Barras harmónicas O tema das barras harmónicas é tão extenso que merece um livro, contudo vou simplificar algumas das ideias principais sobre barramento de tampo e o que uso para estes ukuleles e cavaquinhos, a forma em X
Consideraçõs estruturais
Os tampos e as costas são afinados afim de serem mais responsivos, mas têm de aguentar uma enorme carga de tensão. Remember that 200 lbs of string tension on the neck? Well that tension is more or less being split between the neck and the bridge. Imagine the whole guitar wanting to fold upon itself like a taco! The first thing the bridge has to do is spread that tension out to the sides to keep itself from ripping off the soundboard. The bridge achieves this through a system of trusses or “braces” whose primary goal is spreading this load out to the sides. This is the job of the X- Braces. At the same time as the bridge wants to rip off the soundboard, the tension on the neck creates a torqueing effect on the fretboard tongue, which places some of the tension dealt out to the neck squarely onto the upper bout region of the soundboard. The Upper Transverse Bar and the Fretboard Graft spread out this load. The Soundhole Braces connect the Upper Transverse Bar and the X-Braces so that the load from the fretboard tongue and the load from the bridge are connected to the bracing system as a whole. The Soundhole Braces also serve to strengthen the soundhole area because, well, there’s a giant hole there! Another goal of the bracing system is to keep splits in the soundboard, sides, or back from spreading, if they happen. That is why braces always run either perpendicular to the grain or at an angle to the grain. Also notice that there are no large open areas without cross-grain bracing. Acoustic Considerations The X-braces carry not just the tension out to the sides, but also the sound. Every time a string is plucked, the sound travels through the dense material of the saddle, the hardwood bridge and the hardwood bridge plate to the softwood of the X-brace, where the tone spreads out to the rim, the finger braces and the aptly named Tone Bars. The entire soundboard is activated.
The primary purpose of braces, both on the sound board and the back of the instrument, is to add structural integrity. A limited tuning range is a by-product not the primary purpose. A top for instance, is under a great deal of tension for the entire life of the instrument. In order to get quality sound out of a top it must be relatively thin. This creates a dilemma. The top is quite thin yet asked to support a great deal of tension. To met the challenge braces are added to the under side of the top and back to give it structural integrity.
In order make the top strong without adding an inordinate amount of weight a very light but strong wood is used – spruce. Spruce was used to make planes at one time for this very reason. Its has the best strength to weight ratio of just about any wood. This light weight is important on the top where the mechanical energy of plucking a string is translated to sound by a vibrating plate. Spruce is used for braces regardless of choice of soundboard. Back braces are typically made of mahogany or Spanish cedar. I have seen then also made of spruce as a well as a few other light but strong woods. Since the back is not under tension like the top is you have more choices. The back braces are mainly for strength, shape and to keep the back rigid.