Doug, just to add to what you has said.
When you “align” a system you have two choices, as you mentioned, you can go for a “flat phase” phase response or you can go for a “smooth phase” response. (I am excluding first order filters and FIR filters)
If you go for a flat phase response what you try to do is to get the lo, mid, and hi frequency energy to arrive at the same time – basically you align the acoustic centres of drivers and in this case they would be about 10 feet apart or 11.5ms. This sounds great until you look at the crossover points. There are other delays in the system caused by filters, drivers, box design etc; the result is that the summation at the crossover is often poor. You can try to minimise these problems by trying different asymmetrical filter functions but its never quite right through the crossover.
In the end what you get is a good result, it sounds very dynamic, the majority of the energy all arrives at the right time but there are problems around the crossover points. Its made worse if the crossover points are in those more critical frequencies, in the middle of your vocal region or in this case 80 hz, your kick drum. Its interesting that you select this for playback DJ stuff, I suspect it makes it sound more “live” ?
The other choice is to go for a smooth phase response. You do this by trying to time aligning things at the crossover point so that the drivers sum perfectly and are “in phase” at this point and you get a smooth transition though the crossover point. In this case you would use 6 – 7 ms, the problem is that the low frequencies will arrive a bit late compared to the high frequencies.
In the end you get a sweet smooth sound and a system that’s easy to work with but lacks a bit of the dynamics of the flat phase alignment. This is usually considered the best compromise.
In addition you also need to consider how flat the power response is, is the directivity plot smooth and how it behaves as you move off axis. These will also be affected by the crossover type and alignment.
One example is if the top boxes are flown and the subs ground stacked there is the possibility of significant parallax time alignment problem throughout the venue. In this case it may be better to select different filter types/alignments (eg lower order, over lap the crossover frequencies and time alignment) to achieve the best compromise in terms of bass “lumps and bumps” throughout the venue.
Back to the lab subs, I use my 18 inch version with EAW KF 750s which have a sort of reflex/horn loaded LF so there is about 6 ms of delay associated with the box tuning. I find 6 to 7 ms of delay seems to work best which is what EAW suggest for their BH760 (10ft horn sub).
Peter - typing after 3 hours sleep.
