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Setting the proper splay angle

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Rob Spence:
My A rig has a pair of QRX 212/75 on each side.
I am experimenting with various splay angles for the best sound.

The other day I tried doing it by ear by walking past them while playing pink noise. I could certainly hear the combing and as I spread them out it seemed to improve.

Today I tried the same thing except using the Spectrograph display in Smaart and walking the measurement mic back and forth.

Before someone asks, I was only running one side of the PA and the subs were off.  The tops were up 4ft so the horns were up at 7ft or so. The tests were done outside. See the thread on "getting the tops up" for what the configuration looked like.

I walked back and forth at about 6ft in front of the stack, looked at the display then spread them a bit more. There was a point where the backs were touching and the fronts were about 3" apart where the fingers of the comb were pretty fat and the cancellations between them were thinnest but even going wider I didn't see it go away.

So, any advice on my procedure? Should I be farther away?
Should I expect the fingers to merge?

Folks that were at CRAS in June have any observations?

Thanks.

John Masters:
There is only so much you can do when splaying cabs, and you appear to have found that point.  FWIW a gap of 3-4" seems about right with my trap tops.  The fingers will never merge at all locations, unless the drivers are a 'point source' which is of course impossible.

Stacking the second top box (horn to horn) on top of the first top box will get rid of more comb filtering in the horizontal plane, but would give you a 10ft stack with less horizontal coverage, but you could still swivel the top box in or out a bit.

John.

Chase Reynolds:
Hey Rob

I'm no expert but it seems like it would be good to think about the fact that as you spread the speakers apart from each other, the area where they are interfering is moving away from the speakers.



(sorry for the crude image)

So my thinking is that unless the speakers are arrayed really far apart (almost back to back)they are always going to interfere somewhere at some level.

It might be good enough to just try and make sure the speakers aren't interfering in your audience area.  Lets say you have an outside gig in a small field. If the interference is out beyond the audience area then who cares?

This thinking may be waaaaaaayyyyy off but I know someone more qualified will join in and help us both out with this.

Rob Timmerman:
I've just set splay angles by ear, but the method that Dr. Don taught in his classes on system optimization is as follows:

Place your microphone a little ways away from the speakers (about 6', you want it far enough away to get the entire box, but close enough to get a good SNR from the speaker), on the centerline of the 2-box cluster.  Now, turn on one box only, and point it directly at the microphone.  Rotate the loudspeaker until the level at the microphone is 6dB down from on-axis.  Save both amplitude and phase plots.  Now, turn off the speaker that is on, and turn on the other box.  Rotate it until the amplitude and phase plots match the saved values for the first speaker.  Congratulations!  You should now have coherent summation between the 2 boxes.

You should, of course, double check this by ear once you're done.

Kevin Maxwell AKA TheMAXX:
Rob Spence wrote on Sun, 02 September 2007 23:20
My A rig has a pair of QRX 212/75 on each side.
I am experimenting with various splay angles for the best sound.

The other day I tried doing it by ear by walking past them while playing pink noise. I could certainly hear the combing and as I spread them out it seemed to improve.

Today I tried the same thing except using the Spectrograph display in Smaart and walking the measurement mic back and forth.

Before someone asks, I was only running one side of the PA and the subs were off.  The tops were up 4ft so the horns were up at 7ft or so. The tests were done outside. See the thread on "getting the tops up" for what the configuration looked like.

I walked back and forth at about 6ft in front of the stack, looked at the display then spread them a bit more. There was a point where the backs were touching and the fronts were about 3" apart where the fingers of the comb were pretty fat and the cancellations between them were thinnest but even going wider I didn't see it go away.

So, any advice on my procedure? Should I be farther away?
Should I expect the fingers to merge?

Folks that were at CRAS in June have any observations?

Thanks.



I have found that with those boxes with the backs touching, the fronts inside corners need to be more like 9 – 10 inches apart to sound best to me. It seems to be farther apart then it should be but that’s what seems to work for me. I haven’t had time to do extensive testing on the spread of these boxes yet, just listening and moving them.

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