that driver selection becomes especially challenging.

And why would you want to maintain phase coherency? I'll leave that for someone else.

Great thread, Dodge.

Ahhh, but it would seem you do have the phase chops for this Drubin...coherency (is that how it's spelled?) is what we are all after and why some turn to single driver systems...they can hear the X-over.

Dave

Time/phase coherance is the primary advantage of a first order design. There was a thread here about this topic from Roy Johnson of Green Mountain Audio a while back. Look here:
http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1032037028&openusid&zzgma@pcisys.net&4&5#gma@pcisys.net

"Time/phase coherance"
As Viridian notes above, 1st order helps with phase. However, it's one thing to time align the drive units & anther thing to tackle electrical phase. Cheers

Darn, Marty, apart from the humor you so graciously provide to us all, you've conferred a tremendous amount of good information in this thread.

Not much I can add beyond saying that the more complex the crossover becomes, the more oomph an amp must have to push the music through these parts.

Richard Hardesty is a big advocate for 1st order crossover speakers. You can visit the www.audioperfectionist.com to read more. Like Viridian was saying above it appears to take about 8' to 10' for first order speakers to intergrate.

6moons did a piece on the Green Mountan Continuum last November, which is rich with discussion of these issues. One of the best audio articles I've read. Read it here.

Songwriter, thanks for alerting me to the earlier forum discussion. I see now that this is really much deeper than I had anticipated. We have a full throated ideological struggle going on relative to this topic. Though I learned much from the discussion I still feel ignorant of some of the basic concepts and terminology. For example, Jeff Joseph's contributions on the benefits of steep slope crossovers seem pretty compelling--why would you want a driver to operate outside the frequency band it is best suited to reproduce? It seems that phase coherency is the primary benefit of a 6db per octave design but I cannot determine the value of this since I'm not really sure what "phase coherency" means. Can anyone enlighten me and discuss why this is so important?

Viridian. You are no doubt correct. My reference to Joseph's "contribution" was to the thread, not the technology.

Ah, the JSE Infinite Slopes...

I really liked the smaller ones (1, 1.2(?) and 1.8), and owned a pair. As you moved up in the food chain (2 and 4), I found them to have a tendency to sound boring, and goodness knows, I gave them a chance. Heard a pair of 4s a year ago, and still found them lacking in excitement. Overall, nice speakers and I miss the company a lot. That notwithstanding, Jeff Joseph is a prince of a guy, and his speakers are pretty special.

I find it curious how little mention there is of Joseph Audio speakers in these forums. Given how great they always seem to sound at shows, I would expect them to be more popular. Limited distribution, I guess. Too pricey? Maybe. But look at Verity, who get discussed fairly often here.

Dogelealum, your very welcome. For a better understanding of phase and it's effect on music and human hearing, check out the link below. It looks interesting and well-performed. The test tones and limited music tests were good choices for "having to start somewhere", and the bibliography shows they studied a great deal of other professional research. The headphones results were higher than the speakers', I think, because the speakers they listened to had a great deal of phase shift to begin with (Genenlec). It's pretty involved.....but really interesting reading.
http://www.music.miami.edu/programs/mue/Research/dkoya/title_page.htm

That paper concludes:

"... the phase distortion audibility results in this thesis research did not seem to be as significant...
The human auditory system was found to be extremely tolerant of even gross phase distortion effects"

Compare that with the Rich/Cochenuer AES paper on The Audibility Of High Order Crossovers (115th AES)

"The second-order LR network received lower
audibility scores than the fourth-order network.
Statistically, all but one test subject could detect it
with noise or music and only two subjects were
unable to detect it with music. This result clearly
points away from the second-order network in the
design of loudspeakers."

(lower audibility scores in this context mean it was more noticable and bothersome to listeners when compared to an ideal source - higher is better)

"It is well established that driver interactions in first order networks create frequency response
disturbances with changes in driver path delays and,
for this reason first-order networks were not included
in our study. These networks also fail to provide
cutoff band attenuation to the extent that drivers may
be pushed towards nonlinear operation by signals
present well outside the crossover region."

"Other than their low cost, phase coherence is the sole reason given by designers for the use of first-order networks."

"Our test results could identify no sensitivity of the
test subjects to phase effects; these effects become
increasingly more apparent as the driver order
increases and our controlled study found the subject's
ability to identify the crossover followed an inverse trend."

"From the auditory experiments presented in this
paper, the authors suggest that speaker designs
should incorporate fourth-order notched crossovers,
with the eighth-order notched crossover made
available in high-performance cases."

