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 |
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 |
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 |
The output of a crossover network is a vector sum with real and imaginary components in polar coordinates. What you have in a 1st order at the crossover freq is one vector at .707, +45, and the other at .707, -45, which adds to unity in vector space with a combined phase shift of zero. But because the vectors rotate together with frequency, they are always 90 degrees apart, and they always add to unity voltage and zero phase in vector space, no matter what the frequency.
To say it differently, the combined output of the two drivers is always unity at zero phase, even though the two vectors are always 90 degrees apart. This is difficult to conceptualize, but the math behind it is relatively simple.
For obvious reasons, this is called a constant-voltage minimum-phase transfer function, and the first-order is the only crossover type that has this characteristic. I should note that this presumes identical drivers mounted very closely together and resistive loading, which is hard to achieve in the real world. But with some effort, one can come close, and the effort is well rewarded in the listening. |
Applejelly:
You are indeed correct that recorded phase is a serious problem, and one which is utterly ignored by many recording engineers. However, some of them do take it seriously, along with many other aspects of their craft. So, in my opinion, it is well worth having a system that preserves time and phase. It can't fix the bad recordings, but at least it doesn't screw up the good ones.
About the step response: You are correct that music is not a square wave, but I will make the argument that the square wave (or step function) is the best test signal yet devised for predicting musical fidelity in a loudspeaker. The square wave, after all, is simply the mathematical summation of an infinite number of phase-matched sine waves in a specific frequency progression. As such, any transducer that passes a clean square wave is eminently qualified to perform well on any musical signal it will ever encounter.
The reason that first-order designs change sound when going from sitting to standing has to do with lobing patterns, as discussed in previous posts. First-order designs have more overlap in output between the two drivers, so the lobing effects are more noticeable than with higher-order crossovers. However, there is an important point to keep in mind: The higher-order crossovers have non-uniform phase characteristics by definition, resulting in audible distortions of a different kind, even when listening on-axis. In other words, first-order is bad if you want to listen critically while standing up; higher-order is bad no matter where you are.
I think that many people who listen to Vandys (etc.) and end up buying something else are doing so because of issues other than time-and-phase coherence. This is not the only thing that matters, not by a long shot. The Vandys are built to a specific budget point, and in their price range, there are other speakers that give higher resolution, tighter bass, more treble extension, a more neutral tonal balance, etc. Anyone who values one of these specific things highly could easily decide to buy something else.
However, having said that, it is hard to imagine a better "real-world" compromise at a given price point than what Vandersteen has achieved. As I am fond of saying, the 2C may not be "the best" in any one area, but it is the cheapest speaker ever made that can truly lay claim to having addressed all of the important fundamentals in loudspeaker design. Its status as the best-selling high-end speaker of all time supports this conclusion. |
Golix: I have to back up Roy on this. In a first-order, both drivers are at zero phase in their PASSbands, and at 90 degrees in their STOPbands. (Close, anyway. The only places either of them truly reach 0 or 90 is at DC and at infinity, both of which are well outside the audioband.)
However, when either driver is at 90, the output amplitude is ZERO, by definition, so it contributes nothing to the sound. Its major contribution comes within its PASSband, where its phase is close to zero.
Now, in beween DC and infinity, both drivers make a contribution depending on the frequency relative to the crossover frequency. In a first-order, they are ALWAYS 90 degrees out of phase, regardless of the frequency, and they ALWAYS sum to unity and zero phase. If this isn't clear, you need to look into the math (including complex variables and vector addition).
At the crossover point, for example, one driver is at .707, +45, and the other is at .707, -45, as I stated previously. Due to the fact that this is vector addition, they sum to unity at zero phase. And they do this not only at the crossover frequency, but at every single point from DC to infinity. The first-order is the only crossover that does this.
If you wish to prove this to yourself, it is easily proved by doing some math. If you want to avoid the math, it can still be proved by simply drawing sine waves. First, draw a single sine wave with amplitude of 1.0 and any phase you choose. Next, draw two identical sine waves, each of amplitude .707, one shifted 1/8 wavelength to the left of the original one, and one shifted 1/8 wavelength to the right. Now simply sum their values. What you will find in that the summation is an EXACT replica of the original sine wave, in both phase and amplitude.
This principle works the same way with speakers. As long as your ear is equidistant from the two drivers, you will be utterly unable to distinguish the crossover. This is because the output summation IN THE AIR is identical to the original signal (in both time and amplitude), no matter what the frequency. (Again, this is true of the first-order only!)
Now, it must be said that the real world is not so perfect, and most drivers have rolloff-related and impedance-related phase shifts that add into the equation, giving a less-than-perfect end result. For this reason, designing a good first-order crossover is harder than it sounds. |
Skrivis, great link. As always, a graph is worth a thousand words. |
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 |
