Musical instrument makers and physicists have studied the acoustical properties of wood for many years. They have determined that different species of woods sound out differently when struck or vibrated. For this reason, wood selection is critical for the best sound out of an instrument. Furthermore, it was also discovered that the age of the wood affects the sound as well. Musical instrument makers, called luthiers, spend countless hours looking for just the right piece of wood, since using the wrong piece will make the instrument sound terrible.
Different species of woods have different densities. For instance, spruce is a lighter-density wood than maple. The wood's density affects how sound waves travel through the wood. Lower-density woods absorb higher-frequency sound waves more than lower-frequency sound waves. The end result is when struck, only lower-pitched sound waves come out. The opposite is true for higher-density woods. According to Encyclopedia Britannica, higher-density woods allow higher-pitched sounds to escape out, but trap lower-pitched sounds. For this and other reasons, luthiers use spruce for violin tops, but maple for violin backs.
By and large, softer woods absorb sound waves at a greater rate than harder woods. The end result is sounds are muffled coming out of a softwood, but brighter coming out of a hardwood. This is neither good nor bad, but rather the sound is altered, depending on the type of wood. Luthiers understand the sound reflection factor, and use a soft wood for the top, and a hardwood such as maple for the back. The emanated sound waves from the top reflect off the bottom. You want a highly-reflective hardwood at the bottom, so maximum volume comes out of the sound hole.
Every sound makes wood vibrate. Within a certain narrow frequency range, the wood vibrates a great deal. This is resonance, and every piece of wood resonates at different frequencies. Luthier Jeremy Locke explains that luthiers tap wood, listening for sounds coming out. Luthiers listen for the resonant properties of a certain piece of wood, and pick the piece with the correct sound qualities. It's important to note that even within the same species, two different boards can have different resonant qualities.
Old musical instruments somehow sound "better" than new musical instruments. The age of the wood influences the acoustical properties a great deal. Much scientific research has been done into this phenomenon, and in 2011, research is ongoing. Researchers have discovered that as wood ages, the internal sap hardens. This affects the sound emanating or reflected off the wood. Another reason is the moisture content, and how old wood absorbs moisture from the air. Researchers at the California Institute of Technology, citing violin experts David Boyden et al., state that a component of wood, called hemicellulose, degrades over time. As it degrades, the internal moisture content decreases, which in turn affects acoustical properties.