First Order Crossovers: Pros and Cons

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.