the three way nude swinging dipole thread

[twest820]

Because, after all, it would be a shame to let one of the best unintended double entendres in audio go to waste. I was tempted to title this the ménage à trois thread, but that seemed a bit over the top.

I know of two and a half way swinging dipoles (mige0), a few minimally baffled three way dipoles (notably cuibono and keyser's builds), and StigErik's four way technically has got a three way swinging dipole embedded within but doesn't really count. I don't know of any three way swingers and so a new thread on them seems in order. All build threads broaden into a general discussion of various speaker designs sooner or later so I figured I'd just scope the thread to the category and kick it off with my current build.

The build in question is a BG Radia Neo3, BG Radia Neo10, Selenium 18SWS1100 crossed linear phase LR6 at 200Hz and 1.7kHz. Crossover and equalization are via a Focusrite Saffire 40 and Thuneau Allocator on my Dell Inspiron 1505 Core Solo laptop running Windows XP. The first couple figures are a shot of the right speaker to give you a sense of the build and how the swinging is done along with a detail of the lines on the left speaker. The next two are front and rear wave symmetry for SPL and phase, followed by front and rear directivity. All four measurement sets are in room, ungated, and 1/16 octave smoothed. The smoothing's mainly so it's easier to tell which curve is which and the chuckhole around 150Hz is the usual first reflection from floor, ceiling, and walls (mostly ceiling as I'm renting and hence haven't mounted a diffuser). The next three figures are SPL and THD of the individual drivers when equalized flat for nude dipole operation within their passbands. SPL on top, THD below, and the crossovers turned off. These are in room, ungated, and unsmoothed. SPL for the green 0dB curves is 5 to 10dB above peak SPL at my usual listening levels. I don't have an SPL meter but the measurements are around 80dB or so, give or take a few dB. The red +10dB curves are peak SPL at about the loudest level I ever listen at---it's somewhat uncomfortable to listen while the measurements are being taken---and the blue +20dB curves are loud enough I give up, put in my 35dB chainsaw earplugs, and wonder why I didn't put the earplugs in already for the +10dB curves. Granted, the +20dB setting's quieter than my chainsaw. But not by a whole lot.

DIYAudio only allows 10 attachments per post, so see the next post for the crossover, equalization, and a couple other interesting things. In the meantime, some discussion of the speakers seems in order.

In short, I like them. Quite a bit. Best setup I've heard yet. But then I'm 1) obviously biased since these things are sitting in my living room making dents in the carpet, 2) unable to afford a plane ticket to Norway to listen to StigErik's rig since I blew all my money on drivers, and 3) too polite to show up on cuibono's doorstep and invite myself in. There's also the minor logistical difficulty I don't actually know where cuibono's doorstep is, but that's beside the point. I believe the measurements mostly speak for themselves, but a few remarks seem in order. One is the front to rear SPL asymmetry above 10kHz due to the Neo3 shifting somewhat off vertical when I was turning the speaker around to measure the rear wave and I'm too lazy to redo the measurements. The asymmetry below 1kHz is due to slightly different mic and speaker placement and room interaction along with, possibly, some shadowing by the 18SWS1100 magnet at low frequencies where the diffraction radius is large. Another remark is the 0dB THD curves are higher than the +10dB THD curves because they're sitting at the ambient noise floor of my relatively quiet neighborhood on a quiet night---the floor would be higher if I'd left the fridge on. THD doesn't actually drop in the +10dB curves; it's that turning up the volume raises the SnR. The exception to this is the Neo10, where the +10dB curve becomes excursion limited below 400Hz. At +20dB all drivers are showing rising distortion from excursion. But they're also pushing 100dB SPL each and if all six of them that ran hard the system would be around 105dB. Respectable performance for not having a baffle.

The Neo10s make this system. I originally started out designing a four way with five drivers---the fifth being a second sub---which eventually shrunk to four drivers as my preference for keeping speakers vertically compact asserted itself and I realized a single big driver was shorter than two smaller ones. I eventually chose the 18SWS1100s for their good bang for the buck, generous xmax, and low (for an 18) Mms and Rms. Neo3s were a given for the tweeter as AMTs are expensive, Zaph's shown the Neo3 handily beats most ribbons, and Rudolf's amply demonstrated getting good directivity out of back to back dome tweeters is hard. But no matter how I looked at it I could not find a mid for a three way which didn't involve more sacrifices than I was willing to accept. Until BG Radia finally made Neo10s available for DIY and Meniscus got them back in stock. Greater vertical dispersion would benefit the Neo10s the Neo3s cross low enough to mitigate the problem. Otherwise the Neo10s are to die for. Excellent front to rear wave symmetry, low distortion, detail as good as you'd expect from their low Mms, and no cone breakup. Spendy. Though cheaper and more compact than two mids and the extra pair of power amp channels needed by an actively equalized four way.

