Posted by Wayne Parham [ 70.234.129.7 ] on February 09, 2009 at 14:03:22:
In Reply to: Art Array Improved Crossover posted by FredT on February 09, 2009 at 11:23:38:
Thanks for the link, Fred.
A few thoughts:
The measurements that were made weren't close mic'ed, they were done at two meters, if I recall. I would have preferred a little more distance, but I was working in an area where I couldn't really throw the power at them so I wanted to be sure and be above the noise floor. I chose a power/distance that seemed to be the best compromise.
Also, perhaps more importantly, the measurements weren't done with the speakers on their sides. They were measured outdoors, standing upright with the microphone on axis with the tweeter. See the thread about ART Array measurements.
I think you might be thinking about the fact that I did lay them on their sides and measure them to get a comparison, to see the effects of ground reflecton mitigation from the array. I never published that dataset, but I am certain I commented about it, probably privately in an E-mail. I often comment on how vertical arrays smooth floor bounce, much in the same way that multi-subs smooth room modes.
The Vifa tweeter does benefit from a damping resistor, but I don't see a huge peak in acoustic output near its resonance. It works very well with a simple first-order filter and small amount of resistor damping. Of course, reducing excursion in turn reduces IMD. Then again, there are advantages offered by first-order filters too.
The woofer array has rising response, probably the result of collapsing directivity, ultimately resulting in a hump around 700Hz, just below crossover rolloff. I think we discussed this, years ago, back when we first looked at the measurements. You'll remember I preferred the version of the crossover with a first-order LP filter for the woofer array. Amplitude response was good but it did have a slight rise around 700Hz.
I remember thinking it might be good to attenuate that hump with compensation in the woofer circuit. The trouble is, the hump is just below crossover so if you do it by overdamping the low-pass filter or simply shifting the crossover frequency down, then the Vifa tweeter would have to be used at lower frequency too. I don't think that's a good idea. You could use a notch filter, but that has never been an approach I particularly liked doing. In the end, I personally thought it was best to leave it simple. It sounds good.
Don't forget that unsmoothed LMS measurements are brutal. Lots of people are not used to seeing them. Most speakers measure far worse than your arrays did. Reducing the 700Hz hump would be possibly worth doing, but the rest of the curve is pretty good. Other than possibly smoothing the 700Hz hump, I would not expect to improve amplitude response by any significant amount with a crossover change.
One thing is for certain. Measurements will tell you if a crossover change is an improvement or if it is just different. I think it is worthwhile to try the notch filter to see if it helps, but I would not feel confident calling it an improvement until I had measured it to see. I would also want to do it under the same conditions, because just 1/3rd octave smoothing would make the response curve of your speakers look very smooth. So be careful to compare apples to apples as you proceed. Start with a new baseline for comparison on whatever measurement platform you choose. Measure the stock crossovers, then compare proposed changes with the same measurements system, environment and settings.
Good luck and keep us posted!
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