Why not horns?

Macro, you are a bit off base on that one. It is one of the common misconceptions in audio. Your description of being physically aligned is correct but your description of phase and polarity isn't.

A difference in phase means a difference in time. A difference in polarity means one signal is going positive while the other goes negative. Phase and polarity are two entirely different things.

It is confusing because if you reverse the wires on one speaker (black to red) in a stereo pair then everybody says the speakers are out of phase. That is technically incorrect. The correct phrase is you have reversed the polarity to one speaker. One will be going in while the other is going out. They still happen at the same time so they are in phase but they move in opposite directions so they have opposite polarities. Unfortunately it is common practice to describe it as the speakers are out of phase, and it is awkward to say that one has its polarity reversed, so we are stuck with a phrase that is technically incorrect.

Same situation with balanced cables. While one line is going positive the other is going negative. Some people incorrectly say they are out of phase but actually one has inverted polarity.

If the speakers aren’t time aligned then there is indeed a phase shift, a difference in time.

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Ralph, I'm glad we have found common ground on the horn issue after butting heads on single ended Vs. balanced, and I understand your point about phase shift in crossovers, but trying to do time alignment by phase shift in crossovers seems to me to be a hopeless affair. The amount of shift is frequency dependent so while I suppose you can shift a driver closer to another as you approach the cutoff frequency wouldn’t that leave you out of phase in the passband of each driver? I’m no speaker/crossover designer so maybe I’m missing something.

Herman - Thanks, I needed that. Polarity and phase are different topics with different remedies. And polarity is fairly easy to diagnose and remedy. How about phase problems? What are the causes? How do you diagnose phase problems and how do you fix them?

Phase shifts can occur because the drivers are different distances from your ears. This can be corrected physically with driver placement. That's one reason you see speaker boxes that slope back to get the tweeter voice coil aligned with the woofers. That's pretty easy to do or you can do the digital delay. If you look at various front loaded horns you almost always see that they are built with the mouths of the horns different distances away from the listener to align the voice coils.

The harder problem is phase shift caused by crossovers. In a simple first order crossover as you get close to the cutoff frequency you also start getting a phase shift. At the cutoff frequency there is a 45 degree phase shift. Different frequencies get shifted by different amounts. There are other filter configurations that have less or more phase shift but each is a compromise in some way.

I use a digital crossover and delay that they say has no phase shift but I tend to believe it must be screwing with the signal in some other way. I don't know enough about digital filters to prove that but I'm in the "you don't get something for nothing" camp.

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Herman, I leave the design of speakers to someone else to prevent headaches. I was just pointing out that the crossover can be used to correct time alignment (sometimes this results in the need to reverse the phase of certain drivers as a result), and in fact a crossover will introduce some time alignment issues that might have to be otherwise addressed. I think its a bad idea to rely on that exclusively- doing some physical alignment is important too.