(Infinite Slope falls under the fourth order notched with finite zeros category)

One has to take into consideration the fact that the speakers used in the test had a pretty high degree of phase problems. "The audibility of phase distortion in audio signals was also highly dependent upon individual ability, although for statistical analysis individual data was not considered.... Specifically, a few subjects seemed to hear clearly the presence of phase distortion in the jazz-vocal test signal for the headphone listening test, while a few test subjects seemed to perceive phase distortion better than others during the loudspeaker listening test" If they were using a time/phase aligned speaker system, i'd bet the results would be similar to the headphones. Designers of non 1st order systems will obviously have a difference of opinion. My suggestion would be to listen to both time/phase aligned systems and non time/phase aligned systems. I'd also suggest you listen to the so called "bad recordings" like old Janis Joplin recordings as well as the audiophile pressings most people use to evaluate speakers. A speaker with phase issues will sound bad on these lesser recordings. Then decide for yourself where you stand on the issue. To me, a speaker that essentially prevents me from enjoying many of my recordings is not one i'd ever consider purchasing. YMMV.

After you hear a true full range point source omni driver, anything else sounds less like music and more like a speaker. This is true regardless of the crossover slope used : ) Sean
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Do the models in the current Walsh Mk II series qualify?

Drubin: Those aren't Walsh drivers, they aren't omni's or a point source, there is a crossover network and they are two way designs. The only thing "Walsh" about them is the name. Sean
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Thanks. Okay, so who makes a "true full-range point source omni dirver" loudspeaker?

Drubin: I don't know of any that are currently in production. Even those that come close ( German Physiks, MBL, Huff, etc...) use an additional driver of conventional design ( woofer ) to cover the bottom end. The original Walsh's are very time consuming to build the driver and require tons of hand labor. Three different types of cone material have to be formed, bonded together and internally damped to duplicate the Walsh driver as found in the Ohm F. That's why it has been called the most difficult dynamic driver to rebuild. Sean
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It's true, today's Ohms are Walsh-inspired. They are, however a very cabable sounding design, comfortably outperforming most of today's cone 'n dome varieties.

Viridian: All drivers exhibit some type of roll-off that is mass related. Too heavy of a cone for the capacity of the motor structure and suspension and you lose high frequency response. Too light of a cone for the capacity of the motor and suspension and you lose low frequency response.

These aren't crossovers, they are electrical limitations that are brought about by the mechanical design traits of the driver. Given that the entire cone is excited relatively equally across the audible bandwidth, the individual sections of different material types found in the driver are all sharing the load equally.

As far as the ringing of the driver goes, much of this has to do with Ohm's poor choice of type and quantity of damping material inside the cabinet and the horribly attrocious speaker cable used. Fix these two things and you go a long way towards having a much better speaker. Just remember that Ohm used different voice coils on the A's & F's, different frames / support structures, different types / quantities / placements of damping material. As such, each speaker system might be different and would require individual amounts of TLC of different types and in various areas.

Once you've got all of that straightened out, then all you need is an amp that can produce gobs of current and can deal with a high level of reflected EMF and you'll be even better. You have to remember, just as the amp directly modulates the voice coil in this design, the reflected emf from the driver directly modulates the output of the amp. There's no loss or "buffer" from ANY type of a crossover on this design. Not all amps are load stable. As such, some will change tonal balance, distortion characteristics, harmonic structure, transient response, etc... as the drive levels and amount of reflected EMF varies.

Now factor in that this is a very low impedance design, so your "high bias" Class AB amp will be switching over to Class B mode extremely rapidly. In effect, 10 watts of Class A power will only get you about 2.5 watts of Class A power with the F's. If you've got a typical Class AB amp, your 2 watts of Class A power will get you about half a watt of Class A power before switching over to Class B output. Now you get to listen to the constant crossover distortion that is so prevalent in most amps. This is one of the reasons that very high bias AB or straight Class A amps tend to sound so much "sweeter" and "cohesive" i.e. less crossover distortion.

Given that these speakers are appr 82 dB's in sensitivity, unless your amp is of a VERY high bias design, you'll be operating in Class B mode for the majority of listening. Needless to say, this is a very demanding speaker and most amps simply aren't up to the task.

For sake of reference, the amps that i'm runnning with my F's are rated at 250 @ 8 and 500 @ 4. I've conservatively guessed that i've got appr 800+ wpc @ 2 ohms, which the F dips noticeably below. According to the manufacturer of this amp, the first 50 watts are Class A @ 8 ohms, leaving us with 25 watts of Class A @ 4 ohms and 12.5 watts of Class A @ 2 ohms. This keeps the amp from constantly crossing into Class B operation during normal listening levels. I've performed quite a few mods to these amps i.e. cleaned up the signal path, pulled appr 4 feet of wire from each amp chassis by re-routing the internal cabling, added bypass caps as needed, etc... All of the internals are wired with Cardas cabling too. For the record, the Cardas was installed at the factory, not by me.

