Time coherence - how important and what speakers?

Any single-driver speaker, whether cone, walsh, electrostatic, or whatever, is automatically time and phase aligned. That's one of their biggest advantages.

I may be wrong but I don't believe that Karl's statement is correct. Highs come at you quicker where lows are much slower. A flat diaphragm is not timed aligned by looking at its impulse response and other factors. The frequencies are reproduced at the same time but do not arrive at your ears at the same time. I am aware of only a handful of time aligned (and/or phase correct) speakers. Going back through a lot of specs, etc., nobody has ever called an electrostatic time aligned that I can find. They are certainly not phased aligned. Look at the electrostatic impulse response and phase graph. The phase angle is all over the place.

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.