It is the primary function of the soundboard to amplify as accurately and completely as possible the sound of the vibrating string. In the real world, however, every board adds a certain flavor, timbre, or tone color of its own, all other things being equal. Much like fingerprints, no two woods are exactly alike. While the board doesn't actually produce sounds, it can and does modify the relative intensities of the tone components (harmonics), thus altering the sound, and giving each and every piano a tonal characteristic of its own.

The sound of the struck string is transmitted to the soundboard via the bridges, which be discussed in the "Bridges" section. For now, let us assume that the string vibrations have already been accurately transmitted to the board.

A piano string vibrating without a soundboard would be like listening to an electric guitar without an amplifier - barely audible. The reason being that the sound waves, or air pressure fluctuations set in motion by the strings alone, cover only a very small area, and are without any reinforcement. It is the purpose of the soundboard to enlarge this area, setting a considerable mass of air into agitation, thus increasing the volume substantially.

Since this area is increased to such a large extent, it is important that the sound waves travel the length of the board as quickly as possible, in order that one section of the board doesn't vibrate out of phase with another. This would cancel out those certain frequencies vibrating out of sync., resulting in a board with uneven response. It is mainly for this reason that the soundboard must be made out of wood which transmits sound as quickly as possible.

Norway Spruce, in which sound travels at the speed of 3 miles per second, is ideally suited for this purpose. At this speed, vibrations run the length of the board in .002 seconds. Across the grain, the speed is reduced to about 1/3 of this. The speed of vibrations traveling across the grain would be even slower if not for the ribs glued to the bottom of the board and across the grain. The purpose of these ribs is twofold - not only do they help transmit sound across the grain, but they also give the board support from the downward pressure of the strings, thus preserving the crown, or slightly concave shape of the board.

As mentioned earlier, Norway Spruce was best suited for piano soundboards because of the speed in which it transmits sound. Another unique feature of Spruce is the presence of tiny membranes throughout the wood - millions per square inch. In live trees these membranes contain a resinous liquid. When the tree is cut, this liquid volatizes and leaves a dry membrane. Figure 2 shows a close-up of these little diaphragms magnified 650,000 times.

As you can see, it consists of a hollow chamber, which acts like a tiny resonator. These spherical resonators add a certain richness, or tonal resonance which is unique to Spruce. Without these membranes, or tiny resonators, the tone is flat, and dull, lacking the live sonorous quality of a well made soundboard.

On a much larger scale, these microscopic membranes might be compared to the Mormon Tabernacle in Salt Lake City, Utah, one of the premier acoustical structures in the world. It is a 1/2 egg shape, with no angles - collecting and reinforcing the sounds so that a person speaking in a normal conversational tone can be heard distinctly and clearly at the farthest seat. There are literally millions of micro Mormon Tabernacles in a spruce soundboard.

It is important that the acoustic function of these diaphragms remains intact during any exposure to excessive heat, as in the artificial seasoning process, or while being exposed to any other undue stress, such as excessive pressure during assembly. Studies have shown that the loss of these tiny resonators makes for an acoustically unacceptable board. The board would sound lifeless, lacking the ability to carry the tone and reproduce the intricate patterns set forth by the vibrating strings.

Pine has all the qualities of spruce, i.e. fast transmitter, light, strong, etc., but lacks these tiny membranes. It is for this reason that pine soundboards don't have the resonance of a spruce board. Nor do they respond uniformly to the partials - a good tone having approximately 50% fundamental.

Another important tonal quality of spruce is the fact that it is a highly compliant and elastic material, and is therefore able to be set into motion from the slightest vibration. This translates into a very responsive piano. In an instrument where the player is so far removed from the actual production of sound, (that is, out of physical contact with the strings themselves, as with the violin, or guitar) responsiveness to the slightest touch variation is of utmost importance. It creates a more intimate relationship between player and instrument, enabling one to play very soft pianissimo with a tone that will carry to the farthest reaches of the concert hall.

One other characteristic of spruce which makes it particularly suitable for soundboards is that its resonant frequency lies outside that of the entire range of the keyboard. This helps to maintain a uniform response to all of the partials.

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