This is another great myth of the piano. While cracks, or more accurately: breaks, are unpleasant cosmetically, they usually are meaningless musically.
But before we go any further we must be sure we all know where and what is a soundboard and why it cracks. If you look under your strings in a grand piano or at the back of your upright behind the posts you will see the soundboard.
This mysterious piece of vegetative product is the most misunderstood part of our favorite instrument. The soundboard is made up of fletches of quarter-sawn sitka (usually) spruce. Traditionally tight grained spruce is preferred but with the loss of such old growth trees wider grain spacing has been used more in recent years and increasingly multi-laminate wood products (plywood).
The function of the soundboard is to take the energy from the string through the bridge and transduce that energy so that enough air is moved we can hear the sound. It doesn’t amplify the sound. Amplification implies energy being added to the system. The soundboard actually increases the rate in which energy is used. Without it, the string would vibrate softly for a very long time, slowly using up its energy. The soundboard uses the energy given to the string by the hammer at a faster rate allowing a large enough mass of air to be moved to produce an audible sound.
The soundboard has three primary components; the bridge receives the energy from the strings and provides contact with the soundboard. Underneath the soundboard panel are the ribs which move the energy across the grain of the panel and support the crown or upward bowing of the panel.
There are two methodologies for constructing a soundboard, for today’s purposes we will ignore the more modern method and look at the traditional method of soundboard construction. Traditionally the soundboard panel is dried to a level between 4% and 5% EMC (equalized moisture content) which is simply a measure of the water present within the wood. The panel shrinks in this process. The ribs are then glued onto the panel, perpendicular to the direction of the grain. As the panel absorbs moisture it expands and literally bends the ribs. The ribs prevent the panel from expanding across the grain on the bottom of the panel. The top side is freer to expand and expands more. The effect is the crown of the soundboard. The crown provides the soundboard positive resistance to the downward pressure of the strings.
Both the top and bottom sides of the panel will be in a permanent state of compression. Through the normal environmental swings the levels of compression will rise and fall with the relative humidity. Two things will occur over time from this situation. The first is compression set. While initially the wood fibers expanded and contracted within their compressed state, eventually they will take the compressed state on permanently. After being compressed one too many times during a damp season, during the following dry season the soundboard will release its excess moisture but because the wood had suffered compression set, instead of relaxing intact it breaks along the compression. Sometimes you can feel a compression ridge on your soundboard. This is a grain of wood which has been pushed up during an excessively high compression period. It does relieve some of the compression but you are looking at next winter’s crack.
So that is where cracks come from, what do they mean? Initially they mostly represent just a loss of surface area on your soundboard. In most cases we are talking about a .1% loss. In other words it means nothing. As more cracks develop the soundboard will lose more if not all of its crown which usually sounds like a warm, long sounding piano without much punch. Even a very bad board will still act as a transducer.
However if the ribs break off of the panel, you can see this as a gap between the rib and the board from the bottom or a raised portion along a crack on the top, you can develop some very loud buzzing as the rib, panel and glue residue vibrate against each other. This condition is very repairable and the soundboard can continue in service for many more years.
If you have any questions about the condition of your soundboard or piano, just drop me a note.