Right.
I did find a solution: A piece of hexagon mesh aluminum from some long-forgotten project. Removed Neo8 driver from one Pentagon tower & mounted it with the mesh sheet, zip-ties & wires over the bass driver. The bottom edge of it ended up a bit closer to the woofer cone -- about 1" above the level of the surround vs 1.6" or so on the original offset Neo8, which was left untouched on the other speaker. The original wood supports for the Neo8 were used to anchor the mesh.
Disconnected the PTmini-6 & passive crossover components, put both towers in the middle of the room about 2' apart. Measured FR of just the Neo8s with mic 2' away, both front and back. A 300Hz 48 dB/oct high pass was set in the minidsp 2x4hd to protect the Neo8s.
The above curves are on-axis only.
I find it curious that contrary to drewmc's concerns, the offset Neo8 has virtually identical front & back FR, compared to the centered Neo8, whose back directivity is considerably different from the front. Of course, the wooden supports left on the center Neo8 speaker could be responsible, and it's also possible the mic wasn't at exactly the same distance.
I will try more measurements off axis.
Front & rear FR curves for Neo8 centered over woofer on Pentagon LXmini woofer (about 1" above level of surround)
Front & rear FR curves of Neo8 mounted offset off the back, not actually OVER the Pentagon LXmini woofer. rather more over its "back" edge.
The front & back curves on both of the above were done at -20 and -15 dB sweeps to try and make them easier to see.
I know leaving the support structures for the Neo8 in place may impinge on the clarity of this test/comparison, but... 😐
It's a lot of data but you can see that...
1) the stretch 1~5kHz is more linear on the offset Neo8, both front AND back.
2) Both offset & centered mounting show increased peakiness around 7kHz for the rear waveform.
3) The above peak begins at a slightly lower frequency for the offset Neo8 and has a higher amplitude.
Given the flatter 1-5 kHz response of the offset Neo8, it's my preference. I don't think the linearity of the back radiation is as important. Also mounting the Neo8 in the offset position is much easier.
PS -- Admittedly, I haven't considered the differences between these 2 samples of the GRS Neo8. 😐
Front & rear FR curves of Neo8 mounted offset off the back, not actually OVER the Pentagon LXmini woofer. rather more over its "back" edge.
The front & back curves on both of the above were done at -20 and -15 dB sweeps to try and make them easier to see.
I know leaving the support structures for the Neo8 in place may impinge on the clarity of this test/comparison, but... 😐
It's a lot of data but you can see that...
1) the stretch 1~5kHz is more linear on the offset Neo8, both front AND back.
2) Both offset & centered mounting show increased peakiness around 7kHz for the rear waveform.
3) The above peak begins at a slightly lower frequency for the offset Neo8 and has a higher amplitude.
Given the flatter 1-5 kHz response of the offset Neo8, it's my preference. I don't think the linearity of the back radiation is as important. Also mounting the Neo8 in the offset position is much easier.
PS -- Admittedly, I haven't considered the differences between these 2 samples of the GRS Neo8. 😐
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Not sure about "a bit of eq". More like a lot, methinks. Between 1.1 kHz & 2.2 kHz, the front FR of the centered Neo8 has >5 dB difference. That's a whole octave -- 2nd to last on the treble side of a piano. Much smaller difference with the offset Neo. The higher freq peaks are actually much narrower if you view them musically, in octaves, and affect mostly overtones. The 7-12 kHz peaks seem nearly unavoidable -- which is why I tried the PTmini-6. Actually, all the curves look kinda bad.
But the last config I had of LP filter on Neo8 + high pass on PTmini-6 sounded about the best of all the configs I tried so far. Going to go back to that and keep working on the filters & PEQ.If someone has a brilliant scheme to simply and securely mount the Neo8 over the center of the woofer w/o acoustic impedance front or back, I'm all ears and eyes. I suspect it will have to be formed from metal.
So after these variious explorations, I've come to the following conclusions:
1. The Neo8 performs audibly better than the cone full-range driver of the original LXmini design
2. Like the cone driver, the Neo8 doesn't extend up high enough, nor linearly enough (ie, the on-axis peak), and not wide enough -- to satisfy me
3. Positioning the Neo8 centrally over the woofer might lead to benefits, but this is a headache to implement, so I'm OK with the directivty/FR achieved with rear offset Neo8 mounting.
4. The PTmini-6 in a simple 1st crossover mounted alongside the Neo8 solves #2.
5. The overall performance with #4 implemented & speaker positioned >4' from the front wall is very good.
6. The <50Hz bass is not quite I'd hoped for.
The question I am pondering now is:
Will this Seas 6.5" driver in a transmission line cabinet 30" tall, 7" wide and perhaps 12-14" deep (for the folded line) audibly improve the bass?
The woofer would still be facing up, the mid/tweeter still facing forward just behind the woofer.
Thoughts?
