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!

Time Cohesion is making sure that the sound from the tweeter gets to your ears at the same time as the sound from the woofer. Lets say theres a piano sitting 10 feet in front of you and a cello sitting 6 feet in front. The sound of the cello hits your ears faster than the sound of the piano. Its the ratio of the speed that needs to be conserved by the speaker. This can only happen if the time cohesion is tuned perfectly. 

The master has spoken.

Time Cohesion is making sure that the sound from the tweeter gets to your ears at the same time as the sound from the woofer. Lets say theres a piano sitting 10 feet in front of you and a cello sitting 6 feet in front. The sound of the cello hits your ears faster than the sound of the piano. Its the ratio of the speed that needs to be conserved by the speaker. This can only happen if the time cohesion is tuned perfectly. 

The master has spoken.

Not exactly… master.

If the piano and the cello are playing the same note, or using a full range driver, then they are already tome coherent from a single driver.

And the cello will still arrive 4 msec before the piano.

If it is a piano playing many notes then we want them all to be time coherent with each other. That is a harder to do with multiple drivers, but pretty common these days.

There is no “ratio of speed”, as the speed of sound is the same for cellos and pianos.

I generally find that speaker and sub coherence is best with the front-firing sub pushed a bit in front of the main speakers. Same thing, first with the big Velodyne DD18 and Dynaudio Consequence speakers, in my main system some years ago, and now, in a 'micro' version, in my home office, with single driver Arche FR2 speakers and a small Fostex Submini. Dragging the sub ca 8 cm in front of my speakers and screen looks a bit strange at the desktop but gives the most 'spot on' time alignment, to my ears. I mainly listen for timbre, juice, liveliness, emotional involvement.

It seems to me, however, that live music isn’t time aligned. Suppose I’m playing the piano and you’re sitting across the room. When I stretch out my left hand to hit the low notes, those notes travel along the same long, slow wavelengths as the notes from Wilson’s woofers. Similarly, the treble notes I play with my right hand move quickly through the short wavelengths. The notes from the piano are naturally out of alignment.

 

Something must be wrong with my line of reasoning. Would someone please point out where I’ve gone wrong? Nicely?

The time it takes for the low notes to arrive at your ears is slower than the highs no t because they are inherently long but because the voice coil of the woofer is mounted behind the baffle whereas for the tweeter it is mounted almost about the same level as the baffle hence the tweeter is closer to yours ears than the woofer. 

With a real piano, various sounds come out of it from various distances. These relative distances must be preserved by the speaker hence the requirements for time cohesion.