***As the frequency goes up, The solid conductor will begin the process of skinning, so it's inductance will begin to drop because the center of the conductor will have less current, so there will be less magnetic field within the conductor. A stranded conductor will have less conductivity with respect to eddy currents, so it's inductance will drop slower than that of the solid conductor. Litz will hold it's inductance the longest. Here, the size of the conductor will factor, as the smaller wires will go to higher frequencies before the effect starts to be noticed.
When wire pairs are considered, there are more effects as a result of proximity.
That's all technically correct, don't forget the
application: speaker wire.
Twenty kHz is still a "low" frequency as far as signal transmission goes, so all that stuff, while true, really doesn't mean anything
in this context.
Assuming a simple twisted pair cable ("zip cord"?). ***
I've always thought the term "zip cord" referred to a "shotgun" (side-by-side) arrangement rather than a twisted pair. I only throw that out there because I may be wrong.
A stranded wire will inherently have some air in the cable cross section (which the solid wire will not have). To achieve the same cable DC resistance a solid cable consequently will have a slightly smaller diameter than an equivalent stranded wire?
Sometimes images are worth more than words.
Here is a (bad, but useful enough) picture of two 14AWG wires with PTFE ("teflon") insulation.
The first one is solid core (Esoteric Audio Isopath). Note that I had to prop the outer jacket in the heatsink of a component to get it to lay the way I wanted it to.
The second one is a mil-spec (MIL-W22759-11) stranded (19 strands of 27AWG) silver-plated copper. Note that it is flexible enough to just go where I wanted it to.
I haven't taken a caliper to them, but the former doesn't seem materially thinner than the latter does, by eyeball.
The second one (along with a 16AWG version of same for shorter runs, and a double-run of either one for very long runs) is mostly what I use for speaker wire. No, it doesn't sound any different than any other wire. But the silver plating makes corrosion a non-issue and the teflon jacket makes the outer diameter of the wire thin for a 14AWG wire. Thin wires may look less audiophool-impressive, but they are easier to route and hide. And at surplus pricing, one can generally find spools of the stuff, in 2-conductior pre-twisted form, for around a quarter per foot for 16AWG and 35-cents a foot for 14AWG. For some reason 12AWG and up rarely comes up around here as surplus, and when it does it's a lot more expensive than 14AWG and smaller.
Furthermore, any differences between the thickness of stranded v. solid in terms of center-center distance will be completely swamped by likely differences in insulation thickness. Here's another (awful) picture that demonstrates such differences:
The wires are, from left to right:
-16 AWG stranded (mil-spec M22579/11, silver plate OFC copper) with teflon insulation.
-14 AWG solid core six-nines OFC copper with teflon insulation (Esoteric Audio Isopath, same as above)
-12 AWG stranded six-nines OFC copper with PVC insulation (Esoteric Audio -"Streetwires" car-fi marketed wire, purchased ca. 1994)
-8 AWG stranded six-nines OFC copper (JL Audio XA-BLUSCS-8; I don't know what gauge the individual strands are, but they are visibly finer than the 16AWG mil spec wire's 29AWG individual strands) with PVC insulation
Note how much thicker the PVC jackets are, compared to the PTFE jackets.
All of those (except the 8AWG stuff, which while sold as "speaker wire" is really beyond overkill for that application and more suitable as a power wire for car audio amplifiers. That's how I use it) are perfectly suitable for speakers. Assuming a run short enough and loudspeaker nominal impedance high enough to result in de minimis loss from the 16AWG one, nobody will ever be able to tell a difference between them.
