Time coherence - how important and what speakers?

"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.

I didn't read every word in this thread, but it started by listing the Joseph speakers as being time and phase correct. I don't think so. They use patented, ultra steep slope crossovers as their main design concept, which would preclude time and phase alignment. (If someone knows something different, please post.)

Karls, I believe that John Bau of Spica designed time coherent speakers with other than 1st order cross-overs.

Well, I'm going to take one last technical stab at this. What we are talking about is the time that elapses between the arrival of the electrical signal at the input of the driver and when the acoustical signal leaves the driver. This will vary depending on the size and frequency range of the driver. This is not the time the acoustical waveform travels through the air. If it doesn't leave at the right time, it sure can't get to your ears at the right time. That is why I said that the baffle slope doesn't mean a lot, relatively speaking. We must be phase correct to assure that the amplitude of the fundamentals and harmonics are not altered and time aligned. By adjusting the acoustical centers of the drivers, we can compensate somewhat for the time alignment. It has to be precise.
The crossover network may need to be very complex to accomplish a 6db/octave ACOUSTICAL roll-off because the electrical and mechanical characteristics of the drivers must be considered. Single caps will not do the job. Phase has to be considered in this equation because phase will create acoustical timing errors. Also, we are talking about acoustical phase, not electrical phase. We are looking at wavelengths here. If you know the frequency of a sound pressure wave you can calculate the "Wavelength" of each cycle. We can also add in "transfer function."
This could be a very long discussion but I hope this gets us all on the same page.

Time coherence is difficult to achieve because the acoustical center for a moving coil driver is not a fixed physical point, but actually varies somewhat with frequency. Source - The Loudspeaker Design Cookbook, sixth edition, page 113.

Phase coherence in a multi-driver system requires first order acoustical slopes for both the low-pass and high-pass driver. The acoustical slope is the sum of the inherent driver response and the crossover response. Note that time and/or phase coherence can only occur in a single plane with a multi-driver system.

Human hearing characteristics aren't linear, but are full of thresholds and masking effects. For example, after .68 milliseconds the directional cues from a repetition of the original signal (an echo) are suppressed, and this suppression (masking effect) lasts for about 40 milliseconds. Now, if we knew precisely the thresholds that apply to time and phase coherence, we could take them into account in loudspeaker design. Alas, the published research is inconsistent on the audibility thresholds of phase and timing errors.