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Optimal tuning of classical guitar
part 2: balancing the bend

by Dr Tom Chalko

The increase in sound projection of modern classical guitars has been achieved by thinning guitar tops and their bracing, resulting in more fexible (and also more fragile) top membranes.

Fig 1. below illustrates the key problem. Forces of tensioned strings and force that holds the bridge attached to the guitar soundboard top do not act in the same plane. These forces produce the bending moment F*h that bends the guitar top.

Since timber and other composite materials used for guitar tops are not perfect springs, bending deflections caused by this bending moment increase in time and become permanent.

Symptoms of this bending include

  • guitar becomes out of tune in high positions, because bridge tilts, which reduces string lengths
  • Sound response of guitar becomes compromised, because soundboard that is subject to bending load no longer acts as a "optimal sound-projecting membrane".
  • guitar and its tuning become increasingly sensitive to temperature/humidity changes, because timber under bending load is more sensitive to these changes than timber under pure tension or compression.

Solution

My solution involves installing 2 carbon-fibre tubes 4mm in diameter through guitar soundhole as shown in Fig 2. Total weight of these tubes is about 4 grams.

The vertical tube is installed in a 4mm hole through the bridge, missing bracing beams and string-tying holes.

The horizontal tube, parallel to strigs, is installed under compression to produce force B on the tip of the vertical tube approximately equal to B=F*h/H (see Fig 2 for notation). Silicone tubing "tube feet" increase friction that holds loaded tubes in place.

 

After I installed carbon tubes in my Juan Hernandez 2012 Luthier model guitar I held the guitar without strings for about 4 weeks to let the timber "unbend" itself. After I installed strings - the guitar top remains unbent.

Prior to "bend balancing", the Hernandez guitar was powerful, but required a soundpost to control "wolf notes" on G and B strings. After "balancing the bend" the soundpost has become redundant. The acoustic response of the instrument has become extraordinary. I could not stop playing it for several days, ~10 hours each day. "Bright" sound typical to "lattice-braced" guitars has become very rich and controllable, like in best guitars. The longer I played the guitar - the richer sound it produced. It seemed as if the timber was "learning" to vibrate after it was relieved from bending load. Photo 1 shows installation details.

Then I implemented the "bend balancing" system to my second guitar Yulong Guo Chamber Concert 2015 double top (timber and nomex laminate top). This guitar has almost no bracing (see Photo 2) and after 12 months of playing - the increased top deformation has caused the bridge to "raise" and alter the guitar "action".

Effect of "bend balancing" on my double-top guitar has also been impressive. This is now my preferred guitar for recording, because it has ecceptionally powerful and yet uniform response in all registers.

On the basis of my testing, the "bend balancing" method described in this article has potential to become a new method of restoring sound to older guitars and preventing deterioration of new and old guitars.

It is quite likely that the "bend balancing" technique can open new frontiers in guitar design and construction with luthiers pushing for even thiner tops and adventurous bracings and using pre-load in carbon tubes to "optimize" performance of their guitars.

Carbon-fibre tubes are removable. You can also choose to balance "part" of the bending moment (50% or 80% for example).

Instead of patenting I decided to share my method freely, but I would appreciate acknowledgment of its implementation. Please contact me for more details.
 


 


Photo 1. "Bend balancing" system of carbon tubes inside Hernandez "Luthier" guitar. Note "lattice" bracing. On the outside the carbon tube entry is barely visible.

 


Photo 2. "Bend balancing" system of carbon tubes inside Yulong Guo Chamber Concert double-top guitar. Note minimal bracing. On the outside the carbon tube entry is barely visible.

 
 

 

Fig 1. String load F and the counter-acting "bridge holding force" F act in different planes, producing the bending moment F*h that causes the guitar top to bend. This bending (shown above schematically as a red line) accumulates over time

Fig 2. We can reduce the bending moment applied to the bridge and the guitar soundboard top to ZERO if F*h=B*H. The tension force in the guitar top behind the bridge increases by the value B=F*h/H, which is typically equivalent to the tension force of one string. Increased tension in the guitar soundboard top behind the bridge and elimination of bending load noticeably improve guitar sound projection and articulation.