I'm also pleased with the hanging. The mechanical benefits of swinging drivers are covered at length in StigErik's Open baffle with Beyma TPL150 build thread (more accurately titled the no baffle without TPL150) and mige0's Beautiful Swinging Speaker thread. I've little to add here but agreement and it doesn't hurt swinging saved me a bunch of MDF dust and spending two hundred bucks on a router. Good deal. mige0 used a cable and bolt system for hanging and StigErik progressed from bolts and chain to bolts and rubber straps. I'm not a fan of either approach, but my previous solution of picture wire was subject to creep and I had to keep readjusting the drivers. So I picked up 2mm, 3mm, and 6mm climber's auxiliary cord and set to with bolins and rolling hitches. Way better. The frames are obviously prototypes and I've not trimmed the lines yet but the reason I included a detail of the hanging is so you can see how the Neo3 is supported with a pair of lines, one for the driver itself and one to support the cable so that it doesn't pull the Neo3 around. The Neo10 is light enough to be subject to same problem and the combination of using parallel lines to get a pantograph type of motion in the swing works well for it too.

 

[CLS]

Where're the attachments? I mean the photots.

 

[twest820]

The next batch of figures is for the crossover and equalization. The first is the overall VST setup in Bidule followed by a screenshot from Allocator which shows the equalization for all three drivers, the crossovers, and the arbitration to get the crossovers to linear phase. As has been my usual experience, using warp phased equalization for the minor driver wobbles produces flatter phase than using linear phase equalization. However (as has been discussed over in the Arbitrator forum at Thuneau) dipole rolloff does not induce a phase shift and wants linear phase phase equalization. Additionally, drivers under a phase shift around Fs which wants correction. My current solution is to use a peak/dip Q = 0.5 biquad centered at Fs and then correct for both the biquad and Fs phase shifts with a fourth order Arbitrator dedicated to that particular driver. Hence the additional three Arbitrator VSTs for the Neo3s, Neo10s, and 18SWS1100s. They operate at 575, 150, and 39Hz, respectively, though as the yellow line in the Allocator screen shot shows I've used a soft dipole equalization on the Neo3s to avoid excessive excursion. The difference in levels between low, mid, and high is minimal as the 18SWS1100 and Neo10 are a good bit more efficient than the Neo3 in dipole operation. However, there's still a goodly amount going on to get the output SPL flat. I've also EQed the room interaction at 150Hz so it measures flat but so far it's sounding better to me without EQ. That's about what I'd expect as the 18SWS1100 is pretty flat between 100 and 200Hz and reflections within the first 3ms are minimal.

Preringing is an issue which arises with all linear phase methods---time reversed IIR (what Allocator uses), FFT, and FIR---so I've included the impulse response as well. I'm at a loss as to why HOLMImpulse shows more preringing 60 degrees off axis, but I find I can't hear the preringing shown here. Seems unsurprsing to me as it's 30+dB below the causal impulse response and short enough to fall well within the 2-3ms window within which events are percieved as simultaneous.

I hope others find this data useful. While there was pain in selecting a midrange most of the unknowns were in how much sub was needed to get reasonable SPL at reasonable THD and hence some guesswork was involved in selecting the 18SWS1100s. Personally I tend to listen somewhat softly so I might be able to get away with Selenium 15PW5s or 18WS600s but I wouldn't want much less driver than the 18SWS1100s for nude bass. At the risk of stating the obvious, a baffle to pull the dipole peak on the low channel down to 200 or 250Hz would be more efficient and the resulting decrease in excursion would reduce distortion. However, I was more interested in what it'd take to get nude bass on a scale more modest than StigErik's 117dB at 16Hz and the 18SWS1100 seems sufficient for most purposes. I've personally chosen to EQ them flat to 35Hz for no other reasons than 1) it's low enough for me, 2) happens to be what falls out naturally from the biquad at Fs method described above, and 3) I've not hit problems with hearing harmonics instead of fundamentals despite the boost low frequency distortion gets from dipole EQ. With digital it's trivial to extend the equalization lower in frequency and so my last figure is of the 18SWS1100 running nude and flat down to 20Hz. Curves from top to bottom are SPL, phase, and THD. Distortion rises significantly so I suggest multitone IMD testing to see how badly deep bass harmonics bleed upwards but, depending on your criteria, it may be feasible to extend nude operation below 35Hz.

