2015 – Rosewood / Sitka OM

Description

This guitar is the first of a series of two OM guitars constructed with traditional Martin style bracing. The soundboard is Sitka spruce, back and sides are EI Rosewood, curly maple binding and back strip, and a rosewood rosette with double abalone rings. It has a five piece neck with the center strip of mahogany. The fretboard and headplate are ebony, with abalone diamond inlays in the fretboard. The finish is French polish.

Voicing

I over-thinned the top on this guitar. For the past several guitars I have thinned the perimeter of the soundboard to improve the mono-pole response. In this guitar, I made the mistake of using an RO sander to try to achieve a gradual taper. I ended up with top thickness of about 2mm for about 3 inches around the perimeter. Apparent Helmholtz frequency dropped to 85 Hz and monopole response was below 165 Hz. The guitar sounded good with light fingerpicking but lacked crispness in the low end.

To correct the issue, I made of map of soundboard thickness using my homemade Hacklinger gauge. I then used the thickness map to place eight strips of spruce inside to stiffen the top. I also thinned the bridge to 9mm reducing the weight to 19g and replaced the 4.5g bridge pins with 2.7g boxwood pins.

These corrections helped. I can’t say that it’s my best guitar but it’s one that friends and I gravitate toward when choosing a guitar to play.

These images show the monopole, cross dipole and long dipole resonance patterns after completion, along with the coupled frequency spectrum.