Your concern is warranted. The pitch of a string depends on these two things: (1) its tension and (2) its mass-per-unit-length (that is, the
linear mass density). Now, the (volume) density of titanium is 4.5 gm/cm^3, whereas the density of steel is about 7.6 gm/cm^3 (it ranges from 7.5 to 8.0 gm/cm^3 for different types of steel). In other words, titanium is 1.7 times LESS dense. This means that a solid-core string made purely of titanium would need to be the square-root-of-(1.7) = 1.3 times thicker than a comparable steel string to have the same mass-per-unit-length, and therefore sound the same note at the same tension.
Of course, the lower strings (G and D) on a mandolin, and sometimes the A, are all
wound. In a wound string, the core material (steel or titanium) supports all the tension, whereas the winding material (usually brass, bronze, nickel, etc.) supplies most of the mass-per-unit length. In the Martin titanium core strings, this wrapping is made from nickel,
not titanium. Also, the unwound strings (A and E) are
not made from titanium, either: these are made of stainless steel (Martin says this is "cryogenically treated stainless"). Not much different from the strings you currently use.
In summary, the cores of the wound strings are the only things with titanium in them in this set. I don't know what the actual core diameters are. They certainly don't need to be any thicker than steel cores, though, because titanium is about 3 times stronger than steel: the tensile strength of titanium is about 1500 mPa, compared with about 500 mPa for stainless steel. So the cores could be made thinner, in principle, without sacrificing strength. Maybe these wound strings break less often? But on my mandolin, it's always the UNWOUND strings that are most prone to breakage, especially the E! So, there is little room for improvement there, in my opinion, in terms of durability. Of course, titanium is more corrosion resistant than steel, but the most problematic corrosion happens on the surface of the windings mostly, not inside on the core, so any claims about "superior corrosion-resistance" that Martin makes seem largely irrelevant to me. The string longevity will mainly be governed by the nickel windings, and not by the titanium cores. But it's true that nickel tends to corrode more slowly than phosphor bronze, anyway. So these strings might be expected to have longer longevity than, say, D'Addario J-74's. But that would not be because of the use of titanium.
Again, the mass comes mainly from the nickel winding. As you change the relative thicknesses of the cores and the windings, all while keeping the mass-per-unit-length about the same, you will alter the tone a bit. So maybe this gave Martin some room to explore slightly different sounds and core/winding combos? Since nickel is a bit denser than bronze, they could even make a
thinner string made of nickel-wound titanium that supports the same tension as a bronze-wound steel string.
Anyway, I have not heard what these strings sound like, yet, so I am reserving judgment about tone quality. Maybe they sound great? But advertising space-age materials like titanium seems like a sure-fire
marketing gimmick, regardless of whether these strings truly sound better or last longer. So maybe this will be a winner? And you can be sure that at least some folks will write in to the Mandolin Cafe and swear they made their "The Loar" sound more like a 1922 Loar.
Hey, what about using adamantium cores?
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