Questions Re: Time & Phase Alignment

To supplant, perhaps, the Thiel info:
1)& 2) (Time alignment): in practical terms, the acoustic centres of the drivers (i.e. where sound originates) are aligned in the vertical plane. As tweets are typically more shallow than, say, woofers, their acoustic centre is more forward vs. the woofers'. So, sloping the baffle helps bring the acoustic centres CLOSER to being aligned. Soemtimes even, aligned. Snag: one listens to OFF axis sound (axis being the line of sound emmission from the transducer -- being vertical to the plane of said transducer).

(Phase shift & 1st order xover): all xovers intriduce phase shift -- that's what they're used for, it's part of their job, that's how they attenuate the frequencies you ask them to attenuate. The 1st order is considered less deleterious because of the gentle slope (90d /octave on paper) which, it seems, is less annoying to our ears. Snag: this puts a strain on the two drivers playing together and can cause intermodulation distortion. "Puts a strain" also means that the drivers+speaker system are/is engineered to play well in a 1st order situation: expensive or very cheap (and poorly performing).

To imagine phase, consider how we depict a sound wave -- i.e. two consecutive semi-circles on either side of a horizontal axis. Consider the crossing point on the axis as "zero" time. If the two drivers cross zero at the same time point, you are in phase.

3) There are many obvious advantages to coincident drivers, sound emanating from the same point ("single point source" i.e. "coincident") being a major advantage. There are many challenges: there is a xover anyway, intermodulation distortion, the outer cone can be a wave guide for the inner driver... (as Theil notes).
The Theil driver, BTW, is quite good and very expensive.
Cheers

Some simple comments, which I think are reasonably correct.

(1)Speaker inductors and capacitors in crossovers do not pass through electrical signal instantaneously. For example, a capacitor is two parallel plates. Electrical charge builds up and electrons flow across. With alternating current, the energy goes back and forth. The build up and passage of energy takes time, not much, but a discrete amount of time. Therefore, frequencies can be time delayed. With time delay comes phase misalignment. So time misalignment causes phase misalignment in this case.

Other things can cause misalignments too. For example, no passive crossover is perfect. Some frequencies will come from both drivers. If the drivers are different distances from your ear, the same frequency reaches your ear at different points in time. Your ear perceives this difference in timing. The same frequencies from different drivers may also interfere in phase so you can get phase misalignment too.

(2) A sloped baffle can help to alleviate misalignment. You typically sit with your ears at tweeter height because higher frequncies are more directional. Off-axis seating is more problematic for tweeters than woofers. Sitting with tweeters at ear height means that the woofer is farther away. If you slope the tweeter back so that it is the same distance away from your ear as the woofer, when sitting with the tweeter at ear height, you can compensate for the phase misalignment somewhat.

(3)First order crossovers do not eliminate misalignment. In fact, it is the opposite. With a more gentle roll-off, and with some frequencies produced by both drivers, you will get greater overlap. Time and phase errors will be exacerbated, not reduced, in the absence of other compensating factors in the speaker design. If somebody tells you that first order crossovers, by themselves, eliminate misalignment, tell them to send their research to the Nobel prize committee for evaluation so that the world can share this revolutionary discovery in the laws of physics.

(4) The answer about coincident drivers should be obvious from the points above. If you have the tweeter and woofer radiating from the same point, one cause of misalignment will be eliminated since any overlapped frequencies from the two drivers will be the same distance away from your ear.

Call Roy and ask the technical questions. He has always explained anything I have asked and is one of the most intelligent people I know. Great company and service.
Enjoy!

Ears are roughly 6 inches apart....a lot of phase correction is done when the brain combines both signals....so don't get too hung up on this issue alone, especially above 2Khz where our ears will quite likely get out of phase sound at certain specific frequencies anyway (depending on precisely where a person is seated...nobody keeps seated to within an inch position).

The lower mid range, however is an area that is more sensitive to phase issues....particularly the cross over from mid range to woofer in a three way design (two ways have an advantage here). At frequencies below 600 Hz the wavelengths are similar and then larger than the distance between our ears. Notice that at roughly 90Hz we completely lose the ability to locate sound. Misalignment in phase between two drivers in the lower mid can produce "comb filtering", a bumpy frequency response at the crossover and an "airy" unfocused sound; if male vocals lack the same focus as females then this may indicate a problem.

At 340 Hz the wavelength is around 1 foot; spatially speaking, mid and a bass woofer drivers that crossover near this frequency should not be more than a few inches different in distance from the ears (at least less than a quarter wavelength or 3 inches). This is rarely a problem if the drivers are mounted vertically....as the listener must go to extremely odd seating postions to have one driver more than even an inch different distance to the ears compared to the other driver.

Provided separate drivers are reasonalbly aligned in phase and spatially, most designers believe it is enough to have a smooth gentle phase behaviour over several octaves rather than preserve absolute phase (hence the preference for 1st order crossovers). Nothing abrupt. This approach is supposed to preserve the timbre/transient reponse of individual instruments by not smearing their partials with respect to their fundamental harmonics.

Active speaker designs have helped speaker manufacturers tightly control phase between separate drivers, gaining the advantages of separate drivers (less distortion, less beaming, less cone breakup, better dynamics and efficiency) whilst minimizing the drawbacks of having a crossover region.

Roy gave me this very sensible response to the coincident driver issue: "Regarding co-incident drivers: Any 1" dome tweeter, below 5kHz, wants to radiate in an omni-directional pattern. This means that much of the sound you would hear from a co-axial mid/tweeter driver in the lower treble comes to you secondarily by way of bouncing off of that mid's cone. Even a short horn around the tweeter cannot prevent this (because it is short). A high-order crossover can help reduce that secondary 'splash' off the mid's cone, by chopping off the low-end of the tweeter even more rapidly."

Hopefully he wouldn't mind me posting it (can't see why not).