It's generally accepted that the max spacing between drivers is...

[454Casull]

one wavelength at the crossover frequency, but that applies to drivers that are laterally spaced. What about drivers in front of each other? Suppose you have a tweeter mounted in front of a woofer - how far ahead can the tweeter be mounted without dispersion problems, assuming that the signal to the tweeter is delayed such that the sounds from the woofer and tweeter arrive at a given distance at the same time?

Let's say that the distance is 1m from the woofer. Assuming that the delay compensation is fixed for phase coherence at 1m and 0-degrees off axis (i.e. directly on-axis), what would the polar response look like? This, I believe, depends on how far apart the tweeter and woofer are, and is the basis of my question.

[joe carrow]

In general, the problem with that is that the sound from the tweeter would reach you before the sound from the woofer. That's going to give you problems with coherence. You could always put some delay on the tweeter, though, and that should solve the problem. Take a look at active crossovers- I think that'll be the key.

[454Casull]

Yes, I forgot to mention that there would be electronic delay compensation.

[Andrew S-]

On axis I dont see much of a problem. But get off axis any bit and its going to start separating the drivers.

[BillFitzmaurice]

You're going to have off-axis phase problems that will lead to comb filtering. Even if you have the tweeter perfectly phased and time aligned those conditions will only be preserved on-axis; the farther the tweeter is from the woofer the smaller the off-axis angle you'd have before pathlength differentials really foul things up in the crossover area. Then there's the matter of imaging, which again would be fine on axis but would collapse off-axis.

[454Casull]

Originally posted by BillFitzmaurice
You're going to have off-axis phase problems that will lead to comb filtering. Even if you have the tweeter perfectly phased and time aligned those conditions will only be preserved on-axis; the farther the tweeter is from the woofer the smaller the off-axis angle you'd have before pathlength differentials really foul things up in the crossover area. Then there's the matter of imaging, which again would be fine on axis but would collapse off-axis.

Let's say that the distance between the tweeter and woofer diaphragms is 20cm - would the degraded off-axis performance be better or worse than a tweeter and woofer mounted conventionally, with a center-to-center spacing of 20cm?

[BillFitzmaurice]

Let's say that the distance between the tweeter and woofer diaphragms is 20cm - would the degraded off-axis performance be better or worse than a tweeter and woofer mounted conventionally, with a center-to-center spacing of 20cm?

Yes. Assuming you do it correctly, which is to place the tweeter above the woofer, there is no comb filtering on the horizontal plane as the distance from the two sources remains constant as one moves from side to side; vertically there is a combing, but that's usually only problematic if you're speakers are in a gym and you're doing jumping jacks in front of them. You can get no combing at all with a coax set-up, but you have to keep the HF element in tight, Tannoy being an example of how to do it right.

[paulspencer]

This doesn't really answer your question, but rather steps around it but ... have you considered having a look at the Seas coax drivers? They have the tweeter in the centre time aligned. Looks very promising.