what are peoples views on using different frequency HPF and LPF to make a crossover?

[Puggie]

been playing with designs in Bass Box Pro and Xover Pro (on a side what are peoples opinions of this software?) and have come up with what looks a nice design; BUT. the response has a definitel trough at the crossover point, now if it keep my woofer low passed at 3kHz and drop the high pass of the tweet to 2.4kHz (both 18dB/Oct Butterworth) I now get a response that is nice and flatthrough the crossover frequency but my relative phase response has shifted to about 10-15Degrees apart, is this a major worry? which should I go for, flat frequency and mismatched phase or frequency dip at the Xover point and a nicely matched phase response around the Xover point?

slightly confused at which way I should go?

[BillFitzmaurice]

The way you propose to configure your crossover is precisely how the best crossover designers go about it. If it works it's right. Your ears are sensitive to response, insensitive to phase in moderation. You will get a bit more IM distortion than is desireable, but again that is not as audible as a response dip, especially one in the midrange.

[Swedish Chef]

Bill,

why would you get more IM distortion by doing this?

Cheers
/Magnus

[Saurav]

I've played with software simulations where I tried separating the two 3dB points further apart. I never thought about trying to make them overlap, i.e. the lowpass of the upper driver below the highpass of the lower driver. Something new to play with tonight

[BillFitzmaurice]

IM distortion has a number of sources, one of which is having two drivers reproducing the same information; one of the advantages to high order crossovers is a minimizing of IM. This source is far less serious than, say, not isolating the midrange driver in a sub-enclosure from the woofer rear wave, but it is there. In this particular case it's definetely the lesser of two evils.

[DSP_Geek]

Originally posted by Puggie
[....] BUT. the response has a definitel trough at the crossover point, now if it keep my woofer low passed at 3kHz and drop the high pass of the tweet to 2.4kHz (both 18dB/Oct Butterworth) I now get a response that is nice and flatthrough the crossover frequency but my relative phase response has shifted to about 10-15Degrees apart, is this a major worry? [...]

Whether phase response can be heard or not around 3kHz (the jury's still out on this one), a trough in the frequency response will definitely be more audible. I'd say go for the overlap.

Most crossovers don't look much like their ideological prototypes once they've been optimised, by the way, since drivers tend to roll off not far from their crossover points; that's why they're being crossed over in the first place!


Cheers,
Francois.

[Swedish Chef]

Since the original question has already been answered (i.e. go for flattest response) I would like to discuss the IM issue somewhat.

Bill,
I can see potentially lots of IM being generated with a non-insulated mid driver in a sub enclosure. But why would two drivers operated in the same frequency range give rise to IM distortion? Assuming air as a linear medium (valid for these low SPL:s or not?) and the two drivers radiating in free space, well there is going to be spatial comb-filtering effects but why IM? And if so, why would there not be IM if they were operated at different frequency ranges?

/Magnus

[mr_push_pull]

Originally posted by Puggie
been playing with designs in Bass Box Pro and Xover Pro (on a side what are peoples opinions of this software?)

BassBox is nice, I haven't played with XOverPro but I've heard that it can't import FRD and ZMA data and that sucks IMO. My recommendation is that you should go with XOverSim instead, it can do that. And it's free.

[BillFitzmaurice]

why would two drivers operated in the same frequency range give rise to IM distortion?

The problem is that the two drivers aren't identical, so while they're both reproducing the same frequency range in the overlap zone they aren't doing so in precisely the same fashion. They're also receiving their respective inputs from two different sources, one being the HP filter output, the other being the LP filter output, and those aren't going to be identical either. You end up with a mix from two sources that are slightly off from each other in terms of phase and response, and what you hear is the intermodulated combination of the two.

[Swedish Chef]

Bill,

are we talking the same type of IM (InterModulation) distortion here?
In my world IM distortion can only be the result of a non-linear system. Well, you know the classic two-tone test for an amplifier for example were say 10 kHz + 1 kHz tones will generate sidebands at 9 kHz and 11 kHz.

So unless the two drivers are affecting each other (like the mid placed in the sub) linear effects like differencies in phase and amplitude is not going to produce IM products under the assumption that air is a linear medium.

If this indeed is the type of distortion you are talking about, could you point me to a reference?

/Magnus