As was posted in a previous thread, Bill at Millersound considers the Ohm A's and then the Ohm F's to be the finest speakers he's ever heard once he's done modifying them. Coming from someone that has repaired / modified everything from Wilson's to garage sale specials, this is pretty high praise. Then again and based on what Bill told me over the phone, he changes the voice coil, the spider and the foam surround. In effect, the only thing that remains stock is the actual cone of the driver.

Bill says that this brings the nominal impedance up quite a bit, making it an easier load on the amp. The sensitivity is also drastically increased, resulting on a further reduction in strain on the amp. Transient response is also improved due to less driver sag and keeping the voice coil in the magnetic gap of the motor structure. This in turn reduces the ringing since the efficiency of the motor is increased, therefore increasing control over the cone itself.

Other than that, i've got a very large set of E-stat's with dipolar woofers, large 5 driver four way towers, time-aligned monitors with subs, several different horns, etc... The F's, even without Bill's "re-design", are still my favourite speaker. They just do some things that no other speaker that i've ever heard can do. That is, once i found some amps that could do them justice and tweaked the tar out of them : ) Sean
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Sean, next time I'm in Chicago, can I come over?

Drubin: One set of my F's are "smoked" and awaiting a "full tilt" driver rebuild as per Bill at Millersound's suggestions. The other set is packed up. I am currently down to using what was primarily my HT system for both music and movies now. After relocating, i hope to have everything up and running, but i'll have to install a new AC system. My latest purchase ( 450 lb iso transformer / voltage regulation / surge suppression system ) should be here next week or so for that house. Only problem is, i don't know where i'm moving to as of yet. I may end up becoming a Hoosier and moving out of the "Land of Lincoln" if things keep going the way they are here. Illinois is becoming more and more of a "police state" as each day goes by.

Viridian: I hear what you are saying about each individual "layer" of material within the F and / or A, but this is completely different than how a whizzer cone works. The whizzer cone has a completely separate level of output / radiation pattern / frequency response than the main cone. On the other hand, the segmented Walsh design uses multiple types of material that pass signal from one into the other. If any one of the segments didn't pass signal into the other in a relatively uniform manner, you would end up with a loss of surface area. When you lose surface area, you end up with the associated reduction in low frequency extension. As anyone that has ever had a decent running pair of F's or A's know, weighty bass is definitely not their problem.

As to the Threshold 400A that you had, i looked up my notes on this amp. As most of you know, i'm a pretty vocal supporter of Nelson Pass designs, so what i have to say about this amp may come as somewhat of a shock.

First of all, the 400A is definitely not a stable amp. That is, it changes frequency response / transient response / distortion characteristics as impedance varies does so to a pretty noticeable degree. Sonically, the bass is solid so long as the amp isn't pushed. Once you start to throttle the amp, the bass gets soggier. This has to do with the lack of power supply reserve ( not enough filter capacitance ). While the warmth region and lower midrange are pretty decent, as frequency climbs, the upper mids and treble regions were noticeably forward and bright. This tends to highlight any type of lean recordings, exacerbating the problem. With most SS preamps, this amp would tend to introduce a very noticeable sibilance and glare problem.

Once into the top octave, the treble response fell off sharply. This was not as noticeable as one would think though due to the high frequency emphasis just below this point. A definite lack of brass shimmer to cymbals with more of a pronounced "raw & steely" sound.

Due to the increased upper midrange / treble output, transients seemed to be "in your face". The harmonic structure seemed slightly disjointed because of the lack of balance between the lows and highs.

Much of this could have been DRASTICALLY improved by working on and improving the power supply. After all, everything that the amp is capable of ( or doesn't do well ) starts in the power supply. This is why so many aftermarket modifications center around cleaning up / stiffening ( increased reserve ) / improving speed in the power supply of gear. If you've read any of the overly long AC based threads that i've posted, i've commented time and time again about how important the proper design of the power supply is.

As a side note, Nelson Pass has an article about how important the power supply is on his website. As to the Dayton-Wright's, Nelson also used these speakers to test various loudspeaker cabling on. He mentions these speakers in another article on his website, as they used to send some amps into convulsions. Using such a reactive / high demand speaker provided an excellent test bed to see how an amplifier would respond to changes in the load that it saw via altering the speaker cables & their electrical characteristics. As mentioned in that article, some cables allowed the amp to drive these speakers with no problems whereas other cables caused the amp to shut down. As such, there's an obvious audible and measurable difference there i.e. sound from the speakers with some cables and no sound with other cables : )

As far as ribs go, i'm trying to cut down on eating meat. When it comes to good barbecue though, i don't know how i could turn down either beef or pork. That is, until i get serious about my health and dining habits. My midsection is starting to protrude and as hot as it gets in the summer, i'll only be miserable if i don't lose some weight. Either that or convert some of that "mid section speed bump" into muscle on other parts of my body. Can anyone else relate??? : ) Sean
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One of the major advantages of the first-order crossover, which isn't mentioned often enough, is the fundamental simplicity of the network. Every increase in crossover order is accompanied by a proportional increase in the number of network elements, and the audibility of this problem is severe. Even a single high-quality inductor or capacitor in the signal path is audibly degrading when compared to none at all, which is why so many people decide to live with the severe compromises present in single-driver speakers. It's hard to describe this effect until you play around with it-- my best description is that it "sucks the life out of the music". And the higher the slope, the worse this problem gets. Not very scientific, I agree, but the degree to which this is true is stunning when you hear it.

