A Question About Time Alignment

Two other things I can add to the discussion:

1- An interesting situation exists in the case of line-source loudspeakers, a good example being the Magnepan MG3.7i. This speaker has a long, vertically-orientated ribbon tweeter (a real good one), with magnetic-planar drivers for midrange and bass frequencies running along side the tweeter. In the instruction manual for the 3.7i, it is advised that the speaker be positioned so that the tweeter is slightly further away from the listener’s ears than is the midrange driver. The reason for that is that the speaker’s crossover creates a slight time lag in the midrange driver. With the tweeter and midrange driver equidistant from the listeners ears, the two drivers are not quite time/phase aligned. 3.7i owners need to experiment with varying degrees of toe-in, until the highs and mids sound coherent. With a dynamic loudspeaker (cone & dome drivers in a box)---with the drivers aligned vertically, as most are these days---tilting the enclosure forward or backward can sometimes be used the same way. Raising or lowering the enclosure instead achieves the same result, of course.

2- In a number of his YouTube videos, Danny Richie explains why tweeter and midrange drivers should be mounted as close together as possible. And why the higher the x/o frequency between them, the closer they should be to each other. That is because at the high frequencies tweeters are producing sound, the wavelengths are very short. Danny explains it all far better than can I, so if interested do a search on YouTube for GR Research. A free primer in loudspeaker design basics!

@carlsbad (edit: and @erik_squires) has/have it exactly correct: all frequencies travel at the same speed---the well-known "speed of sound".

As to time alignment of the drivers, a major objective in a multi-driver loudspeaker is to get the different drivers to be in phase with one another, especially at the crossover "point" (read on). At that crossover point (not a single frequency, but the frequency range withing which driver outputs overlap), if two involved drivers are in phase, their combined outputs will "fill in" the declining slopes of both drivers, thus producing a flat freqency response (for instance, the falling output of the low end of the tweeters output and the falling output of the high end of the midrange drivers output. Those declining outputs are the consequence of the two drivers raw responses combined with the crossover filter slopes). If the two drivers are not 100% in phase at the crossover frequency, that won’t occur "completely", and there will be a "dip" in response. It’s complicated, but drivers are referred to as having certain "degrees of phase rotation", a subject far too technical to explain in this post. The information is available in the literature.

But there is another consideration. "Time aligned" drivers are aligned at a given listening position, that position in relation to the drivers. In his excellent YouTube videos, Danny Richie of GR Research demonstrates how moving a measuring microphone (which is a substitute for ears) effects the phase relationship between drivers. Two drivers in phase at a listening position equidistant between two drivers can become somewhat or even significantly out-of-phase when the mic is moved above or below that equidistant position.

"Time aligned" has become a marketing buzz word, but it is an over-simplification of the complex relationship between drivers in all multi-driver loudspeakers with crossovers. Just buy a Sound Labs or Sanders full-range ESL, a planar-magnetic dipole (Eminent Technology or Magnepan), or a loudspeaker designed by Danny Richie ;-) . The latter are available only as DIY kits, but you can do it!