Time coherence - how important and what speakers?

Stehno, the 1st order crossover alignment, which consists of a single capacitor between drivers is a 90 degree phase alignment. It allows the bass driver to run full range, to its natural limits, and blocks the tweeter from operating below the frequency specified by the capacitor value. This protects the tweeter from harmful high power that may be present at the lower frequencies, and keeps intermodulation distortion to a minimum in the tweeter. The tweeter will be rolled-off at 6db/octave, from the crossover point downward, in this configuration.

The main advantage in this alignment, is that there is the fewest number of crossover parts to screw up the signal. And the presence of crossover inductors is eliminated, which is a major cause of phase shift. For this alignment to be successful, it requires careful matching of the drivers, so that there is not a "bump" in the response where the tweeter crosses in, and the woofer does not cross out. The woofer should be naturally rolling-off on its own at around 6db/octave, right at the crossover point, in order for this alignment to be optimal. Even though this is considered a 90 degree alignment, there is much to be said for the simplicity and signal preservation of this design. There are going to be phase-shifts in any crossover, and preserving the signal integrity is a valiant thing to do, considering the necessity of having a crossover in a multi-driver system. Having no crossover in a multi-driver system, with the drivers having matching natural roll-offs at a certain point would seem to be optimal, but the protection of the tweeter is vital, and intermod is present in unacceptably high amounts, as the tweeter vainly tries to respond to low frequency information that it cannot reproduce. So the single capacitor which functions primarily as a tweeter protector is a good compromise, if you can match the drivers well enough to keep the response curve smooth. If you want to avoid the crossover phase shift altogether, you must go to single-driver, which presents its own set of difficulties.

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.

Karls, thanks for the clarification. I was under the impression that the capacitor or coil shift would be additive to the reactive element, and not be cancelled by it, in a first order configuration. Is this fully proportional, or are there some varying degrees of shift that are not cancelled ?

I think we could discuss this subject for a long while and get very technical. I feel that anything that helps preserve the waveform is beneficial. TWL is right about the waveform. You want it all to arrive at the same time. You can go into a chamber and easily prove that a lot of speakers do not deliver the sound as a "On time" acoustical wave. This is what time alignment tries to do. Science has proven a bumble bee can't fly but it does. Acoustics has a lot of issues just like electricity. Vandersteen tries to get his speakers to perform in a manner that preserves this waveform and even though the phase angle does change, it is not going far in a positve or negative direction unlike some speakers. It is just closer to an electrical ideal. Does it make a speaker "sound " better? Well, there's your debate.

"Science has proven a bumble bee can't fly but it does"

While I won't disagree with most of your statments, science never proved a bumble bee can't fly. Technically, science can't prove anything. We/science never proved all objects fall to the ground at a constant speed (or that they'd all even fall to the ground) since we can't possibly drop all the objects. An assumption is finally made that the hypothesis being tested is 'true' when nothing disproves it after a certain number of experiments (and we decide when enough experiments is enough). And this is why we also create the null hypothesis for the sake of formality/consistency and this is called the falsification principle that science progresses under (Wasn't it Popper who write of this?). Helicopter's fly, and do so in a completely different manner than airplanes do-and we have explanations for both. Dragonfly's create little tornadoes under their wings by disrupting the air current, a completely different manner of flight than most birds. The Bumble flies and for a reason--we just may have not found a way to explain it yet. But nothing in science says a bumblebee can't fly, because this would be defeating to the whole purpose of what science is about: to know why everything around us is the way it is-to understand all of cause-and-effect. (I don't know what's in the literature about bumblebee flight).

Returning to Science & Audio:

"Just because a speaker system has a sloping baffle does not mean it is time aligned."
My bad, this is a correct statement. If there's a new generalization to be made, possibly: a slightly sloping baffle is usually not time-aligned, but one of a greater/steeper angle is.

"There are a few manufacturers who would like for you to think there speakers are time aligned. This has been noted in Stereophile's reviews of speakers from time to time"

I'm not sure about the first sentence-it gets into other peoples' intentions and I can't live in someone else's head. But then again I don't know what all the manufacturer's out there are doing so you may be right-there may be some doing it. Ruark had a little floorstanding model years ago in a Stereophile review that was alluded to as not being time-aligned. However I didn't think anyone was insinuating that Ruark wanted you to think it was. If anything it cost more money to make a cabinet with a sloping baffle and why a manufacturer would do it but not go through with the xover design is a little weird. Especially, since xover design is *simplified* (I believe) once the acoustic centers have been lined-up. But I'm thinkin, maybe, in the case of the Ruark its baffle was slightly sloped just to get the sound up to the listeners ears (and maybe add a little visual appeal), so Ruarks intentions may have been sincere. 30" is pretty short for a floorstander and while some drivers are better listened to off-axis, 28" above the ground tweeter might be a little low regardless. And it may be effective in getting rid of internal standing waves which become a big problem with many floorstanding designs (although cabinet stuffing is the most effective way). But, there maybe alot of models out there marauding/falsely advertising as 'time aligned' creatures. I don't read the fashion rags because they're still recommending $10k speaker cables with $12k speakers. Although from what I hear there may be an article or two in there I wish I had read--Stereophile reportedly had a good one on Opamps.

Incidentally, BigTee, some of Bose's models are, more or less, time and phase coherent. My clock radio is.