Also, I would take issue with the research quoted by Joseph. One of the basic facts about second-order crossovers is that they require at least one of the drivers to operate in inverted electrical phase, to avoid a null in frequency response at the crossover frequency. This inversion alone is enough to utterly destroy the integrity of the musical signal, and any comparisons to fourth-order crossovers at that point are completely meaningless. Since no one in their right mind would use second-order networks in the first place, it doesn't say much that fourth-order sounds better than second-order. This paper, like a lot of quoted research, might be true in its own limited environment, but it doesn't even begin to tell the whole story in the real world.

The main drawback to first-order networks, as stated above, is the need for very wide bandwidth and very high quality drivers, with no severe "breakup modes". Thankfully, these are available at a price from Scan Speak and Audio Technology, among others.

Disclaimer: I am the manufacturer of the Ultimate Monitor, a two-way speaker using first-order series crossovers and Scan Speak Revelator drivers.

Best,
Karl

Nice to see you here again, Karl. (Maybe you've been around and I just missed it.)

Hey Sean: this is my first posting ever, but I couldn't resist. If you're watching the midsection then stay away from the Italian beefs--they'll give you a first-order speed bump in no time flat.
Say hi to Chicago for me,
Charlie

I'm going to let everyone know when Bill finishes his own Ohms, and then you're all going to have to get into Philly so that we can all go over to HIS house for a bar b que. Believe me, it's going to be an absolute blast.

As Sean stated, Bill's favorite speakers are probably the Ohms. He has told me several times he considers the Ohms the finest speakers ever produced, once he implements his ideas into them. I spotted them once in his shop, and he just lit up. I asked, "But, they sure are hard to drive, aren't they?" His reply was, "No, when I get done with them, they are incredibly sensitive. They'll play like crazy with no power."

No, he hasn't tackled his own yet, as he dreads working on them due to the labor intensity. He just doesn't want to take time away helping you and I via his business. You must know the man in order to understand. When he does something, he does it better than anyone else has ever done, otherwise he just doesn't do it. Last summer, he painted his wife's car, at what he claimed to be twice what a quality body shop would charge. Why? Because it needs to be to his own standards or else he will drive himself crazy, and no one beyond him can meet those standards. If anyone ever wants to see how detail oriented he ever is, just ask him a couple of questions regarding automotive transmissions. You'll get an education the likes of which no one has ever gotten before.

I strongly believe Bill is the finest speaker guy in the country, if not the world. It's amazing who we hold in such high regard in this industry, while failing to recognize the true genius of the craft.

I haven't read this entire thread, but it's not clear that a particular speaker which is or claims to be time and phase coherent will have a simple crossover network with few parts, although that seems to be the case with Meadowlark and Green Mountain.

Vandersteen and Thiel have involved crossover assemblies, for example.

I think the only real problem associated with first order crossovers, meaning the one problem which can't be mitigated by engineering or setup quality, is the off axis lobing. This lobing will exist, but may or may not be bothersome to a particular person in a particular situation....

As for whether the recorded music itself has any time or phase coherence by the time it winds its way through the rest of the recording and playback chain...who knows? I just think Vandersteen 2Ce Sigs are still a great bargain.

Off-Axis Lobing. Sounds ominous. Frankly, I honestly cannot say the "off-axis" response of my GMA's is a whole lot different than on axis. In fact, these babies sound fine dead center; 30 degree's off axis or three rooms removed from the speakers. Sure, the image is deaper dead center, but they sound fine off axis too. Can't begin to tell you how many people have made that observance with my GMA's. I get the "We are in the Den but those speakers sound great" comment. Three rooms removed. Can you elaborate on what said "Off-axis lobes" might sound like, Suits me? I heard the same comment from Bobby at Merlin, but he would not elaborate. Will you? What do "off-Axis Lobes" sound like?

