XO Group Delay

[SQdude]

So I've been reading The Loud Speaker Cookbook and am confused on the section about XO group delay. The author mentions that a LR-2 XO has a flat group delay yet the graph he refers to starts flat then has a negative slope in the XO region and returns back to flat. What am I missing here?

Also, say a XO has a flat group delay albeit with a non-zero magnitude, does this mean that essentially the drivers are in-phase but one is lagging the other? If so, this is bad and I would think the only way to correct this is to offset the horizontal driver distances (so that leading (in time) driver's voice coil sits farther back).

[joecarrow]

The thing with phase is that it's a different time delay for each frequency- it changes continuously.

The period of time as a difference between the two drivers in the crossover region is inaudible, unless it causes some sort of interference effect. Matching the phase of the two drivers assures that this does not happen. What's difficult, however, is getting to drivers to conform well to the LR2 response. Drivers have their own roll-off, behavior, etc, and most drivers will require additional consideration to be forced to this response. It can be done with good effect if the drivers are of really high quality. Zaph's ZD5 project does exactly this.

To convince you that the time difference doesn't matter, consider a 2khz crossover, with the 180 degrees of phase difference (half a cycle delay), it's 1/4000th of a second difference, or 0.25 milliseconds.

At this point, I guess you'd have to take my word for it that this is a very small delay. I started to look for a reference to show that this is the case, and I came across this old article:

https://web.archive.org/web/20070928140759/http://www.trueaudio.com/post_010.htm

I guess I could have just linked to that to start, but I haven't had my coffee yet

[Dennis H]

I don't have the book but I think he probably means the LR (Q=.5) filter's group delay is "as flat as you can get" or critically damped. Note the peaking in the Butterworth (Q=.7) filter's group delay just below the 1kHz crossover frequency.

[SQdude]

Cool thanks guys. Dennis what pgm is that?

[Dennis H]

That's the free software to control the QSC digital crossover/EQ unit. I use it for simple stuff like this 'cuz it's so quick and easy to use.

https://web.archive.org/web/20070220024053/http://www.qscaudio.com/products/software/signal_manager/signal_manager.htm