The Amplifier of the Sound
The soundboard is a large, thin wooden plate which acts as an amplifier of the sound produced by the vibrating strings – its purpose is to radiate a large volume of sound over a wide frequency range. All stringed instruments have some type of soundboard – most are built directly into the body of the instrument, such as the violin or the harp. These soundboards amplify the sound produced by the bowed string of the violin, or the plucked string of the harp. In the case of the piano, the string begins to vibrate when it is struck by the hammer. As the string vibrates back and forth, it displaces the air, creating sound waves. The string alone, however, has a very small surface area, which means that it cannot displace a large amount of air, making it difficult to hear. When the strings are linked, or coupled, to the soundboard by the means of the bridge, they are able to transmit their vibrations into this large wooden resonator, which is then able to displace a much larger amount of air than the string alone.
The soundboard is not one solid piece of wood, but is made of many smaller planks that are glued together. The wood used for these planks is Sitka spruce – a wood that is lightweight and flexible, yet still very strong. Sitka spruce has a long, straight grain which allows for the quick transmission of sound vibrations. The planks are cut so that the direction of the wood grain runs the vertical length of each plank. The planks used are about ½ an inch thick, and 4 to 6 inches wide. They are glued together edge-to-edge in order to form one large sheet of wood (notice the diagonal planks in the picture at the right). After the glue dries, the large sheet of wood is cut into the proper shape, which must match the shape of the rim precisely. After it has been cut, the soundboard is then planed down to the proper thickness – usually only three-eighths of an inch. The top of the soundboard has a crown, or a slight dome, that curves up towards the strings. The crown allows the strings to set firmly upon the bridge by pushing up against the downward pressure being applied by the strings. The strings are pulled over the bridge at an extremely high tension - so high that over the course of many years, the downbearing force of the strings can cause the soundboard to lose its crown.