That is a claim with modifications that's not entirely true , depends how you view it, Not counting the top plate (5), above the copper sleeve then yes.
What can you see from the 2 copper sleeves?
There is a inverted, extended T-pole, encircled in a large copper sleeve (6), all the way down to (3).
Immediately below that, there is (3) a NdFeB magnet slug, as you saw in my previous post, it's magnetic permeability is close to air at 1,05.
And on the opposing side of where the NDfeb ends, you again see a full length copper sleeve (7).
Also do notice that the copper sleeves overlap from one side to the other.
So they have gone very far with 'inductance control', and "mimicing air core behavior" in the motor design.
On the very bottom of the gap below (3) there is again magnetic permeable material, but that is by design to let the field reduction and suspension parts, stop the coil from destroying itself on the backplate.
You should also note the voice coil wind is not of a "standard flat topology" with a even layup over the whole length.
I do not believe you can make a "air core" motor design with Ferrite magnets, but im not updated on magnet tech.
Børresen and Focal (mids) does it with Neo. As close to a air core as is possible to get with todays tech.
here is some high end hifi commerical showing one:
You will find the Aurasound whiepaper, and the Super Flux Magnet document added explaining in more detail.
And you are far from done.
AE:
JBL DD has barely been touched upon.
There are version with shorted turns.
patents for copper cap/sleeves and demodulation rings.
There is also a few -ND versions (2216ND-1 fx.) using the Low TCR voice coil alloy to reduce power compression.
Also take notice that all they're "top of the line" HF drivers used to use internal Neio magnets, as opposed to the traditional topology, (except for the 476 who had to compromise on that to fit in the copper sleeve).
The HO "midrange" drivers (2012 and 2020H) had some interesting motors.
The 2242 (only reamining) SVG JBLs incorporated a copper cap back when.
1400/1401 ND used a interesting motor structure too.
W15GTI has a interesting motor for being such a old "car"sub .
And last but not least the 1500AL/1501AL top of the line, underhung Alnico drivers.
Do notice the alternating copper/steel gap rings.
Also take notice of the magnet, it is internal where the pole piece would normally sit.
Those 2 features again makes the driver close to a "air core" Alnico 5 has a permeability around 2, just with a internal magnet on the inside of the coil, utilizing the basket as demodulation and heat conduction. And they largely designed away the Alnico heat related demag issues in the same time.
Also look at the magnetic circuit and see how much copper the path has to go through.. There is a reason it is expensive and have such a insane magnet strength, with a fairly normal gap density.
And there are still loads of design variations to look into.
AE Lamba motors overhung and underhung.
18S has the AIC (added) Tetracoil in different configurations and sizes with demodulation rings etc., there are also so copper sleeved drivers with demod rings (15MB1000, 12MB1000, 12NMB420, etc).
Børresen as mentioned.
Focal mid range in the top line.
Kef has a 'air core' driver concept, name escapes me.
B&W both the FST that's added, and they're dual coil 800 series bass driver that has a interesting motor topology.
Beymas LEX is interesting in terms of quiet excursion and cooling.
Kartesian has some interesting designs out too, with "assembly drawings"
For pure subwoofer Duty Stereo Integrity has showed some impressive BL(x), Kms(x) linearity, and good Le curves on the top of the line drivers.
Pioneer and Rockford also made some interesting motor topologies over the years in theyre top of the line drivers.
Including Pioneer/TADs unified frame 1102 and 1601C.
BMS 18N862 is still excellent today for what it is, nothing radical just good design.
US designed, China made Redcatt, has made a few 18/21 with they're XR motor, that has an impressive amount of xmax, for the sensitivity, and well controlled inductance, judgung by datasheets.
Chinese A&D uses copper and alu demod rings, but it's not found in the marketing, Klippel tested in R&D. Can be read in the Impedance curves. and spec sheets.
The list is "endless"
You barely touched the surface. And i am by no means a expert in the subject.
There are far more knowledgeable people about this, then me on this forum.
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The highest efficiency driver listed there, the 15" JBL 2220H/J has an 8.7% efficiency (half space) due to it's high Bl driving a light weight cone and underhung voice coil (7.2mm height in a 9mm gap), incapable of much excursion or deep bass, Mms is only 70 grams.
Higher excursion requires longer, heavier voice coils, and heavier cones to withstand the stress induced when pushing and pulling more air mass further, lowering efficiency.
The differential drive 15" JBL 2265 Mms is 125g, with Bl less than half of the 2220, dropping efficiency to 1.99%.
I forgot but from what i have seen of Wavecore, mostly measurements that is. Just the WF midwoofers, and the new carbon/kevlar woven cones really.
Well behaved in the TIme domain performance, fairly easy to filter.
BD is better controlled inductance then a normal version generally.
Kilippel result have not been super impressive.
Look at the distortion measurements at mfg. website for a indicator.
and be aware that most TS specs and data sheets is based on mathematical Xmax = 70% of BL point.
It does not touch on any form of linearity over excursion like a Klippel test.
And it usually does not account for the suspension limitations or other factors.
There really isn't a lot of information on the Wavecor drivers. They don't seem popular and I can see why. They are expensive for what they look like.
However, I am interested in the for their bonkers QMS specs. I have found a trend. Every driver I have heard that plays the 30-500hz hynamics I praise above all else seem to all have a high QMS. I know this isn't end all be all. But it is definitely related.
Some of the Wavecor drivers have a QMS as high 19! That's nearly 10 times the QMS of a 6" Dayton RS. A driver I found to be quite dull sounding.
I know the alternative is a huge cone like a Nero. Something I do not have space for.
Getting high QMS like that is mainly from using a nonconductive former. I have seen QMS that high on some of my motor designs from simply winding the coil on .1mm grade 2 titanium former.