It may be reasonable to consider the guitar as a machine. The top being a moveable piston inside a musical air pump. If you decrease or increase the the size of the hole you can tune the fundamental frequency at which the top vibrates. For example, if we increase the size of the sound hole we can usually see a rise in pitch due to a decreased mass in the center of the piston.
The pistons energy comes from the vibrational energy from the strings which vibrate in a circular motion from the nut down to the saddle. Every luthier strives to craft a pleasing and inspirational voice by converting the kinetic energy from the vibrating strings into kinetic energy of the piston, which in turn results in the kinetic energy we describe as tone.
Spruce (Picea abies) has been used for tops on stringed instruments for hundreds of years. It was considered ideal because of the stiffness along the grain and overall flexibility across the grain. North American parent species of Spruce include Sitka,Engelmann,White, Adirondack and Lutz. Sitka Spruce is favored most widely for guitars due to it’s massive size of the boles of the harvested trees, which makes it easier for luthiers to obtain perfectly quatersawn sections for use in instruments tops. But Spruce is not the only popular top to acoustic stringed instruments. In 18’th and 19’th century Europe guitar makers used frequently used Pine,Cedar,Mahogany,Redwood for guitar soundboards. Koa and Fir are also widely used as a soundboard for guitars.
Instrument tops are either carved into an arch shape, which is self supporting like an Arch Top or Violin, or are flat so that the top must be supported by braces underneath. For ArchTop guitars the starting block of bookmatched wood is usually around 6 cm. Bookmatching is a desired characteristic of acoustic instrument tops because the two nearly identical panels will share exactly the same vibrational characteristics. Bookmatching tone-wood gives bilateral symmetrical sound waves which allows the two sides to work in unison rather then fighting with each other.
Carved top instruments rely on their architecture for strength and do not loose strength as they get older, due to the convex surface supporting the strings. The same is not true for flat top guitars. The tendency for all flat-topped guitars is for the bridge to fall into the sound hole.
Carved Top instruments have thickness patterns in the carvings that are specifically designed to allow the portion of the guitar between the F holes to vibrate. The thickness of the top is graduated in a very precise way to allow for the thinnest part of the top to serve as a fulcrum for sound vibrations. The soundboard on a carved top instrument is actually quite thick under the bridge then thins out about 2mm before the F holes, and then thickens again near the edge where the top is bound to the top.
Flat top soundboards are very different. The glued bookmatched pair of panels will produce a very large,very strong,very weak,very flexible soundboard. Guitars use only quatersawn wood. The reason been that the tiny tubular cells that make up the body of the tree run up and down the tree (cells are called tracheids in softwoods) In hardwoods there are tracheids but they are mostly vessel cells, which are generally much larger then tracheids and have a different anatomy. These cells are supported by thick walls containing lignin,hemicellulose and cellulose. Once the wood is dried and cut into lumber it continues to gain and loose water forever, but the swelling and shrinking is almost entirely concentrated in the thickness of the cell walls.
For a musical instrument, shape change is lethal. If the top,sides,back,bridge,neck, or fingerboard warp the instrument is useless. Cheap manufactured guitars solve this problem by using plywood; which has layers and layers of wood alternating orthogonal layers that prevent movement.
So for all good guitars, everything, every single piece is quatersawn in order to build a strong, great sounding and long lasting instrument. The other property of wood that forms the top of an instrument is the width of the annual rings produced by the tree when it was alive.
Fast growing soft wood tree produce weak wood because each annual ring is big, therefore the cells are large which gives the wood a low density. Low Density=Low Strength. Slow growing softwoods have small annual rings with thick walls which results in a denser and stronger soundboard.
Wood with wide rings will be softer and less stiff and hence have a more mellow, bass rich tone. A dense top will be more stiff and have a more defined treble sound.
A lot goes into finding the perfect top because it’s role is absolutely crucial in creating a pleasing instrument for generations of enjoyment and inspiration.
(source: Storyteller Guitar by Doug Larson)