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In Reply to: RE: "a cancellation effect of this resonance as you increase the length of the driver" ... posted by Satie on June 05, 2016 at 14:36:48
Can you post your measurement of a single (or two) driver operating in the array, and then a measurement with all 6 drivers operating?
(Same microphone position obviously and equivalent electrical drive voltages to the transducers.)Dave.
Edits: 06/05/16Follow Ups:
I don't have the old measurements anymore. But I posted other's and some of my own before. The old laptop they were on died suddenly some 5 years ago I didn't back up since that was the backup pc for work.
I have this left from 1/3 octave warbles from about 8ft. It should be on axis from the equidistant arc setup. The leftover peak is smaller at the listening seat @11ft by about 2 db. I will look for those. There should also be one channel sweeps somewhere.I can look through the spreadsheets to see if anything else interesting is left. The old REW sweeps I don't have any longer.
Edits: 06/06/16
The area of interest has very low resolution on that plot, and the primary peak (around 8khz) is not the driver cavity resonance.
Regardless, I still think I would apply the appropriate notch filter to all the drivers. The cavity resonance is inherent in the transducer construction and it can't be "averaged" away by acoustic "coupling." If you move your microphone closer you will start to see it. I know they allude to this coupling "alleviation" characteristic in the driver datasheet, but I'm not buying that's the most preferable implementation.
Anyways, extending on your hypothesis......if you were to apply the 12khz notch filter to the array configuration, you would create a dip/anomaly in the free-field response, yes/no? I don't believe that to be the case. I think the measured acoustic response would look even better/flatter. And if you moved your microphone close it would look even better/flatter too. Win/win.
Cheers,
Dave.
I posted a Neo8pdr 9 segment line array from a DIY forum that was done with better equipment and there is nothing remaining of the cavity resonance. It should still be on the asylum. The forum is defunct so I can't post the notes from the projects Yes, the use of a notch filter produces a dip in the line array. The 12khz resonance is entirely gone but the residual portion at the onset of the peak requires more distance to dampen.
The resonance peak does get cancelled at a distance, most of it is gone by 1 m. I already knew to expect it from other's line array projects so never used a notch filter. As you can see in the FR plot the combined drivers are not showing anything. The plot I posted is the LR4 symmetrical at 5khz. I got very similar results with 1st order at 6-8 khz though a tiny bit of the 8khz peak is visible in the combined speaker I will see if I can find a trace, but the 1st order work was done 5 years ago. The LR4 and B3 crossovers were from nearly 10 years ago. So I can redo measurements to show you how it works but it very obviously does.
The FR at the listening seat is not showing any need for a notch filter. You should not view the resonance as an inherent part of the driver's FR but as a noise with its particular behavior. At normal listening distances of 2-3 m there is no need to pay any attention to it. Definitely no notch filter.
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