1. The Neo8 performs audibly better than the cone full-range driver of the original LXmini design
2. Like the cone driver, the Neo8 doesn't extend up high enough, nor linearly enough (ie, the on-axis peak), and not wide enough -- to satisfy me
3. Positioning the Neo8 centrally over the woofer might lead to benefits, but this is a headache to implement, so I'm OK with the directivty/FR achieved with rear offset Neo8 mounting.
4. The PTmini-6 in a simple 1st crossover mounted alongside the Neo8 solves #2.
5. The overall performance with #4 implemented & speaker positioned >4' from the front wall is very good.
6. The <50Hz bass is not quite I'd hoped for.
The question I am pondering now is:
Will this Seas 6.5" driver in a transmission line cabinet 30" tall, 7" wide and perhaps 12-14" deep (for the folded line) audibly improve the bass?
The woofer would still be facing up, the mid/tweeter still facing forward just behind the woofer.
Thoughts?
Without wishing to derail the thread, I'd be really interested @silent mike in the implications of what you've learned from varying the relative positions of the woofer and high-range/tweeter.
I am thinking about another LXMini derivative that would have an upward-facing woofer with the tweeter on the front of the woofer enclosure. The woofer enclosure would be shaped so that the tweeter was effectively without a baffle in the front and its radiation absorbed in the rear.
So from what you've learned, do you think that woofer-fullrange positioning is flexible as long as (a) time delays are set to get the phase relationship correct, and (b) the distances are small relative to the listening position and to boundaries?
Linkwitz wanted the LXMini to look transparently like what it is (function apparent from form). Partly this is a question about other aesthetic forms, and partly about greater flexibility in drivers as you've explored here.
Any thoughts appreciated!
I am thinking about another LXMini derivative that would have an upward-facing woofer with the tweeter on the front of the woofer enclosure. The woofer enclosure would be shaped so that the tweeter was effectively without a baffle in the front and its radiation absorbed in the rear.
So from what you've learned, do you think that woofer-fullrange positioning is flexible as long as (a) time delays are set to get the phase relationship correct, and (b) the distances are small relative to the listening position and to boundaries?
Linkwitz wanted the LXMini to look transparently like what it is (function apparent from form). Partly this is a question about other aesthetic forms, and partly about greater flexibility in drivers as you've explored here.
Any thoughts appreciated!
If the rear wave was completely absorbed, then yeah, I don't see why this would not work. Obviously need to add delay to the woofer. But not sure why you'd make the fullrange monopole when dipole is intended. If the tweeter (or fullrange) is monopole, then the cardioid pattern in the crossover transition range would not happen.
I think so. If using active crossover & DSP, a "reasonably close" match of the omni+dipole seems to work OK... though I really have tried only 2 positions, with one planar dipole driver for the fullrange.
Yeah - that's right. My mistake. There would need to be a tunnel through the woofer enclosure. I've never been fully clear on the function of the heavily stuffed fullrange tube and how it's meant to modify the dipole.
Back to watching your adventure...
LXm tube is mystery to me. The motor/magnet blocks the tube so it is just a waveguide and the method to position the driver over the woofer.
Erin has measurements here https://www.erinsaudiocorner.com/loudspeakers/linkwitz_lx_mini/
but I'd like to see comparison wthout the tube
Midrange tube through a box - I haven't seen measurements from backside. I suspect dipole pattern is weak and at low freq. Stuffing this tube kills the rest of it.
Erin has measurements here https://www.erinsaudiocorner.com/loudspeakers/linkwitz_lx_mini/
but I'd like to see comparison wthout the tube
Midrange tube through a box - I haven't seen measurements from backside. I suspect dipole pattern is weak and at low freq. Stuffing this tube kills the rest of it.
The stuffed tube magnet mount is to prevent a hotspot directly behind the speaker when used close to a wall. Without it, the speaker needs to be pulled out into the room a considerable distance to mitigate that strong reflection. It's designed to function in a smaller room than a full blown dipole design will allow.
Right. From Linkwitz...
If I've followed correctly, the current experiment involves placement a safe distance from the back wall.
...and...
So his intent was monopole + dipole = cardioid in the crossover region, then a modified dipole, more dispersed at the rear, to temper issues with relatively close boundaries to the rear.
If I've followed correctly, the current experiment involves placement a safe distance from the back wall.
Yup. I'm not sure just how successful that is.
Brent's champagne LXm are usually placed fairly close to his front wall, probably 1' from the back of the speaker. It doesn't sound great in that position, imo -- decent but nothing special. He pulled it out 3' before having me do any serious listening. By then, I'd pretty much given up on the idea of close-wall placement for my project & resigned myself to speakers that are easy to move back and forth from close-wall.
yes. I've been listening with them 3-5' from the front wall. Never did hear Brent's LXm close to the wall in my studio.
Didn't measure Brent's speakers' from the back -- should have! His fullrange + tube is built very closely to the original. I did listen closely to the back output. A lot of the sound goes not go through the tube. The gap between the back of the cone & the start of the tube is around 3cm, more than enough for at least half the back wave to progagate freely, imho. Even at the far end of the tube, a lot of sound still gets through, and the treble does not sound that rolled-off, mostly just a bit lower in amplitude.
