You can over consider skin effect yes but there are other properties that yields better current efficiency along with skin depth. Proximity effect has influence at lower frequencies and higher current too. We also have Vp linearity that can be adjusted with wire resistance (size of the insulated wire). Mother nature can allow all three, skin. depth, proximity effects and Vp linearity, to get better with a good design. Cable is not a one shot attribute pony.
I know it is easy to fall into the type of looking at a single attribute of the week type of thinking. Stuff goes boom in the real work doing that.
If you sweep the resistance, Rs, of a cable, you can see that conductor get less efficient at higher frequencies based on the wire size. Bigger wires will see larger Rs changes than the same aggregate AWG size of smaller insulated wires in parallel. The current through the larger wire is not using the center cross-section where smaller wires more evenly distribute the current across a wires cross section. This is a fact, but like you said is it heard “alone”. No, not so much as tertiary variables smaller wire allows. Better has to improve everything so it is important to keep those calculations in a designs considerations.
A hollow wire, unless it is extremely thin (less than 20-mils) does not do anything to improve skin depth at audio other than the wire efficiency of the tubes wall thickness. It will fail to improve proximity effects or Vp linearity based on the resistance of the wire cross section or CMA area and how many are used. Use multiple wires and L and C are going to have to be managed differently to avoid one or the other rising exponentially,
The EM field is on the OUTSIDE of the conductor so that the “air tube” inside the wire does near nothing. This is the exact same issue with RF cables where the center is simply wasted area for using copper. So we use some material that might benefit a different attribute like strength, or weight for instance.
At higher frequencies you want the wire cross section to carry the signal (skin depth). An RG11 has lower attenuation than a RG59 because the CROSS-SECTION of the copper portion of the wire is larger. The copper THICKNESS is still 40 micro-inches, for instance. Make the wire LARGER in circumference increases the CMA area to lower attanuation. The signal won’t penatrate any lower than the skin depth at that frequency. To trick it to work, we hold the thickness and increase the wire diameter. This works best at RF. So skin depth isn’t as influential at audio frequencies since our current is more diffusion coupled or closer to the same across the wire surface through the wire. We can improve it, but that improvement has to also consider the other variables in play to be worth the work.
RF wire efficiency with plating thickness can be easily seen with coaxial cables that use IRON, STEEL or aluminum center wire material to add strength and/or remove worthless material for attenuation INSIDE the wire, and below the wire’s skin depth frequency. Those “filler” materials are terrible for the EM signal but don’t have any effect on the signal at all as they are INSIDE the wire. Air won’t change that at all, except it makes the conductor harder to make and is only appropriate for larger “wires”. A TEM, Transverse Electromagnetic Wave, is traveling “in” the dielectric material between the inside surface of the shield and on the outer skin depth surface of the conductor ONLY. It doesn’t use the wires center region, ever.
Capacitance (governed by the dielectric material BETWEEN the wires SURFACE and the distance between them) and Inductance (distance between the wire surface / loop area. The dielectric has no effect on inductance) don’t see the center properties of hollow wire either, so I’m not sure where the hollow wire is helping L or C.
Does the design support proper R, L and C, lower Rs and improved Vp linearity with proven measurements and calculation? You comments seem to be hoped for, not actually proven in design, calculation or use. What properties are improved and HOW are they being improved…where is the beef in the design? If we pay for it to work, we should ask to be shown WHY it works. Does it really lower inductance? How? Example?
Great materials scattered all over a poor design don’t make a good cable. But, we all seem to want to not know how it really works to believe in what we “want” to be working. Not sure why do that, but we do.
Remember early high speed plane designs? Ya, they were built BACKWARDS! A water drop forms a tear shape falling through the air, the most efficient shape for low drag coefficient (low turbulence or vacuum effect). The best shape is fat on the leading frontal area and tapers at the rear. The air has to keep snug against the surface of the “shape” being forced through the air. Air has to be brought back together smoothly and with zero turbulence at the trailing end to be efficient.
Car design suffers the same, but a way better fate as we do know more now. Notice how Mercedes Benz cars all look the same? Well the SHAPE to get a 0.23 drag coefficient is set by mother nature. But our eyes want to see “different” things. For a car, there is ONE ideal shape for the lowest CD, and once that’s defined there is no better “look”. To get the look we want, we sacrifice efficiency and as little as we can, to keep a car’s MPG and noise (turbulance) as low as we can. The end result is that most cars are 0.28-0.30 up from 0.20 (or the ideal value we can get to) to LOOK different. We toss away some performance for marketing “our” product. True, some just copy MB shape.
We seem to do this in a lot of areas. Audio cable is one of those areas. I design for calculation and measurements, that’s it. True, the form/fit/function will be biased to performance ONLY. If we want to do something else with the cable we can remove some performance to get there, same as the car or plane example. I did that with the BAV XLR cable as a cable example. I made it more durable and flexible but had to change the EM properties of the dielectric and conductor DCR and wire design to do it. It is all in the design explainations.
The air speed does change the TAPER and curvature of an aero object but the fact remains, we can’t wish it to work because it looks cool.