The plate can be either built-up structural steel or cast gray iron.... [/b]
Hmm, that's interesting. Structural steel? You mean like regular old "A36" structural steel? I thought that steel didn't have acceptable damping qualities (pardon if this is not the correct technical term) that a good plate material should exhibit, compared to cast iron. Could you elaborate? [/QB][/QUOTE]
What is meant by exactly "acceptable damping qualities? This is the character of gray iron that always comes up in this discussion. It has been promoted for decades as a desirable thing. But why on earth do we want the piano plate to have “damping qualities?” Gray iron is used for industrial machine bases because it readily absorbs vibrating energy. By definition this means that the material readily absorbs and converts vibrating energy into heat. Personally, the last thing I want the piano plate to do is absorb and convert into heat vibrating energy that could be going into acoustical energy.
Yet, whenever this subject comes up I hear this argument. It has been proposed since time immemorial by piano makers and by piano technicians as a desirable thing. Yet very few have actually have had any hands-on experience with steel plates. I have. All prototype pianos at Baldwin were built using plain, off-the-maintenance-room shelf hot-rolled steel stock. My piano plate drawings were converted to accommodate the standard available thicknesses and the plates were welded up in the maintenance shop. Some grinding and hole drilling later and we were ready to build a piano. No, they were not at all pretty but they worked. Indeed, they worked so well that, without exception, generally sounded better than subsequent pianos of the same design using cast gray iron plates.
True, there is a hypothetical problem with braces ringing at some odd harmonic but I never found this to be a problem in real life — we never once had a ringing problem. Which is not, of course, to say that it couldn’t happen. It even happens occasionally with cast gray iron plates. But if it does there are simple and attractive ways to deal with the problem.
The structural advantages of steel are obvious. The tensile strength of the common gray iron mix used to pour piano plates is around 24,000 to 26,000 psi. I don’t recall the exact specification off hand, but I think even hot-rolled steel is about three times that. With various cold-rolled steels it’s pretty easy to get into the 100,000 to 150,000 psi range. Much is made of the supposedly great compression strength of gray iron and it is true that this is somewhat higher than its tensile strength — typically about double for the grade of iron used in piano plates, not enough to make it strength-competitive with steel. So even here it loses out to steel by some considerable margin. Finally, much is said about the supposedly great rigidity of gray iron. But this is a mis-understood phenomena. What gray iron really has is a lack of plasticity. It will bend fairly readily up to its proportional limit — then it breaks. Catastrophically. Steel is both stiffer and lacks this rather nasty habit of breaking like a dropped glass or piece of chalk.
All this aside — actually, because of all this — steel simply makes an acoustically superior piano plate. Perhaps its most significant acoustic advantage over gray iron is a marked and measurable increase in sustain time. Its primary disadvantage is not any tendency to ring, but its cost. Steel plates are decidedly more expensive to make, especially in high volume where cast gray iron shines. Fabricating a built-up steel plate takes time and man hours. It is more difficult to drill — the graphite in gray iron makes this easy — and to finish. Though automation and high production can definitely narrow this gap. They also look a bit unusual since the longitudinal struts are rectangular and do not have any draft. This could be machined in but it would be fairly expensive.