Any single-driver speaker, whether cone, walsh, electrostatic, or whatever, is automatically time and phase aligned. That's one of their biggest advantages. |
Far fewer speakers are really time-aligned than you might think. For example, the Wilson Sophia has a slanted baffle, but one look at the Stereophile review's impulse response tells you that it is not even close to being aligned in either time or phase! Speakers that really come close are the Dunlavys, Meadowlarks, Thiels, and Vandersteens. The impulse response is extremely easy to measure and tells you almost everything you need to know.
And yes, it is very important, it is the difference between merely a good-sounding speaker and one that sounds like real music. |
Twl, only first order filters are phase coherent, no exceptions. First order usually refers to the electrical filter, not the inherent slope of the driver's response. Therefore a first order filter consists of one reactive element, either an inductor or capacitor, in series or parallel with the load (the driver). And the phase shift caused by the reactive element varies with frequency, as a function of the ratio of reactive impedance to load impedance. In an "idealized" first order filter, there is a woofer inductor and a tweeter capacitor, each of which has a 45 degree phase shift at the crossover frequency, but in opposite directions. Over the rest of the frequency spectrum that each driver is intended to cover, the phase shift caused by the reactive elements is less than 45 degrees. Above the woofer's crossover frequency, and below the tweeter's crossover frequency, their phase shifts gradually approach 90 degrees, again in opposite directions.
Higher order filters add 90 degrees of phase shift per order. For example, a second order filter will have a 90 degree phase shift on both woofer and tweeter at the crossover frequency, approaching 180 degrees in each driver at the extremes. The two 90 degree phase shifts at the crossover frequency in opposite directions gives a net 180 degree phase shift between the drivers, meaning that if the drivers are wired in the same absolute phase, there will be a null at the crossover frequency. This then necessitates that the input to one of the drivers be run in inverted phase, meaning that throughout the entire range covered by the two drivers, they are operating in opposite phase. Talk about screwing up the integrity of the waveform!
Twl is right, there is no variation in the speed of sound relative to frequency. Each driver has a different "acoustical center", which is the point at which sound can be said to originate. It is not a physical property but an acoustical one, although a good approximation for any given driver is to use the point where the voice coil former meets the cone or dome. |
I'll take a stab at Joseph Audio's post: First of all, no one ever claimed that time and phase coherence is the only important parameter in loudspeaker design (at least, no one with any sense). There are many, many factors, all of which are extremely important, and any one of which, if ignored, will detract in a significant way from the end result. THAT is the crux of the matter.
So how do you know that time and phase coherence is critical? So few speakers have these characteristics that many people are fully acclimated to the sound of noncoherent designs, and so will not notice that anything is wrong. All the individual "sounds" are there, and in the right volume proportion, and they all have excellent attack and decay, etc., but the speaker itself does not sound "coherent". I hate to use such a nebulous term, but it truly sounds as though there is not a musical whole, but rather a bunch of separate musical events occurring more or less simultaneously. It is the difference between "natural", which allows you to relax into the music, and "a really great stereo system", which may be fantastically impressive at a hifi show but never actually fools your brain into thinking that you are listening to the real thing. That is my personal opinion, but it is nonetheless exactly what I hear. When it is right, you simply forget that you are listening to speakers. And it occurs with vastly disparate design philosophies, including multiway cone/dome systems, electrostatics, and Walsh systems. Although they are instantly distinguishable from one another due to their vastly different dispersion characteristics, all of them allow your brain to relax and feel as though you are listening to real music. I have never heard a noncoherent system that could do this, not one, in over twenty years of being obsessed with hifi and music. Sure, I've heard plenty that sounded like great hi-fi speakers, but that's an entirely different subject.
That, in a nutshell, is my personal answer. Do not presume that I or others are just making this stuff up for our own intellectual masturbation. It is very real to us, every bit as real as the anechoic-frequency-response or cumulative-spectral-decay graph is to you. And every bit as important, even if it is only one of many things that are every bit as important. |
I am not about to slam on Joseph's speakers, because I think that he is correct on many points and certainly the results speak for themselves: a very clean and clear decay and very flat frequency response. These are tremendously important factors, and any speaker designer that fails to perfect them is doomed to failure from the start. In addition, I would not speculate that his sales are poor either; there are few speakers in the world that perform better than his in this regard. In addition, I agree with him fully that first-order crossovers are an absolute bear when it comes to frequency response and driver integration in real space. BUT that doesn't stop me from saying that I still prefer the "sound" of a well-worked-out time- and phase-coherent speaker. Granted, there aren't many out there, but they do it for me like no others can. |
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A couple comments about concentrics: It has always seemed to me that the presence of a moving cone surrounding the tweeter would actually frequency-modulate the tweeter's boundary reflection signal, at least in the range where the tweeter is not beaming, leading to some rather strange colorations. That, plus the inevitable horn-loading colorations imparted by the midrange cone onto the tweeter's signal, makes me not-so-impressed with this concept. |
Vandersteens are definitely NOT T-lines, marketing hype aside. They are ported, end of story, and passive radiators are just a variation of porting. True T-lines are VERY big and VERY difficult to build. And I will take exception to Roy's (and Martin Colloms') lumping of T-lines in with ported designs, as they are not the same. The crucial difference is that a properly filled T-line has the best and most uniformly damped impedance curve you will see in any speaker anywhere, better than most sealed boxes and the exact opposite of all ported boxes. Impedance peaks are due to resonance, plain and simple, and the sharper and higher the peaks, the worse the resonance Q and the higher the stored energy. This is directly related to bass transient performance and that is why properly damped T-lines have a legendary reputation in the deep bass (and why ported boxes suck). I say this as a huge fan of sealed-box loading, often the best "real-world" compromise. But beware, a claim by the manufacturer of "T-line" loading is not sufficient to achieve this level of performance. It takes a lot more work than writing ad copy. |
Roy,
I have not investigated T-lines thoroughly enough to have all the answers, but I will say that much of the received wisdom is downright wrong. Examples: (1) The commonly bandied-about equation describing speed of sound changes based on stuffing density is patently wrong on its face, and almost no one seems to notice. (2) There are all these theories about how the stuffing works, from the air causing movement in the stuffing to adiabatic/isothermal changes to who knows what else. From what I have seen, these are 90+% BS. Viscous damping due to air movement past the fibers is almost all you need to understand stuffing.
This is why the impedance curves come out so flat. In an undamped line, you have a whole series of sharp impedance peaks at n/4 for all odd n. (Note that these are pipe resonances just like in an organ, and that this is very different from a ported box, which has only two peaks which are compliance/mass resonances.) The stuffing removes these peaks entirely at even midbass frequencies and damps them extremely effectively at lower frequencies (including at the lowest 1/4 lambda resonance). On the other hand, ported design is specifically intended to function without damping, for all practical purposes.
In addition, although this isn't discussed much, T-line woofers have their fundamental resonance frequency dramatically reduced when placed in the line (as opposed to a sealed box, which always drives it upward). This is most likely due to the effect that at the lowest frequencies, the entire mass of the air in the line becomes coupled almost 1:1 to the cone. This is a very substantial increase in effective mass. An argument could be made, however, that due to compressibility, the initial attack at higher frequencies is much faster than if an equivalent real mass were added. Contrast this to a sealed box, where the only way to drive the resonance down is to add real mass, which hurts the transient response at higher frequencies. (And in addition, an argument could be made that decay at all frequencies occurs much faster as well, due to the high level of damping the stuffing provides.) This increase in effective mass at low frequencies is very nearly "something for nothing", and is probably why T-lines seem to have both "speed" and "weight".
I cannot disagree about the delay of the back wave, but I question whether it is an audible effect at the very lowest frequencies (because, again, a properly stuffed line will absorb everything from the lower midbass on up). The question becomes whether an 8-ft delay is audible at 35 Hz. I can't say because I don't honestly know. It could well be.
I am not trying to disparage the quality of a low-Q sealed box in any way, as I too think it is often the best real-world solution, but I think that there is a lot more going on in a "T-line" than is commonly appreciated, and worse, a lot of plain misinformation floating around.
Cheers,
Karl |
Roy,
Thanks for the long response. Here are a couple links that show interesting data on the above topics:
www.t-linespeakers.org/projects/tlB/radresponse.html
This has impedance data which shows a remarkable impedance flattening at the 1/4 lambda frequency at quite reasonable stuffing densities, in addition to a dramatic reduction in the driver resonance peak itself.
www.t-linespeakers.org/projects/martin/focal/test_line.html
This also shows a dramatic drop in the 1/4 lambda resonance at normal stuffing levels (using Dacron), and also has several other interesting results. One is that the reduction in speed of sound is far less than Bradbury etc's data on wool and fiberglass. You are likely correct that the microscopic fiber characteristics have a major role in this. Also, note that at the higher frequency peaks, the experimental data show near-perfect correspondence with the theoretical numbers, suggesting that there is effectively NO air-mass coupling to the cone at these frequencies. This one plot is what convinced me that there is indeed a strongly frequency-dependent air-mass coupling.
I will still take issue with your (implied) statement that added mass is not a problem. I understand the games that can be played with mass and compliance, but only at the expense of cabinet size and/or efficiency. I also understand that one can say that "you can always make the magnet bigger." But therein lies the real-world problem: you would like to keep the efficiency as high as possible (within reasonable limits), and the cabinet at a reasonable size, while being limited by the reality of the relatively weak magnetic fields achievable with fixed magnets. So added mass does not come without penalty. In addition, my passion for a long time now has been for 2-way systems, so my perspective tends to be skewed by that reality without my realizing the need to state it, and I should have prefaced my comments with it.
Again, thanks for the extraordinary effort you have put into this thread. It has been very enjoyable. |
Cdc, NSM is blowing smoke up your ... There is no way on heaven or earth that the speaker shown in your link is time coherent. As I posted early on in this thread, very few speakers are actually time coherent, and I've never ever seen one with drivers mounted to a flat vertical baffle that comes even close. Sure, many manufacturers would love to have you believe it, but one look at the step response is all you need to tell otherwise.
Roy, great summary of philosophy, which I agree with 100%.
Meadowlark has a simple and easy-to-visualize example of this on their website, www.meadowlarkaudio.com. |
4th order LR does have the benefits described. It should be noted that your particular link seems to be talking mainly about ACTIVE crossovers, meaning (typically) op-amp based electronic filters placed upstream of the power amps. This is an absolute piece of cake compared to designing proper passive crossovers, but very few speakers on the market go this route as it requires an amp for every driver, and makes the overall package pretty spendy. ATC does a great job at this, but their market share is pretty low overall because most people want to be able to mix-n-match their amps and speakers (I'm not saying this is a better approach, quite the opposite, merely that it's what most people seem to want). Implementing an ACCURATE 4th order L-R in a passive crossover with real-world drivers and all their problems is a giant headache, to say the least. Lots of manufacturers will claim to be using this type, and they are in a theoretical sense, but in fact the end result isn't a truly accurate 4th order L-R due to the frequency response/ impedance/ phase variations in the drivers themselves. They can sometimes come close, but the end result is never ideal like it can be when using op-amps. This is why active crossovers are such a piece of cake by comparison. |