Full disclosure: Thuneau supported my investigations into this VST topology with a donation of Allocator Lite and Arbitrator so I've chosen to combine the full Allocator instance I purchased with the donated Arbitrator instances for clarity and as a roundabout way of saying thank you. One could accomplish the same thing with additional full Arbitrator instances or by combining Arbitrator with VSTs like Cockos' ReaEQ (possibly better known as Reaper ReaEQ).

 

[SunRa]

Excellent work. Truly nice stuff.

Congratulations!

 

[CLS]

Hey I like the tree

Well done, indeed. Congratulations.

Say, did you try bringing the woofer lower, to the floor? That can provide some bass support and should be very little negative effect on L-M xover as you cross them under 200.

And, if possible, add another woofer or two filling the very bottom would be significantly nicer. 20-some and 30-some are very different and you'll like the former once you have it, believe me.

I've tried on my own system with/without center sub. I EQ'ed both conditions to the same measured response as possible for the comparison, and I like 3-channel bass a lot more. I didn't push them hard on that test, so 2-ch condition was far under the limit of the woofer, they were not stressed at all. Still, 3-ch condition sounds far more effortlessly and fills the room more evenly and smoothly. I'm not sure the improvement is from the double cone area or the distribution, or both. Anyway it's worth trying. Please make it your next project, or at least bring it up in your to-do list.

 

[StigErik]

Very nice work. Well executed and documented! And I love to see that you actually have an Oscilloscope...

Very interesting thoughts you have there around EQ, phase and impulse response, and combining warped phase and linear phase EQ. I will try swithing to warped phase EQ myself for some of the EQ points in my own setup.

 

[StigErik]

I agree with CLS that bringing the woofer closer to the floor is a good idea. It might cure the suckout you have around 150 Hz as well. I would also try a slightly higher XO between the woofer and Neo10 (like 250 or 300 Hz), to lower the strain on the Neo10 a little bit.

 

[Helmuth]

Nice drivers choices twest820.

The response has 10dB non linearity are your trying to improve this or are you finished for the time being.

Keep on the good work!

Regards Helmuth

 

[studiotech]

I too would try the woofer to mid xover a little higher. I'm using 2 per side and still prefer over 300Hz, but maybe my max SPL requirements are higher than yours.

How much Eq are you needing to bring up the low end of the Neo to meet the woofer flat? With 2 working together on the low end of mine and having some baffle, I'm not adding ANY boost at all and getting great results.

see here:

Raal 140, Neo10, AE Dipole 15 in the works

Greg

 

[twest820]

Thanks for the kind words and praise; glad that small book I composed last night is of some value.

We get into some of the true beauty of DIY and swinging speakers here. Bass on the floor just takes letting out a bit of line and lowering the subs. I've tried it and also done a few experiments with tippping the subs back so the front wave is more direct to the listening position. Got extension down to 30Hz with the sub magnets sitting on the floor but couldn't find a radiation pattern I cared for. That's typical of my hearing; it rolls off around 40Hz but is quite particular about consistent directivity. There's no way I'd go back to two way but my previous build had a compact presentation I quite enjoyed which the three way can't match. I've listened to the 18SWS1100s EQed flat down to 20Hz but can't really hear any difference. Not too surprising as when I'm playing stand up bass I tend to keep track of the E1 by resting the instrument against my knee and listening more via bone conduction than with my ears. On some tracks with the 20Hz EQ I think I hear deep bass harmonics clouding the higher frequencies but I'd have to do IMD testing to tell. StigErik would probably know exactly what's up with a quick listen.

I considered a floor woofer arrangement much like what CLS has suggested but rejected it as a three way on directivity grounds. As a three and a half way or a four way it seems like a good option. One of the cool things about the Neo10s is they enable a dipole four way to use two subs rather than two mids. I think an approach with something like Selenium 12PW5s to 75Hz and the second sub channel below that could be quite nice---my 18SWS1100s are 13 inches above the floor so one could insert a 12 and lower the 18s to floor level without much fuss. However, experimentation's shown I personally wouldn't be satisfied with the directivity of such a setup. Unless I can find a way around that my future speaker projects lie in the direction of building smaller three ways which still have good performance to the mid 30s.