Yes, Linkwitz designed that speaker to be easy to move out into the room for critical listening, and to help mitigate the rear wall reflection issues for those with small rooms that could not do so as well as being easy for diy construction with limited tools. In similar situations I've convinced people to let me build in wall systems instead of the compromises involved with designing a speaker placed close to a wall.
Ehem, SL perhaps tried to find recommendations that fit the acoustic properties that LXm happens to have, rather than did tremendous effort to make it perform that way.
I still don't see how different the tube makes rearside radiation compared to a simple nude cone driver. Here I show Erin's Klippel NFS of LXmini horizontal and my homemade measurements of a 4" Audax HM100 that I was thinking of using as dipole mid for AINOgradients - ten years ago! After seeing these and looking at the relatively big magnet, I decided to order B&G Neo8. Sorry, I haven't saved similar 0-180 data of just the Neo8.
My raw response measurements were normalized in VCAD. Notice the similarity in general pattern. Pure dipole pattern is lost above 2kHz - rearside spl drops down and it's practically monopole with narrow pattern above 3kHz.
I still don't see how different the tube makes rearside radiation compared to a simple nude cone driver. Here I show Erin's Klippel NFS of LXmini horizontal and my homemade measurements of a 4" Audax HM100 that I was thinking of using as dipole mid for AINOgradients - ten years ago! After seeing these and looking at the relatively big magnet, I decided to order B&G Neo8. Sorry, I haven't saved similar 0-180 data of just the Neo8.
My raw response measurements were normalized in VCAD. Notice the similarity in general pattern. Pure dipole pattern is lost above 2kHz - rearside spl drops down and it's practically monopole with narrow pattern above 3kHz.
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I'm using the Sounderlink neo8 clone with LR cover at 500hz (2nd order). There is a 7 ohm pad on the planar. Woofer is Bold North 8inch paper cone with Dayton Pr behind. Way exceeded my expectations. Pairs well with sub < 70hz.
http://store.miscospeakers.com/8-inch-stamped-frame-subwoofer-bna-82132
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I didn't give up on this imitative, though it did morph from the original topic. The question was whether the spacious, open sound of OB dipoles could be achieved with a pair of speakers close to the wall. After trying a couple of experiments with OB mid/top, it seemed unlikely. Then I veered to the LXmini, combining a slightly larger 7" woofer with a NEO8 magnetic planar dipole. This proved successful, though not when placed close to the wall. Realistically, for these speakers, about 18" is the minimum distance to the back wall in my studio.
I also looked at the the Larsen speakers, which are basically omnis, but no one seems to have hands-on experience with them. They are often shown placed closed to the wall. Then I came across the Morrison speakers, which again are omnis combining the resonance-cancelling bass loading tech developed originally by Hegeman and now implemented in a different way for tweeters by KEF with its metamaterial damper. I've never actually paid much attention to omnis, but recently found that those who like them often aren't interested in other designs at all. Why this dedication? They can't all be nuts, right? (We all are, I know.) I needed to find out for myself. It occurred to be that omnis might be placed close to the wall without much negative effect: if properly implemented, the sound is projected in 360 degrees, horizontally at least, so the power response wouldn't be affected regardless of positioning. So...
Here's my latest LXmini+NEO8 LXmini variant, and a 3D printed omni deflector that can replace the NEO8 with a SB TW29RN tweeter facing up toward a spherical deflector. This is the first iteration; a few adjustments need to made -- tweeter deflector needs to come down 1-1.5cm, and the struts made a bit sturdier. Obviously some smoothing of the surface also has to be applied. It's just a start. At least one more deflector has to be made, and a pair of these 7" towers modded to take the deflectors.
I also looked at the the Larsen speakers, which are basically omnis, but no one seems to have hands-on experience with them. They are often shown placed closed to the wall. Then I came across the Morrison speakers, which again are omnis combining the resonance-cancelling bass loading tech developed originally by Hegeman and now implemented in a different way for tweeters by KEF with its metamaterial damper. I've never actually paid much attention to omnis, but recently found that those who like them often aren't interested in other designs at all. Why this dedication? They can't all be nuts, right? (We all are, I know.) I needed to find out for myself. It occurred to be that omnis might be placed close to the wall without much negative effect: if properly implemented, the sound is projected in 360 degrees, horizontally at least, so the power response wouldn't be affected regardless of positioning. So...
Here's my latest LXmini+NEO8 LXmini variant, and a 3D printed omni deflector that can replace the NEO8 with a SB TW29RN tweeter facing up toward a spherical deflector. This is the first iteration; a few adjustments need to made -- tweeter deflector needs to come down 1-1.5cm, and the struts made a bit sturdier. Obviously some smoothing of the surface also has to be applied. It's just a start. At least one more deflector has to be made, and a pair of these 7" towers modded to take the deflectors.