(From an article by Dr. William Braid White, Principal of the School of Pianoforte Technology, Chicago, Ill., on the Steinway Website reprinted with permission from the Piano Trade Magazine, Chicago, Ill.)
In the piano the function of the soundboard is to take up and repeat the vibratory motions of the strings, and thus to set up in the air sound waves of vastly greater size and power than could be generated by the strings alone. The more faithfully the soundboard performs this function, the better soundboard it is.
The layman will better understand this amplifying function of the soundboard if he will think of the relatively enormous area of the board when compared with the very small area of all the strings taken together. Hence, when the piano is played, the soundboard, repeating the vibratory motions of the strings, sets in vibration vastly more air than could the strings themselves.
The more than two hundred strings that constitute the tone-generating element of the piano are stretched, at high tensions, over wooden bridges, or supports, which are rigidly fastened to the surface of the soundboard. Thus, within a small fraction of a second any motions of the strings are transmitted through the bridges to the soundboard, which as it were, accepts them, and faithfully reproduces them over its entire surface. These tiny but intensely complex motions, originating at the strings, are transmitted to the large body of air surrounding the front and back surfaces of the soundboard, thereby setting up powerful sound waves which immediately register on the ear-drums of all within hearing.
So faithfully does the soundboard perform this difficult function, that no matter how many strings may be sounding at one time, their almost incredibly complex motions will always and unfailingly be taken up and reproduced. Thus, the soundboard of the piano acts just as does the parchment head of a drum or the thin steel diaphragm of the receiver element in a telephone. It should be remembered, however, that it is the strings, and not the soundboard, that originate, by their vibratory motions after they have been struck, the sound which the soundboard amplifies.
Construction of the Soundboard
In order to obtain these very remarkable effects of amplification, the soundboard of the piano must be constructed with exquisite skill. Its length and breadth depend, of course, upon the size of the instrument, while its thickness, with some variations between one end and the other, averages one-quarter inch. The pieces of sprucewood from which it is made are matched in such a way that the grain runs roughly parallel to the line of the great bridges upon which the strings rest.
Facts Little Understood
It is strange but true that these simple facts about the effect of the strings upon the soundboard and about the soundboard's responses to the strings, are still very little understood. Thus there persists a common notion that a crack in the wood must in some way cause a deterioration of the tonal output. Actually, no such effect is to be expected.
The erroneous idea that a crack in a soundboard reduces the tonal output is undoubtedly due to the equally erroneous theory that sound "vibrations" in some way travel transversely across the soundboard. But, as has been shown here, the movement of the board is that of the movement of the strings, up and down in the case of a grand, backward and forward in the case of a piano of vertical construction. The glued-up strips of thin spruce, reinforced by bridges and ribs, which constitute the soundboard, become in fact a single unit, so that the whole board vibrates with the playing of even one single note anywhere in the scale.
Effect of Cracks and Checks
For this very reason a crack or check in a soundboard reduces the soundboard's ability to amplify the vibrations of the strings only to the extent to which the crack reduces the vibrating area of the board.
Soundboard areas vary with the size of various pianos, but consider for example a board with an area of 4,000 square inches, counting both surfaces. Now assume that there is a crack in this board 35 inches long and one-eighth inch wide, which would be an enormous crack. That crack would have an area (counting both surfaces) of 8 3/4 inches, and so would reduce the air disturbing area of the board by less than 1/4 of one per cent, an amount utterly negligible.
Here we have considered the effect of an enormously big crack. A dozen ordinary cracks, even if they extended from end to end of the soundboard, the crack or cracks might have about as much effect, certainly no more. So long, in fact, as the structure of the soundboard remains solid, with ribs and bridges adhering correctly to the surface of the soundboard, and with the entire periphery rigidly fastened into the frame of the piano, the question of cracks is utterly unimportant.
Effect of Atmospheric Changes Greater
As a matter of fact, the tonal output of any piano, with no cracks in the soundboard at all, is subject to vastly greater change with every change in temperature and humidity. The alternate absorption and evaporation of moisture affects the soundboard, and therefore the tone of the piano, to a far greater degree than any crack or accumulation of cracks, yet few persons complain of this or even appear to notice it.
I have gone into this matter at some length because the public, and some piano men, are misinformed on the subject. To correct the difficulties that arise out of public misunderstanding of this matter has been the object of this article.