Off-axis lobing causes non-linear frequency response i.e. added peaks and dips at various points along the curve. The more consistent the dispersion characteristics of the drivers as angle of radiation is altered, the less noticeable the lobing will be. The differences in baffle related diffraction also come into play here, so speakers with a more advanced baffle design will suffer less from this also. I haven't seen pictures of all of Roy's speakers, but i do know that he's very aware of baffle related problems and seems to pay attention to such things. Sean
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Suits_me:

You are correct, it is possible to make even first-order crossovers very complex, due to either inherent problems with the particular drivers chosen, or simply an obsessive need to over-engineer the problem in pursuit of a perfectly flat frequency response. This is unfortunately all too common. I used to suffer from it myself, until I learned the virtues of simplicity and elegance in design. :)

All crossovers suffer from lobing in the crossover region, no matter what the slope, but the higher-order crossovers have less overlap, so the lobing occurs over a narrower frequency band. Whether this is less audible due to the smaller bandwidth overlap, or more audible due to the faster rate of change in the lobing pattern and source location with frequency, is still open to debate in my opinion.

D'Appolito (MTM or WMTMW) configurations have a superior lobing pattern (less variation at various angles) than standard one-driver-above-the-other configurations, but only if they are used with odd-order crossovers (first, third, etc.) Concentric drivers eliminate the lobing problem altogether (at least in the M/T crossover region), but at the expense of creating an even bigger problem in another area (modulation of the tweeter output by the midrange cone).

It should also be kept in mind that lobing is essentially a direct-sound-only effect. In other words, it does not significantly affect the in-room power response, so its overall effect on the in-room sound while seated is pretty slight. Unless, of course, you like to do critical listening while continuously standing up and sitting down. In which case maybe you need Ritalin more than a high-order crossover. :)

Drubin: Thanks. I don't have much time anymore, just came across this thread by accident and felt like I could add something useful. I hope.

Best,
Karl

The lobing is actually due to having 2 spaced sources of the same signal at the same frequency. It isn't really caused by the crossover, but by the drivers. (Just being pedantic here.)

I don't see directivity patterns (lobing) as a huge problem - provided that the designer is aware of it and communicates with the user.

As long as you know how the speaker is to be used and the proper listening axis, it isn't really very necessary to worry about directivity. (As long as your listening room meets some minimum requirement, which I'll discuss later.)

The ultimate is omni-directional speakers, with Bose being a good example. They design for uniform frequency and power response throughout the room. In doing so, they utterly trash the signal. They can't accurately reproduce a transient.

Siegfried Linkwitz is also very concerned with frequency and power response. But he ignores time-related (phase) elements. His designs are highly engineered, but I doubt they are very accurate. Hmmm... I guess I would say they are good designs, because I respect all the work and thought he's put into them, but I just don't agree with the compromises he has chosen.

Mithat Konar has some interesting comments on the MTM designs. http://www.birotechnology.com/articles/VSTWLA.html

I don't agree with Mithat on a lot of things, but he also puts a lot of thought and effort into his work.

Would it be best to have flat frequency and power response throughout the room? Probably. But there seem to be more important things to take care of first. Pat McGinty said that once you ensure your design passes transients correctly, the frequency and power response fall right into line.

Stereo requires a fairly limited listening position. You have to be equidistant from both speakers. It starts getting into psycho-acoustics, and that's complex and not something I fully understand, but I know a few general things. You need to accurately reproduce the amplitude and time spectrum of the original signal. That requires phase-coherent speakers.

Phase-coherent isn't the best term. Time-aligned isn't either. Minimum-phase? That gets a bit closer I guess. Maybe I'll just stick with phase-coherent... it gets the idea across without having a connotation of a certain method like time-aligned does.

At any rate, accurate loudspeakers don't really need to worry too much about the rest of the room, just the listening position. If there are major problems with the room, then that needs to be solved separately. I you can't remove reflected sound in the room, then you need to get enough delay so you won't smear transients. (I've seen several papers that detail the amount of delay needed to ensure that a reflection is not perceived as part of the original signal.) You also need some amount of attenuation, from distance or damping material in the room.

If you live in the right area, you can take your stereo system outside and hear it without all the room reflections and resonances. This can be quite revealing.

Where there is disagreement is in whether you design to minimize room effects, or allow for them or even exploit them. A lot of time is spent on this. There is probably room for different philosophys here, because some people just may not have a good room to listen in, and designs that work with this may at least give them decent sound, if not accurate sound.

A typical ballroom is very reflective. You can certainly hear the echoes. But, the delay is great enough that you hear them as echoes, not as part of the original sound. (The distance also reduces them in amplitude.)

A small bare room has little delay between direct and reflected sound. It's probably the worst place to listen. Bose is probably correct in this type of room because you're not going to hear transients properly anyway, so you might as well try to have even frequency response and get some part of it right. :-)

Listening outdoors gives you long delay, plus significant attentuation, without being so anechoic that it's disturbing. (I feel that when you're in an anechoic chamber, you can't hear where you are or what type of environment you're in, and this is what is disturbing. It's very unnatural and not like the environments we evolved in or are used to.)

Most people can't listen outdoors, so they need to ensure their listening room is as large as practical, and also well-damped. (Damping or room treatment can be said to increase the apparent size of the room.)

