Welcome! Need support, you got it. Or share your ideas and experiences.
Return to Planar Speaker Asylum
71.67.188.239
In Reply to: RE: "a cancellation effect of this resonance as you increase the length of the driver" ... posted by Davey on June 04, 2016 at 09:26:23
Can;t separate it out of course, What I did do was leave the mic at the midpoint height at the listening seat and block the bottom half with a board at a couple of distances on the mic speaker line and the closer one ~3ft did not have as much of a reduction in the cavity resonance, but the 6-7 ft distance did not show any different result than the listening position without the board. Just shows crudely that the cancellation effect takes place in proximity to the drivers within a distance the length of the driver line rather than further out.
I did close mic FR sweeps on the 6 driver line but don't recall the results exactly. I think it showed some reduction from the peak on the single driver FR but it was no different than measurements at a distance on a 2 driver line so would have been a 3-4db reduction in the peak.. I do remember setting the distance at 8" to correspond to the driver height.
Follow Ups:
So, you are saying the cavity resonance does not exhibit a minimum-phase characteristic and each driver is different than its neighbor?Dave.
Edits: 06/05/16
The cavity resonance is a trapped air resonance so has to build over time so would have random phase relative to the signal and a time delay of at least one cycle. It is exactly a random noise with amplitude related to freq, not a freq response variation due to any other cause like a driver's breakup and self absorption modes related to non pistonic behavior. The diaphragm itself is not producing the resonance. It is more like a cabinet resonance.So should not be thought of as a part of the freq response of the driver but a noise with its own separate behavior.
What is different among drivers above and below the midline listening and measurement axis is opposing directions, nothing more.
When I started trying out the Neo8 drivers my plan was to use a notch filter if my line did not squelch the resonance as some others had shown in their Neo8 line array measurements. But as I increased the number of drivers in he array the resonance peak progressively declined in magnitude and was pretty much vestigial with 6 drivers per side.
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: 06/05/16
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.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: