Acoustic guitar with a glossy finish placed under a microphone, set against a soundproof room with foam spikes, creating a serene, studio ambiance.

Understanding Q

Determining the Quality factor in tonewood.

Since the days of Stradivari, spruce has been used as an ideal tonewood.

Spruces have low density and high stiffness when compared to most other woods. These characteristics make spruces especially suitable for stringed instruments, but when combined with their low damping (high Q), spruces really shine.

Close-up of Miroslav Tadić playing a classical guitar, overlaid with the text “In Conversation: The Science Behind Building Better Guitars.”

"Now for the first time, we can accurately quantify the relevant acoustic properties of soundboard and bracing, and thus grade tonewood by the criteria that will allow the optimal tonal quality.”

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, both for its robust algorithm in measuring Q, as well as its overall ease of use.

BING sonic grading system analyzing a tonewood sample at Pacific Rim Tonewoods, with spectral data shown on screen and stacks of labeled wood nearby.

A person testing a row of guitars in an anechoic chamber lined with acoustic foam wedges.

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,in guitars with soundboardsat the mid or high Q levels, thesound quality was markedly different from soundboards with low Q wood. In short, if wood has a low Q, there is no point in fussing about it – the guitars sound similar and though they can be good they can't be great.But, when guitars are built using either mid or high Q wood,they will sound better. The differences in the sonic profiles betweenthigh and medium are much less significant, depending upon whether the wood is optimized by other criteria.

Frequently asked questions

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.

SOUNDOFF

Miroslav Tadic

Miroslav Tadic, musician, composer and luthier, tests three guitars built by Trevor Gore, featuring Sitka spruce that has been sonically graded as Low, Mid and High Q.