Determining the Quality factor in tonewood.
“While acoustic testing is widely used to measure the strength of structural lumber, using it for grading tonewoods is a match made in heaven.”
Testing for Q
What is Q and why is it important? Q is the measurement of how quickly a sound signal decays when we induce a board to vibrate. What we find is that a high Q value is very important for tonewood, but, prior to this time, measuring Q has not been practically possible.
Q is measured in several ways, and is known to be difficult to assess accurately. It requires tight control of the moisture content and dimensions of the wood, as well as careful control of the input energy used to set the material in motion. This is where the science of acoustics comes into play. Pacific Rim Tonewoods selected the BING technology to sonically test and grade tonewood, for its robust algorithm in measuring Q, as well as overall ease of use.
Low Damping, High Q
From our research in Germany, we determined that low Q guitars are not preferred, regardless of whether they are made with low or high density soundboards. However, at the mid or high Q levels, the sound quality was markedly different from the low Q wood. This was a profound finding in determining how best to use the science of acoustics to sonically grade tonewood. If the wood is low Q, there is no point in fussing about the wood – the guitars sound similar. But, for the mid and high Q wood, the differences in the sonic profiles between guitars are much less significant, depending upon whether the wood is optimized by other criteria.
Ask the Expert
Q is an “inverse” measure of the damping, or internal friction, of a vibrating material; high Q = low damping. When any material structure is set to vibrate, some of the energy is transformed into movement, which can then vibrate the air around it and produce sound. But any movement is resisted by the internal friction of the material. That friction causes “loss”of the energy to heat, and contributes to the decay of the sound produced – fast decay in high damping wood, slow decay in low damping wood. A bronze bell, for instance, has high Q / low damping.
In spruce or other softwoods, most of the damping is due to the friction generated by the wood cells, or tracheids, sliding against one another as the wood bends ever so slightly during vibration. Spruce in general has quite low damping (High Q), a property it shares with other important tonewoods like Dalbergia spp. and Pernambuco. Low damping wood tends to radiate sound quite efficiently, all other factors being equal.
The particular technology we use is called BING—Beam Identification for Non-destructive Grading. This technology was developed at one of the world’s leading wood laboratories in Montpellier, France. The way it works is that we use a steel ball, dropped down a tube, so that the ball strikes the wood at exactly the same spot with exactly the same force every time. That signal is then picked up by a microphone, and a computer mathematically derives various physical properties which are used to determine density, stiffness and Q.
Higher Q wood is already responsive, whether it is high or low density; it is more active and excitable. Our blinded studies in Dresden, Germany, show that listeners prefer high and medium Q guitars, but that assessments of these two can be mixed, as to which is rated the higher. However, no one prefers low Q guitars.
Miroslav Tadic, musician, composer and luthier, tests a guitar built by Trevor Gore, featuring Sitka spruce that has been sonically graded.