Make Better Guitars: Understanding Damping or Q

What is Q?
You have probably heard about "Q" when talking about wood properties. Well, what does it mean? The Q factor is a measure of how much vibrational energy is retained in a system vs the energy lost from the system, per cycle of vibration.  In the context of guitar soundboards, (both tops and backs), High Q wood retains more of its vibrational energy for producing sound, rather than it being dissipated as friction and heat in the wood itself.  So high Q wood vibrates both longer and louder than low Q wood.  Because there are more cycles per second in high frequency sound, low Q wood will lose higher frequencies (the higher string harmonics) more readily, producing a “warm” or even muffled tone, rather the brightness that continuing high harmonics impart.  Variations in Q do not affect the natural resonant frequencies of the soundboard, but rather the decay and harmonic content of each note.

Why is this important for luthiers?
What matters to guitar makers is that they choose the Q of the wood to match the performance required from the guitar.  Acoustic finger picked instruments, both classical and steel string, generally benefit from the use of high Q wood, as a skillful player has a broader spectrum of sound “colors” to choose from.  But that comes at the cost of needing to have excellent technique, because the guitar will produce more sound, including the sounds you don’t want to hear, like picking and fretting noise associated with less-than-best technique.
At the other end of the spectrum, lower Q woods are excellent for stage guitars with a sound reinforcement system, as they are much less prone to acoustic feedback.  After all, you want to build a guitar, not a microphone.
Understanding Q and how it relates to the instruments you build is a critical part of a good luthier’s skillset. It is important that as luthiers we choose the Q factor that helps us achieve the tonal results being targeted. 
Below are a couple of videos that feature luthier and guitarist Miroslav Tadic comparing the sonic resonance of identically constructed classical and steel string guitars. These guitars were built by renowned luthier Trevor Gore, using Low and High Q sonically-graded soundboards from Pacific Rim Tonewoods. Can you hear the difference between the Low and High Q soundboards used on these guitars? 

 
Until recently all tonewood has been graded by suppliers based on visual aspects. Pacific Rim Tonewoods is one of a few suppliers that is now testing wood to determine Q and other properties. This sonically tested wood is now available to us luthiers and can be used as a tool to build consistently better instruments. There is a wealth of information available on the PRT site that will help you understand more about Q and their sonically grading process,