I've tried crosses up to 500Hz between the subs and the Neo10 but prefer crossing at 200Hz. I rarely listen loud enough for it to be excursion limited at 200 and the lower the Neo10's run the more compact the speaker sounds. The drivers are reasonably broken in but I found that over several months the low end THD rise on the Neo3 softened up. I'm hoping the same will happen with the Neo10s so I can lower the cross to 175 or 150Hz---the LR6 slopes I'm using are helpful here. Those who aren't so fussy about directivity and want to listen louder or use flatter slopes should get fine results raising the cross to 300 or 400Hz. I'd also point out digital makes configuration changes trivial, so if one's feeling like listening loud it only takes a few seconds to load a different config with the crossover points moved up.

How much Eq are you needing to bring up the low end of the Neo to meet the woofer flat?

Someplace between 3dB and 10dB or so depending on choice of reference point. The Allocator screenshot in my second post is probably your best answer, though as a bit of context I find the Neo10 pretty efficient in dipole operation. BG Radia may rate it at 92dB SPL 2.83V/m but I'm running mine 2dB lower than the 96dB efficient Selenium 8W4Ps I was previously using as mids.

The response has 10dB non linearity. Are you trying to improve this or are you finished for the time being?

Oh, this gets complicated. I assume you're referring to the 150Hz chuckhole which, as I remarked above, is mainly ceiling bounce that's not attenuated because I'm renting and hence haven't mounted a diffuser. As you can see from the close mic data in this post there's no raggeness in the wavefront coming off 18SWS1100s themselves, so the direct radiation from the drivers is just fine (curves from top to bottom are SPL, phase, and distortion as usual---EQ for flat far field is in place; SPL slopes due to near field bass boost). One school of thought---probably mostly people with hardwood floors and significant early reflection which is merged with the direct sound perceptually---favors EQ so the mic says flat. In my case carpet minimizes the floor bounce so I'm left with mostly late reflections that are heard separately from the direct sound and mostly ignored perceptually. So there's an alternate line of argument that even though the mic shows a dip there's no need for EQ because the listener perceptions are dominated by direct sound.

I've tried both approaches and found the latter sounds best to me (mentioned this in passing above). I'm not entirely convinced I prefer zero EQ but accurate discrimination becomes difficult for me when listening to relatively narrowband adjustments of one or two dB. I'm high confidence I find 0dB EQ preferable to the 6-10dB suggested by EQ to the mic (the amount needed depends on where in the room you measure). But what about 3dB? Not sure yet.

I will try switching to warped phase EQ myself for some of the EQ points in my own setup.

I look forward to your findings. We're pursuing rather different things with similar approaches and the resulting differences have always proved informative so far.

Hey I like the tree

The ficus is indeed popular. It's a rescue tree but it's been doing well in that corner. I've a somewhat orphaned dracaena deremesis in another corner that's recovered nicely. They both make good corner dampers, too.

I love to see that you actually have an Oscilloscope...

It's quite handy for seeing how much power you actually use while listening and designing power amps accordingly. Plus you learn a few things about music if you sit down and watch some songs instead of just listening to them. I have to confess the scope's not actually mine though, just borrowed from tomchr.

 

[CLS]

Under 30-some, it's indeed not so audible, but feel-able. I do feel a wall of air that is vibrating and hitting me. Usually it's not much, but it's there. Once proven by the RTA display, it's even harder to ignore.

Except for pipe organs, more often are probably those close-mic stuff that LF "noises" are picked up, like the resonance by initial touch or hammering on the string, pedal work of piano, tapping on the stage floor... sort of things. While very subtle, they have that initial LF impact on each note and add a tad of atmosphere of 'real things'...

Oh well, that might be the residual toxication of audiophile-ness in my blood to adore that low bass. My bad.

But, I appreciate it very much more by the clean dipole bass (rather than those boxed ones), which is said to be unable to pressurize the room.

 

[twest820]

I doubt adoring low bass is any different from my fussiness over directivity. One of the nice things about DIY is you're free to slant the engineering towards what's important to you. With a typical speaker a lot of what you feel is vibration coupled through the floor. With a swinger that mostly doesn't happen, though I get some pickup when playing loud and the music hits the resonance frequency of the joists (around 200Hz). I don't personally play loud enough to feel the air, though I've noticed the effect with a friend's sub when barefoot with my feet fairly close to the driver. Also, I live in a desert at 1200m around 45N so I'm usually wearing more clothes than one would in Taiwan. Tends to reduce the effect.