Phase and transient problems seem to be on the "leading edge" of things, with room problems being on the "trailing edge." I feel that it's most important to have an accurate "leading edge," and then you can do whatever you can to improve the "trailing edge." I think Linkwitz explicitly says the trailing edge is most important.

Is phase audible? The studies I've seen haven't insured that there was any "control," in other words there wasn't phase-coherency to begin with. They just took a system with unknown phase properties, then added phase changes to it and asked people if they could hear a difference. There was one Master's thesis on this topic that I saw that was so flawed it's a wonder they let the guy get away with it. I hope to find a more scientific study at some point...

I have found that designs that attempt to insure phase-coherency sound more lifelike than ones that don't. So I'm convinced, even if others aren't. :-)

I cannot really add to the science, but I would like to add to the voice of several others regarding the Green Mountain Audio Europas I own (I previously owned Spica Angelus, and prior to that non-coherent designs).

The Spicas and the Europas both possess great imaging. Absolutely pinpoint images seem to hang in the air, as if not coming from the speakers at all. On orchestral CDs the room behind the speakers appears to melt away and one is left with a sense of scale that I never had from previous non-coherent designs.

The Spicas were not true coherent designs, but an approximation, with a 4th order Bessel low pass filter on the LF driver (to the best of my knowledge). The Europas are true first order on both drivers.

The Europas outclass the Spicas in their ability to define transients. Macro and micro dynamics are much more defined on the Europas. Depth of image is also better on the Europas. Strangely the Europas also load the room slightly better than the spicas sounding uniformly good around the room, and with less of a tendency for the image to collapse once you move away from the sweet spot.

Both the Europas and Spicas are extremely fussy of placement if they are to give optimum performance. You really must arrange the room around the speakers, and be prepared to have them well out into the room. If you can live with this then you are rewarded with music that fills the room, not sounding as if it is emanating from two little boxes.

If there is a downside to these two speakers it is high frequency response. The Spicas did not do high-frequency, with a rolloff above 14kHz. The GMA Europas are more extended, but the treble sounds a little less defined than in previous speakers that I have owned that were 3-ways with a ribbon tweeter. I suspect the treble performance of the Europas could be bettered at the same price, but only by losing some of the dynamics and imaging due to higher order filtering.

I'm not sure that it's all due to the phase coherency, or also a side benefit of having fewer, higher quality crossover components, but I am sold on the result.

Placing the speakers well out in the room should reduce early reflections. (More distance from a reflective surface equals more delay and more attenuation.) That fits in with the idea of wanting to separate the reflection (echo) from the original event so that your ears/brain will hear them as two separate events and not one muddled-up event.

There are definitely people who don't agree with this approach. Some feel that you're going to have reflections, so you should get your speakers close to the walls so that the direct and reflected events blend together. I think Ted Jordan claims that it's best to have speakers mounted in the wall, and I think this is why he says that.

I'm not positive that phase-coherency leads absolutely to pinpoint imaging. The source to microphone to loudspeaker process has some inherent flaws. Binaural and transaural methods attempt to correct this. (With some success IMO.)

I think it's quite possible that phase distortions could result in the type of soundstage that all the reviewers love to describe. "I could hear that the violinist had a rash on his left elbow. Not the right, but the left..." :-)

I have more to learn about this before trying to come to conclusions.

I do remember hearing a remarkable soundstage from a pair of Acoustats a long time ago. But panel speakers are hardly phase-coherent (nor very linear), so there's something more going on here.

I also once heard a demo of signal processing, played on a boombox, where a helicopter moved toward you from the right front toward your rear left, and seemed to pass overhead as it went! It opens up intriguing possibilities, such as listening to a performance in a "virtual" concert hall and being able to choose where in the hall you "sit."

"I'm not positive that phase-coherency leads absolutely to pinpoint imaging"
I think coherency may be one of the more important of many factors that lead to pinpoint imaging. Inert cabinets, stiff, lightweight drivers, minimal crossovers all help ... in short pinpoint imaging is the result of an excellent transducer.

"I think it's quite possible that phase distortions could result in the type of soundstage that all the reviewers love to describe".

I don't think ANY distortion will improve pinpoint imaging, but I suspect it might help to create an artificially broad soundstage. I remember once demoing a large pair of Martin Logans. The soundstage was huge, and almost sounded like 180 degrees wrapped around in front of you. However within that "wall of sound" the placement of individual performers was very vague ... nothing like the spicas or the europas. It was impressive, and I'm sure some people would love it, but ultimately everything sounded a bit too "huge" to be accurate.

>The lobing is actually due to having 2 spaced sources of the same signal at the same frequency. It isn't really caused by the crossover, but by the drivers. (Just being pedantic here.)

This is news to me, as stated. Of course, any two or more driver units will have interference effects, some configurations less than others. (It's even controversial whether a line array of dynamic drivers is truly a line array, for example.)

But in addition to these driver interference effects, the crossover topology certainly does influence off axis lobing. And you could demonstrate this by using the same pair of drivers and swapping a first order crossover and an "infinite slope" crossover in turn.

Every step up in xover order shifts the phase between tweeter and woofer by an additional 90deg. This means for maximum phase-coherence the obvious choices are second order (180deg shift so you just invert the connections to one driver to get it all back in phase) or fourth order (360deg shift: back in phase but not in time, a simple delay circuit will fix this). The ones to avoid are first and third order as a 90deg (270 for the third order) is very dificult to fix.
One advantage of a first order xover is a small number of parts which results in a very good transient response. The other is their immunity to electrical resonance ("ringing"). The higher the order the xover the easier it is to induce resonance with a high enough power input. When such a filter resonates it actually turns into a sinewave generator and produces an output at the xover frequency! Almost all analog synthsizers use this effect very succesfully but you don't really want that happening in your speakers, to this end most higher order xovers contain a damping circuit to avoid this.

The only truly phase-coherent dynamic multiway speaker in existence (Tannoy DualConcentric) uses a second order xover and inverts the tweeter.
It is phase-coherent to within 18deg. The only way to get better than that is a full-range driver, be it dynamic or planar. The result of this is a stereo image
that is far more precise than any other dynamic speaker I have ever heard.
And, being a true point source, you are always in the sweet spot as long as you are somewhere between the speakers and, where ever you are in the room, the sound never changes! After having spent a fair bit of time in recording studios I can say I have never heard a speaker as close to the real thing as a Tannoy! None, at all. Electrostats have better micro resolution and a Klipshorn better dynamics but, apart from those and overall, theres just nothing that comes even close. I put this down to phase-coherence and the point source characteristic. There is also a marked difference if I invert the connection (on both speakers at the same time): the stereo image pretty much collapses ie it goes flat in depth and height and the bass goes soft.
With other speakers I've never heard a difference.

Dear Golix,

We have exchanged some thoughts on the Time Coherence thread- thank you for sharing those. I greatly respect what Tannoy has done for sound quality, as I can tell you do. Certainly working in a studio qualifies you to know when a speaker's distortions are minimized.

Some corrections, please, to your information above, in the spirit of technical accuracy in our comments:

As I described in the recent Time Coherence thread, the phase shifts by 90 degrees, yes, for each additional order of crossover, as you state above.

But do know that the correct expression to use is that the phase DIFFERENCE is changed by 90 degrees for each additional order- at the CROSSOVER FREQUENCY. Those are important distinctions.

You see, the problem caused by higher-order crossover circuits is that this phase difference is ALWAYS CHANGING as one moves away from the crossover point- a DIFFERENT time delay is being imposed on each and every frequency.

This ever-changing phase difference (ever-changing time delay) CANNOT be corrected with any additional circuit.

The math also shows, without question, that this ever-changing phase difference/time-delay distortion cannot be "fixed" by inverting the tweeter's polarity.

Flipping a tweeter's wires from + to - serves only to flatten the frequency response, when one measures using continuous sine-wave test tones or continuous pink noise.

Those are both unchanging signals, without beginning nor end (and therefore carry no information). These test signals do not indicate anything about WHEN things happen- about the time-delay distortions that are occuring, frequency-by-frequency. When the tweeter's wires are reversed, the resulting transient response has only a "different kind" of inaccuracy, even though one will like "the tone balance better".

Only a first-order crossover has a CONSTANT PHASE DIFFERENCE at every frequency above, at, and below the crossver point. That means there is Zero change in phase between them, so the signals out of the high- and low-pass sections are "in Phase", always, on every frequency.

And all of that means, finally, that first-order circuits are the only ones that pass all their signals through with the SAME time delay, so all of those emerge in the same time-order in which they entered. This is called time-coherent behaviour. So you get the original transient, one not smeared out in time.

A speaker that is designed to deliver a time-coherent output is automatically "phase coherent". The converse is not true, as you may know: A phase-coherent speaker is not necessarily a time-coherent speaker. In fact, if you see the advertisement claim phase coherency, you can bet that the speaker is not time coherent. One has to put something in an ad!

On the Tannoys, their "phase distortions" (time-delay distortions) are rather mild. In my experience, once those varying delays are removed (which cannot be in Tannoys), the difference you'd hear is at least the difference between an average mic and an outstanding one. By the way, neither a phase- or time-coherent behaviour can be directly inferred by looking at the electrical phase of a speaker's impedance curve.

