After much internet research and reading of several great books on voicing and interaction with my students I have developed the process i now follow. The Audacity recording program is used to generate FFT analysis of tap tone at specific intervals throughout the build from free top to completed guitar. This information is recorded in my records and used to fine tune future builds of guitars of the same model or allow moving resonence peaks away from scale notes. For example a guitar geared to a heavy flat pick player will be voiced different than a softer fingerstyle guitar. Also I can create a darker or brighter sound or recreate a guitar sound from a previous build.
I will track this on the build of the Flourentine OM and present screen grabs of the FFT charts on this page. I am also quite happy to discuss this process with anyone interested as I am always learning.
The process starts with free top and back evaluation as i work the braces and thickness. The part is held between my fingers at a node point to get a good clean sound. Eight taps are recorded and the FFT graph is generated.
Next test comes after the top is glued to the sides and with the body clamped in the outer mold 8 taps are again recordeds and FFT generated. Included for reference is a note frequency chart
The next time measurements are made is after the body is completd and binding is installed. There are 4 steps in this, the sound hole and sound port if used are blocked and top and back taps are recorderd and FFT generated. Blocking these isolates the top and back from the body resonance so you get the actual frequencies seperated from each other.
Next the sound hole is opened up and the top is taped this will generate the air, top and back coupled frequencies. If there is a sound port it is now opened and test repeated. Usually the air frequency will increase around 4 hertz depending on the port area. The top and back can change slightly but often a change is not seen, The irst 3 highest peaks respond to Air/box, Top and then Back Frequencies. This is the last oportunity to easily modify the frequencies, using my saved data I can estemate the final frequencies of the guitar and adjust if needed.
Once the finish process is completed ready to glue on bridge and assemble the guitar the tests are repeated showing the changes the finish makes to the body frequencies.
Once complete and strung up the last test is repeated. The goal at this point is to not be too close to a scale note. The changes noted across the build can be oe help predict the final frequecies allowing changes to be made as needed. Below is the FFT chart of the guitar we have been testing. I have also included a chart of the note frequencies. If changes are required I can change the top or back frequencies but only in the lowering direction so I tend to peave the top and back slightly higher to allow this adjustment.
The frequencies recorded at each step of the construction can be compared and used in future builds of the same body size to make adjustments to avoid frequencies to close or on a scale note. it also allows me to change the general tone of later builds to match my goals for that guitar. Inthis example the top frequency at 184 hz is a bit too close to F# (only 1 hz) it could be easily lowered by shaving the top braces but is not causing any problems with wolf notes. Since the top will loosen up and lower it’s frequency as it opens up the desicion is to leave it as built.
I am always intersted to discussing the testing process so feel free to contact me.