 

[StigErik]

So you do get some energy transfer between the drivers and the frame? That's what I found with my own speakers, and its the reason I went for rubber straps - to avoid that energy transfer.

I agree with your choice of directivity vs low bass, directivity is far more important, and low bass is really difficult from dipoles.

With a nude 18" just dont try to push it below 35 Hz, the driver simply cant do it, all you'll hear is higher order distortion. I did run my monster Beyma 21" nude, and they could not reproduce cleanly below 30-35 Hz either, no matter if the driver had enough excursion to do it. Because of the massive dipole loss, the harmonic distortion products will be much louder than the fundamental, and your ear's low sensitivity at low frequencies will add to that.

I did end up with four giant H-baffles (aka "refridgerators"), and still I really cant get down to 20 Hz.....

 

[CLS]

StigErik, you listen VERY loud, don't you?

My 4 18" give me 20Hz, loud enough for me, and I don't detect any obvious distortion when they do that low.

In a few occasions, movie stuff, explosive LF makes some rattling. But those are objects in room, like lightings on ceiling, window panes, dishes in the closet, stuff on the shelves... etc. The cones are moving hard, yes, probably exceed 6mm of Xmax by naked eye occasionally. Some wind (or maybe mechanical?) noise is audible if I come near the cone, but not at the normal listening position.

In normal listening level (which is more or less around conversation level), if I cut the LF below 35Hz by EQ, I can immediately tell the difference. (Of course in those recordings containing the bottom octave, many are not.) The sounds with or without bottom end are just giving me different feels.

OK, that's me.

 

[StigErik]

No I dont listen very loud.

Yes - I have plenty of SPL at 20 Hz, and yes - the whole house rattles. But can I hear just the 20 Hz tone without distortion? No... sadly not.

A nice (or maybe not) test of your woofers is to play a 20 Hz sinewave, then a 40 Hz and a 60 Hz sinewave, and then tell me if you also heard the 40 and 60 Hz when you should hear only 20 Hz.....

 

[CLS]

Good point, will try.

 

[Rudolf]

I am at a loss with the directivity issue you are talking about. What is it that the 18" woofer does not do at 300 Hz while the Neo 10 does? Are you talking about beaming of the large woofer?

Rudolf

 

[thadman]

twest820,

Have you considered placing a wedge of 30ppi reticulated foam on the front and rear of the Neo10?

 

[mige0]

Sweet ! - you made my day !
Clever idea with the cables too!

Seems it sound to your satisfaction. How is the big Selenium performing ? I try to get it for weeks now from Selenium Europe but did not succeed so far.


Michael

 

[twest820]

So you do get some energy transfer between the drivers and the frame?

I don't have an accelerometer but I can't feel any vibration in the frame. The resonant frequencies of the pendulums are about 3, 2, and 0.5Hz for the Neo3s, Neo10s, and 18SWS1100s respectively. The nylon aux cord's more flexible than the straps you're using. So the drivers should be well decoupled.

Other than coming in pretty colors there's nothing special about aux cord; pretty much any rope, string, or twine would work just as well.

I did run my monster Beyma 21" nude, and they could not reproduce cleanly below 30-35 Hz either, no matter if the driver had enough excursion to do it.

Yeah, I considered various 18s and 21s including the 21W1600Nds when selecting the subs. And, of the datasheets where distortion's provided, none of the drivers are much below -20dB THD at 20Hz. Without a good source it doesn't really matter what you throw at it; clean bass needs a driver that's at least -40dB THD at 20Hz, preferably closer to -60. I can't see any obvious reason why 20Hz would distort more than, say, 40Hz for a given excursion but the drivers all seem to do that. Makes me wonder if there's something about operating drivers below Fs which increases THD but I'm not aware of any good distortion measurements for the various low Fs drivers I know like the Dayton PA460 and RSS390HF.

Greg, did you do distortion sweeps on your Dipole 15s? I only see SPL data in your threads on them over at HTGuide.

How is the big Selenium performing ?

Hey, Michael! Thanks for the kind words; glad I could make your day. Yes, the sound's to my satisfaction and then some. With crossing off your horn at 500Hz there's not much need for Neo10s, but they easily beat the front to back asymmetry and cone breakup challenges I was hitting with the 8W4P. If I were doing my two way over again I'd probably choose the 8PW3 as well for the slightly longer xmax, slightly higher breakup, moderately lower cost, lower Fs, and smaller magnet. But about the only thing I can imagine that would take me away from the Neo10s in a main system optimized for fidelity is the introduction of a better driver.