That the Tannoy's phase distortion is tolerable for you, is because it is unconsciously ameliorated in those studios by their mic selection, mic technique, the type and tone of the echo/reverb mixed in, the settings of the tone controls on the mic's channel, and by the EQ applied to the monitor system. And in pop music, the phase shift problems are also "danced around" by the sounds created by compressors, limiters, de-essers, and other tools. I speak from many years of recording experience, and of designing speakers with all these different crossovers (and using many others that had all sorts of their own time-domain distortions).

Thank you for your input. I am glad you get to work in studios so much. There are not enough people with that background contributing to high-end home audio reproduction. I hope my information is of help.

Best regards,
Roy Johnson
Founder and Designer
Green Mountain Audio

So by "phase coherent" you really mean that a first order filter remains entirely true to its 90deg phase separeation between tweeter and woofer?
Thats a new definition to me, maybe we have to start with defining every term properly first.
Also a speaker which has the drivers on a vertical axis can only be phase coherent at one point in space. This of course is a purely geometrical problem independent of any electrical feature.

Last but not least a time coherent speaker can easily be not phase coherent if the crossover separates the phase between drivers (as they all do). On the other hand a phase coherent speaker has got to be time coherent as well as a slight time mis alignment MUST result in a phase shift. Indeed a phaser works by splitting the signal in two and then time delays one with respect to the other before merging the signal again. If you modulate the delay you get a phaser/flanger sound , if you don't its a chorus. I hope you get the idea: all phasing is done by playing in the time domain.
Basically a phase coherent speaker is one that is not only in time but also in phase; a time coherent speaker is one thats in time but not in phase.
And most speakers are neither.

Tannoy manages to get theirs phase coherent by fitting the treble coil exactly half a wavelenght (at crossover) behind the bass coil and then using crossover characteristics and a very simple delay circuit. Thus there is no phase separation between tweeter and woofer at all (except some small aberration at crossover). They are thus in phase acoustically and electrically AND independent from the listening position.
And no I am not trying to sell Tannoy, I don't actually like the new ones with their plastic cones.
Personally, I think the aforementioned geometrical phase problem is the reason that conventional speakers have a sweet spot where everything sounds better and the stereo image snaps into place. Tannoy don't really have that: they sound the same where ever you are although the imaging shifts if you stand to the side, like looking through a window at an angle.

Having had a look at your website I see you claim a phase error of 2deg!
A bold statement, care to back that up with some measurments?
I am sure you've got access to an anechoic chamber and the necessary equipment.

Cheers Golix

Step function tests seem to show all of this very well. Stereophile performs these tests on speakers, but they fall down in not drawing any conclusions from the results.

"This speaker simply can't pass a transient correctly, therefore it's crap, no matter how much a listener may like it." :-)

I will chime in here; Roy is correct as usual. It is not possible to fix the inherent phase problems in a second-order crossover, because they are frequency-dependent. This is simply a mathematical absolute, at least in the analog domain. And phase inversion of one driver is a Really Bad Thing, no matter what the reason for it, because it completely screws up the original signal. Now it may be that the Tannoys are good-sounding speakers for many reasons (not having heard them myself, I can't comment), but true time-and-phase coherence is certainly not among them.

Skrivis's comment about the Stereophile step response measurements is right-on and needed to be said. I still remember several years ago when they reviewed the Quad 988 or 989. JA published all the measurements, and then scratched his head in a rhetorical sense, saying something like the measurements were "enigmatic" because most of them looked horrible in comparison to average loudspeakers (as did the ESL-63 when Stereophile reviewed it decades ago.)

After all those years, he still literally couldn't comprehend how a speaker that measured so poorly in the "traditional" ways (i.e., in the frequency domain) could possibly sound as good and as "right" as the Quad does. And all the while the step response, that beautiful, glorious, near-perfect step response, was staring him right in the face.

Of course, this isn't to say that full-range electrostats are perfect; far from it, especially when they have to be put in a listening room. But it can't be argued that what they do well, they do spectacularly. That is, they reproduce the signal in the time domain more correctly than just about anything else ever made. And in so doing, they set a shining example for us all.

Best,
Karl
AudioMachina

A previous poster has made the statement that using a 1st order crossover causes the LF and HF drivers to be 90degrees out of phase and this confuses me. At what frequency ?

If the LF driver uses a 1st order butterworth type low pass filter, and the HF driver uses a 1st order butterworth type high pass filter, with the -3dB frequencies at the same point (to give a flat amplitude response) doesn't this automatically put the two drivers in phase at the crossover frequency ? I must admit it's a long time since I studied filter design (since I went over to the dark side of software).

That would be a phase-coherent time for me ... the two drivers in phase at the crossover frequency, with linear phase response in the low pass and high pass filters as you move away from the crossover point ensuring phase response continuity elsewhere in the frequency response.

Such a speaker should have a good step response, because linear phase filters offer the least distortion of the original waveform.

I am assuming that this is what the Green Mountain Audio designs attempt to approximate (given that everything in real life is an approximation).