Optimal tuning of classical guitar
part 2: balancing the bend
by Dr Tom Chalko
The increase in sound projection of modern classical
guitars can be achieved by thinning guitar tops and their bracing,
resulting in more fexible (and also more fragile) soundboard 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
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 sound is not as good as when it was "new"
- 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
- 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.
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).
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 soundboard"unbend" itself. After I installed
strings - the soundboard remained unbent.
The acoustic response of the instrument has become much better.
I could not stop playing it for several days, few hours each day.
"Bright" sound typical to "lattice-braced"
guitars has become rich and controllable, like in best guitars.
Then I implemented the "bend balancing" system to my
second guitar Yulong Guo Chamber Concert 2015 double top (cedar
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
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.
Pre-loaded carbon tubes allow otimisation of
guitar bracing structure from the pont of view of the final result:
the sound performance of the
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.
String load balancing allows "Structural
tuning of guitar" which can optimise acoustic performance
and reveal full potential of the instrument.