The 8W4P is the largest cone driver I've worked with aside from the 18SWS1100s so I don't have much to compare the 18s to. They measure out well good and, going by datasheet specs, I don't know of any 18s with a better price to performance ratio. I would like to A/B them with 15PW5s and 18WS600s to see if I notice the lower Rms and Mms of those drivers. If you're sticking with low SPL or rolled of bass I would think both those would be good candidates as well. Didn't you have some 15PW5s? How'd those turn out?

I am at a loss with the directivity issue you are talking about.

I'm not exactly sure I can explain it either. But I'll try. You have a good point; now that I think about it vertical time alignment is probably a better description. So, for the moment, quit thinking about frequency domain and radiation patterns and switch to time domain and waves bouncing about. Let me be clear I'm fussing about details and by no means is this a major effect. It's arguably as much a room problem as it is a speaker problem and may just be an artifact of my only having one good ear. As I was typing this up it occurred to me folks with binaural hearing might well describe what I'm hearing simply as less precise imaging.

My build's about as vertically compact as I can get it, but the radiating surface is still 75cm tall. So wavefronts from the top of the Neo3 and the bottom of the 18SWS1100 arrive 2.2ms apart. That's above the 2ms or so threshold where events are heard as separate instead of simultaneous. In a dipole there's little SPL at 90 degrees off axis so this doesn't matter much, but if you sit down and do the math at, say, 60 degrees off axis the difference between when the start of the first reflection off the ceiling in my room arrives at the standard golden triangle listening position and when the end of the reflection arrives is around 3ms. So, while the direct radiation from the speaker and reflections near the horizontal plane are about as time aligned as you can get, the reflected radiation's not. Plus I tend to listen off to one side, exacerbating the problem. The arrival spread from the speaker I'm closest to increases and the reflections from the two speakers don't arrive at the same time.

If you accept Linkwitz's theories about perceptual processing discarding reflections which are copies of the original---which I do---then the implication of the theory is the ceiling reflections will be heard as a distinct source. What sent me down this path of analysis is I kept hearing the band of the ceiling where direct reflections occur sound funny and was trying to understand why. The region which sounds funny extends to the walls somewhat and then ends about where the path length disparity drops below the 2ms threshold. Granted, this is a subjective result, but it's stable, is consistent with theory, and basic tests like turning off 18SWS1100s or moving to points in the room with lower path length disparities yield the expected result. So until a better explanation comes along I'm inclined to lend some credence to this line of reasoning.

The part I don't have a solid explanation for is how the ceiling reflections affect my perception of the speakers. But if I leave the 18SWS1100s running and block off the ceiling it produces that pleasing sense of compactness that my previous Neo3+8W4P build had or that turning off the 18s produces. This is consistent across all sorts of methods to shield my ears from the ceiling. Pillows, sleeping bags, and blankets and well as more sonically reflective objects like magazines, guitars, and violins. Doesn't matter if I hold them close or far and if I only obscure half the ceiling I hear the speaker whose reflections I'm blocking go compact. My guess for the time being is my brain recognizes the ceiling reflections as being a lot like the direct sound but isn't quite sure if they're really the same thing or not. So there's probably some confusion over whether the speaker's sitting on the floor or up on the ceiling and hence the speaker sounds less compact.

It occurs to me I probably could have built a ceiling diffuser in the time I spent typing all that. Balls.

Have you considered placing a wedge of 30ppi reticulated foam on the front and rear of the Neo10?

Not until you brought it up in the push/pull thread. It's roughly the same thing as BG does in the Neo3 PDR, Neo 8 PDR, and---it looks like---the sides of the Neo10. So I don't see why it wouldn't work out OK; if the results don't measure out well it's easy to take the foam off.

However, my requirement's 90dB SPL peak not 120dB (RMS?) so I can cross the Neo3 low and largely avoid both the directivity problems intrinsic to the Neo10 and the volume displacement problems which arise when trying to get above 100dB or so with low distortion. With arrays of drivers the size to consider for directivity is the extent of the array. Cone drivers scale better for this than 'netostats as you've better options for getting volume displacement through xmax rather than Sd. If you gotta have 120 probably your best option is to sacrifice vertical directivity and use push/pull pairs the way StigErik does.