Rational Acoustics

Kip Conner
December 10th, 2008, 06:18 PM
I did some measurements yesterday on a 4 way system and had some issues that you might help me overcome. The speakers if anyone really cares was a OAP CPX1250 3way trap box over an Apogee AE-12 (Dual 18 ported). I went with Mike's XO points that were spec'd for the OAP box- 80->250 / 250->1.25 / 1.25->Out.

1. first off I had trouble signal aligning the mids to the highs in terms of phase. Is it ok that that the traces don't perfectly overlap at the XO point (they do touch at the XO point) I know that the reason that the traces didn't exactly overlap was due to some EQ points that were out of the pass band. The 260 is limited to 4 para-metrics per range and the I needed to use the para-metrics to smooth out the horns. I don't have any of the other fancy filter sets available to me with the 260. I thought that I had done all I could with what I had at the time (knowing full well that I would ask an expert)

2. The system as a whole lags up until 1.25K and then does a flip producing a smiley face in the upper mid band. The XO point is 1.25k. Is this normal or do the HF need delay so that they will join the party- you know those HF drivers always on time and the subs are always late for their own funeral. :)

Arthur Skudra
December 11th, 2008, 09:45 AM
Apart from seeing some screenshots, I think you're on the right track with the limited resources you have in the 260. When you flip the polarity of the HF, do you get a deep cancellation in the magnitude trace at the crossover point? This technique of temporarily flipping the polarity of one of the drivers and seeing a deep magnitude cancellation at the point where the two intersect is a good litmus test on whether you've done a good job aligning the crossover.

With regards to the phase curve taking a turn upwards in the HF, adjust the Smaart delay in small increments to see it flatten out (click on the "down" delay spinner). If there is still excessive slope upwards in the HF, you can correct this condition using all pass filters, however the 260 doesn't offer this as an option.

December 11th, 2008, 12:24 PM
The phase response you describe in part - "The system as a whole lags up until 1.25K and then does a flip producing a smiley face in the upper mid band." - describes a normal phase response for a multi-way system. The X-over filters used to divide up the drive bands basically create that LF-lagging response.

As for part 1, the not being able to get the x-over energy perfectly timed, this may be a direct result of using an a-symetrical x-over - i.e. having different order/slope filters for your HPF and LPF for your crossover. If this is the case, then you just need to make the best comprimise possible with timeing to get the phase relationship through x-over.

If they do truly have the same order x-over filter (remember that passive filters in the speaker may also be in play here - adding orders and phase slope) - then you should be able to get the phase slope overlaying through crossover by adjusting delay (and possibly polarity).


Harry Brill Jr.
December 12th, 2008, 01:00 AM
I'm not sure the 260 will allow this but you could also use different slopes on the high or low pass to get the slopes to match more closely through the acoustic crossover.

Kip, great threads, keep em coming.

Kip Conner
December 13th, 2008, 12:32 PM
The 260 will allow this and I will give it a shot. I popped in on this 4 way system last night for it's maiden voyage. I wanted to give it a little more love since I aligned it outside in a some-what free field. This was being used in a ballroom situation. Needless to say the mix engineer was floored with the response, not just acoustically speaking- but the responsiveness of his console.

There was on other thing, but I'll drop it in a new thread.