Voicing with Frequency Spectrums

The book set Contemporary Acoustic Guitar Design and Build by Trevor Gore and Gerard Gilet provides a more scientific approach to voicing. With my engineering background, I appreciate the technical approach that bridges the link between science and practical methods that help focus my efforts toward learning to build a more responsive, full sounding guitar. I highly recommend this book set.

One of the key measurements is derived by tapping the soundboard with a miniature hammer while using a microphone and computer to record the resulting frequency response. Per the recommendation, I use an eraser attached to a wooden dowel as a miniature hammer to create the impulse. I use Sound Forge software to record and print the frequency spectrums. My current method is to tap five times in the middle of the bridge to capture the monopole response, tap five times on the end of the bridge to capture cross dipole response, and tap five times about one inch below the bridge to capture the long dipole response. This is a typical chart.

We’re interested in the frequencies recorded, not the magnitudes. I’ve observed that the frequencies are repeatable as long as the guitar is in the same orientation and supported in the same way.  The magnitudes of the response will vary based on the strength of the impact and location of the microphone.

A goal in voicing the guitar is to achieve a soundboard monopole response of 170 or 180 Hz, a corresponding apparent Helmholtz frequency of 90 or 95 Hz, and a back main frequency @ 214 or 226 Hz – 4 semitones above the soundboard.

Recorded frequencies of my completed guitars are listed below. These are “coupled” responses with open soundhole, strings damped, top and back undamped, and tapping on the bridge as noted above.

Based on these measurements, I’ve modified several completed guitars by thinning braces and soundboards, including my Cocobolo guitar, Malaysian Blackwood guitar, and Rosewood guitar with subsequent improvement in sound.

A second measurement is bridge rotation. Essentially, a long wooden bar (ruler) is attached to the bridge, parallel to the strings. Measurements are taken at the ends of the bar, then the strings are loosened and the measurements repeated. A change in angle of 2 degrees is about the right amount of flexibility of the top. Much less that 2 degrees indicates a too-stiff top. Much greater than 2 degrees indicates a top that is too flexible and will likely distort over time and may lack focus. A chart of measured bridge angles of my guitars is shown, along with two premium factory guitars that I own.

A third measurement isMonopole Mobility, defined as “a measure of a guitar’s responsiveness, being the mobility of the monopole mode of vibration of a guitar.” The physical measurements involved in recording monopole mobility include the deflection of the soundboard to a known force and the uncoupled natural frequency of the top. Good guitars are typically in the 10 – 14 x 10-3 s/kg range. A chart of monopole mobilities of my guitars are shown, along with a reference of Trevor Gore’s guitar with falcade bracing. Taller bars correlate with a more responsive guitar.

There are several other voicing aspects that I haven’t yet addressed in my efforts, including intonation of individual notes.

I continue to use Chladni patterns in my construction and voicing. Before cutting out the shape of the guitar from the rectangular plate, I use Chladni patterns to determine the long grain, cross grain, and twisting vibrational frequencies for the plate. With the plate dimensions, weight, and frequencies in a spreadsheet, I can establish the target thicknesses according to the formula provided in the book. I use the information to compare and select top plates. Then, after cutting out the profile of the soundboard and back, I reduce thickness until the top and back long grain vibration frequencies are 52 and 43 Hz respectively. This gives me a dynamic measure of the stiffness of the plates.

Then while tuning braces with the soundboard and sides clamped in a mold, I use Chladni patterns to observe the monopole, cross dipole, and long dipole vibration modes to give me visual insight into how the plates are responding.

After the guitar is completed, I use Chladni patterns to record the same vibration modes.