The wooden rim of a grand piano serves two functions: structural and acoustical. Structurally, it's the foundation to which the cast-iron plate is bolted, and as such it assists the plate in supporting the tremendous tension exerted by the taut strings. The acoustical function is less well understood. The soundboard is glued along its perimeter to a shelf formed by the inner rim of the piano case. The vibrational energy in the soundboard thus contacts the rim, and is either reflected back into the soundboard or is siphoned off into the rim, depending on the density of the wood used in the rim's construction, and on the stiffness and total mass of the rim. Dense woods and a stiff, massive rim will reflect sound energy to a greater degree, causing the sound to sustain longer, whereas softer woods and a thinner, more flexible rim will tend to absorb more energy, causing the sound to disappear sooner. This is why most of the best grand pianos have very thick rims made of dense woods such as maple and beech. (Bösendorfer, a high-end piano, has a relatively soft, spruce rim and so may seem an exception to this rule. But spruce transmits sound well, so the tonal energy is not so much lost as spread throughout the structure, which then becomes an extension of the soundboard.) Less expensive grands may use softer woods like lauan (sometimes known as Philippine mahogany, though lighter and more flexible than true mahogany), or alternating layers of harder and softer woods.
Two things are important to note here. First, the hardness or density of the wood in the rim has nothing to do with the rim's ability to fulfill its function of structural support. A rim made with a less-dense wood like lauan won't "fall apart" or cause the piano to go out of tune faster; the issue is strictly one of how the rim affects the tone. Second, the words hardwood and softwood are botanical terms that have little to do with how hard or soft the woods actually are. So when a piano ad touts a "hardwood rim," or a rim made from "select hardwoods," chances are that the woods involved are not very hard at all; if they were, the ad would likely name the actual species of wood instead of hiding behind such general and potentially deceptive terms.
SPRING 2010 -- page 99
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Hybrid & Player Pianos
New-Piano Buyers’ Reference