A Question About Time Alignment

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. 

The problem only occurs with reproduction. It does not apply to live music. All frequencies travel at exactly the same speed depending on the barometric pressure (air density). The problem is cause by crossover design, when certain frequencies are shifted in phase. It can also be caused when drivers are placed in disparate locations such as frequently occurs with subwoofers. Definition and imaging improve when a system is "time aligned." This can be easily demonstrated with digital crossovers that have control over delays. The problem that occurs when adjusting time alignment with driver location is that perfect alignment can only occur along the centerline between the speakers, but then that is where you only get a proper image anyway. The best solution to the problem is a single driver and no crossover except maybe to a subwoofer with digital time alignment.