Time coherence - how important and what speakers?

Two points for you Bigtee.

Number 1 - If the highs travel much faster than the lows, then how does aligning the acoustic centers of the drivers time-align them? Answer: The time alignment attempts to simulate a single source of the sound so that the sound emanates from the same plane. The tweeters are not moved back to compensate for the speed of the frequencies. They are moved back to align the acoustic centers of the drivers for line-source radiation from the same plane.

Number 2 - All drivers have phase charts that are "all over the place". Phase alignment does not mean that there is not phase change. Phase alignment can mean 2 things. It can mean that the multiple drivers are crossed-over in such a way that the phase changes induced by the crossover are within a certain tolerance between each driver. It can also refer to the phase relationship between the driver and the port in a reflex design, or the positioning of the speaker relative to the behind wall, in a dipole radiator speaker. Many drivers will even reverse phase, at or below the resonant frequency of the driver. Every driver that I am aware of has phase irregularities. It is inherent in the electromagnetic transducer design.

From what I have heard, generally "time-aligned" means the acoustic centers of the drivers are aligned. And "phase-aligned" means that the crossovers use a relatively phase coherent 2nd or 4th order alignment.

Some of the Audio Physics and Lowthar(?) designs might qualify.

I just came off of 2 physics websites regarding the speed of sound versus frequency. Both the Cornell physics dept. and the Nasa physics website stated that the speed of sound in air is constant, and does not vary with frequency. The only variable in the speed of sound is the density of the medium(air) which will cause variations according to altitude, and humidity, or barometric pressure. In any case, the speed of sound will be constant for all frequencies at any given condition of the air at the time.

So this idea that the highs travel faster than the lows is not borne out by fact. It seems to be an "urban legend" that grew up around the "mysterious" functions of loudspeakers, that people did not understand.

As I previously stated, the time-alignment is done to simulate a single plane radiation source.

As Karls, stated, the single driver systems, including e-stats which may use a single panel, are inherently time and phase aligned, regardless of what some are reading into some "impulse charts". Any undesirable activity shown on the impulse charts are characteristics of the particular speaker being measured, and not a function of the speed of sound vs frequency.

Twl, I'm in over my head here, but I won't let that stop me. Is it not true that for a speaker to be properly "phase-aligned" by default it almost demands a 1st order crossover?

Supposedly, that's yet another reason why the Vandersteen Model 5's are so highly rated. Time alignment being another reason.

Correct me if I'm wrong.

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.