Sonic Grading

Where art and science meet.

"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.”

Sound, Spruce and Science

Dresden Technical University in Germany has one of the most acoustically perfect anechoic chambers in the world. In a room the size of a small cathedral in which the floor, walls and ceiling are lined with two-meter-long foam wedges that effectively cancel any sound reflection, we recorded guitars meticulously built to the exact same specifications. 

The recordings made in the anechoic chamber were played back under precise conditions in Dresden Tech’s Multi-Modal Measurement Laboratory, an acoustics laboratory lined with 360 speakers in floor, walls, and ceiling. It was here we made a discovery that is revolutionizing the guitar industry—that the known variabilities and relative sonic differences in wood can be proved out. Read more about how this study was carried out.

Fast Facts

5 Years

of research

6 Labs

on 2 continents

18 Guitars

built to exact specification

58 Trained

listeners

Testing, testing, 1-2-3

For centuries, luthiers have worked with the natural variability of wood, and have adapted by modifying their design characteristics to “build to the wood.” In order to better facilitate that process, Pacific Rim Tonewoods conducted extensive research over five years, using guitars that were exactingly built using the same materials in the same way. The only intentional variable in the build was the density, stiffness, and damping (Q) of the soundboard. 

Through rigorous testing and analysis, the results proved out that we can accurately quantify the relevant acoustic properties of the soundboard and bracing. Luthiers still have to build to the wood, but can now actually specify with consistency, tonewood that has been graded by criteria that will allow the optimal tonal quality

We hope that luthiers will be able to utilize the acoustic properties of tonewood from PRT, and to influence their designs with assurance in the reliability of the resulting sound—for the next guitar they build or the next thousand.

Ask the Expert

BING (Beam Identification of Non-destructive Grading) is a software program developed at the University of Montpelier, France. It uses a single standardized impulse with steel ball to induce the spruce billet to resonate. The wood is flexibly supported at its nodes, allowing the board to vibrate freely. The impulse is induced on one end of the board, and a microphone on the other end captures the resonant frequencies and the sustain of the resulting tone. The program then uses these measurements to mathematically derive the properties of stiffness and damping. The process has been refined, tested, and perfected through several years of research.

The density of the wood is the most straightforward measurement of the criteria we test. Boards are simply weighed and the mass is divided by the volume (kg/m3). The density of our most common product, Sitka spruce, varies widely from 340 to about 560 kg/m3. Density is reported as an important variable in and of itself, but it is also used as a direct input into the mathematical formulas that determine the subsequent properties—stiffness and damping (Q).

The frequencies at which a material vibrates can be used to mathematically determine its stiffness, if the density and dimensions are known. This property, known as the ‘Dynamic Modulus of Elasticity’ or MOE, was originally developed by the forest products industry to determine the structural strength of lumber. It just so happens that these techniques are even better suited to measuring the acoustic properties of tonewood.

The combination of stiffness and density of a top and bracing can help to determine the natural resonant frequencies of a guitar top. In addition, stiffness and density interact in rather complex ways to determine the efficiency by which a guitar top will vibrate, and thus the loudness of given design of guitar. Our studies corroborate what skilled luthiers already know—there is a particular combination of density and stiffness for each guitar design that leads to an optimized tone in the finished guitar. It is thus important to know these variables prior to the build process. 

When a guitar string is plucked, the energy of the string is transferred to the soundboard, and the movement of the wood vibrates the air around it and radiates sound. However, any movement is resisted by the internal friction of the wood itself, a property known as damping.

Spruce in general has rather low damping, but as with stiffness and density, there is a great deal of variability in this property. Guitars built with spruce on the high end of the damping continuum will feature a rather “dull” tone that decays quickly and accentuates the fundamental frequency at the expense of the overtones. Guitars built with low damping wood tend to radiate sound quite efficiently, with a relatively “quick” response, and tend to be preferred in blind listening tests regardless of design or build. By convention, scientists express the damping coefficient as a “Q” factor, which is the inverse of damping – thus, low damping = high Q rating.

Jackpot Wood

In testing, there are certain boards that stand above the rest, for their high Q (low damping) and/or their high stiffness. These boards are separated into classes of their own and are exclusively sold in The Vault. When selecting these boards, you are able to view the actual density, stiffness, and Q values before you buy. These specialty products feature:

• High Q: Any board with a Q value of 160 or above is considered high Q. Approximately 10% of our sonically graded wood is high Q.

• High stiffness: A board exhibits high stiffness when the measured stiffness is more than 10 > above the average stiffness at its given density. Only about 2% of our sonically graded wood is considered high stiffness. We only recommend using high stiffness soundboards if you fully understand how the stiffness will affect your design.

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.

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