10 Oct 2023
“Structured sides” refers to a construction method for guitar and banjo sides. This construction creates sides that are strong and stiff, yet very lightweight. This is the method I prefer for both banjos and guitars.
I believe stiffness is very important in banjo rims. A stiffer rim will reflect more of the vibrational energy of the head rather than absorb it. This, in theory, should lead to a louder and clearer sound. A stronger rim should also last longer because, as we all know, tensioning hardware applies a considerable force to the rim and over many years can cause the rim to deform. I intend for these banjos to last a lifetime.
Structured sides are made of three layers. The two visible outer layers are thin veneers that are bent into shape using a hot pipe. Although I refer to these as veneers they are thicker than typical furniture veneers which allows them to provide structural rigidity to the rim. The inner layer is a kerf-bent piece of lightweight cedar. The layers are laminated together in a circular mold. When the rim comes out of the mold, it is strong enough to stand on!
I think of structured sides like an I-beam. When I studied engineering, we learned that in order to make a beam that resists bending, there must be as much mass as possible, as far away as possible from the center of the beam. An I-beam achieves this by being tall and having large flanges on its very top and very bottom. The middle of an I-beam is less important to the structure and can be made thin to reduce mass. In structured sides, the outer veneers are analogous to the flanges on an I-beam - they create stiffness. The middle kerfed layer is like the thin mid section of an I-beam - it is less important for stiffness and can be made to be very lightweight.
I learned about structured sides from my co-workers at the Mile End Guitar Coop in Montreal. My mentor Michael Kennedy has helped me piece together his brief history of structured sides.
We believe that the luthier Alan Beardsell was the first to build guitar sides using this method. He extended the traditional kerfed liners to span the whole height of the sides. At some point Beardsell showed the technique to Sergei de Jonge. Later on, de Jonge asked two of his apprectices, Michael Kennedy and Jeremy Clarke, to build a couple guitars using Beardsell’s method.
Kennedy and Clarke went on to found the Mile End Guitar Coop. A few years later, Clarke decided to try the technique again. While Beardsell’s sides where only two ply, Jeremy added another veneer on the inside of the guitar and flipped around the inner kerfed layer so that the smooth side was facing the outside of the guitar. Another coop member Dan (last name?) came up with the name “structured sides”.
Stiffer sides actually raise the air frequency of a guitar (see footnote 1). Clarke came up with the idea of using a tornavoz to lower the air frequency back to what it should be for a guitar (see footnote 2).
I think structured sides are a fantastic way of building banjo rims and I hope other builders take it up too!
Footnote 1: flexibility in a guitar’s body has a damping effect on the air resonance. This damping lowers the resonant frequency . If you were to model a traditional acoustic guitar as a Helmholtz resonator and try to calculate the lowest resonant frequency, you would get a number that’s higher that the actual guitar’s air frequency. Stiffening the sides pushes the resonant frequency up closer to the theoretical Helmholtz frequency. Source
Footnote 2: the tornavoz is actually a very old piece of guitar technology