For a detailed description see paper:
SALI, Samo, KOPAC, Janez. Brace trimming for tone
improvement of a guitar. V: WICKS, Alfred L. (ed.), SINGHAL, Raj (ed.).
Proceedings of IMAC-XIX: a Conference on Structural Dynamics, Kissimmee,
Florida, February 5-8, 2001, (Proceedings of the International Modal
Analysis Conference & Exhibit, 19). Bethel, Connecticut: SEM, cop. 2001, vol. I,
p. 805-810.
THE EFFECT OF BRACE TRIMMING
It was hypothesised that changing the height of braces
would result in a different sound. The aim was to find such changes in the brace
height that result in an improved sound. All experiments were done with only one
test guitar. The variable was the front resonant board with two cross-grain
braces. Other braces were considered as part of the resonant board and were also
changed when the board was changed. All braces were glued with a glue that was
not resistant to heat. Thus, the braces could be removed with a hair dryer. The
inner side of the front resonant board was made accessible by reversibly
screwing the back board on the rim of the resonance box. The front boards were
glued on the rim. The trimming of brace heights involved three phases, ungluing,
planing, and gluing. All braces and resonant boards were made of spruce (Picea
abies Karst.): At 10% moisture content, the density was 451 kg/m3
(standard deviation: 7 kg/m3). The sketch of the resonant board:
SANDED RESONANT BOARD - SOUND QUALITY MEASUREMENTS
The average sound quality of the tones "F", "B", "g" is
denoted Qm:
Qm = (Q("F") + Q("B") +Q("g"))/3
The quality of the three tones is derived from the rule
of consonance-dissonance. The following four figures show the positive and
negative effect of lowering of brace B for 4 millimeters (from 18 mm to 14 mm).
Positive effect of trimming when brace A is held fixed: (a) Change in the
quality of the three tones from before (1st set of 3 bars) to after (2nd set of
3 bars) trimming of brace B; (b) Positive change of the average tone quality
- from
before (1st bar) to after (2nd bar) trimming of brace B:
An increase in Qm was mainly a consequence of improvement in tones
"F" and "B". The quality of tone "g" either increased, decreased or remained
unchanged.
Negative effect of trimming when brace A is held fixed: (a) Change in the
quality of the three tones from before (1st set of 3 bars) to after (2nd set of
3 bars) trimming of brace B; (b) Negative change of the average tone quality
- from
before (1st bar) to after (2nd bar) trimming of brace B:
PLANED RESONANT BOARDS - SOUND QUALITY MEASUREMENTS
Exactly the same experiments as described above for the single test guitar were
also performed with four planed resonant boards, with dimensions and shapes
equal to those of sanded boards. In contrast to the sanded boards, however,
trimming of the brace B never improved the average sound quality of the three
tones, even when the initial sound quality was bad. After the trimming, the
sound quality remained equal or became even worse. A schematic presentation of
the comparison of sanded and planed resonant boards (tops) is shown in the
following figure:
CONCLUSIONS
Our criterion for determining the quality of the guitar sound is a result of
objective sound measurements and analysis of bad and good
classical guitars. The criterion, the rule of
consonance-dissonance, was expressed in a mathematical form and interpreted
in terms of the physical and musical theory. The essential difference between
the bad and good timbre of a guitar tone is in the relative contributions of the
consonant and dissonant intervals in the frequency spectrum of the tone.
We proposed and tested a procedure for improving sound quality of the guitar
sound; it is based on the assumption that only three tones are enough to
describe the guitar's register of tones. According to this procedure, the
improvement can generally be achieved by trimming of a certain brace on the
front resonant board. In order to refine this procedure, it will be necessary to
perform additional experiments exploring various variables, such as the test
guitar, resonant boards, braces, wood, etc. Planed boards behaved differently
from sanded boards, although sound quality of tones on a test guitar with planed
and sanded boards was similar. It is certain that the cutting process strongly
influences the acoustic properties of a wooden plate. This suggests that a
comprehensive analysis of acoustic properties of an instrument must include the
cutting process as a variable, not a constant. The most probable reason for the
different behavior of the differently machined boards is to be sought in the
differences in their surface layers. Considering that the thickness of tested
boards was only 3 ±0.10 mm, it was clear that the shape of the thin surface
layer was important. It is well known that the surfaces of sanded and planed
boards are different. Sanded boards have torn fibers whereas planed boards have
chopped fibers. Tearing of fibers damages the integrity of the surface much more
than chopping. The different shape of surface layers results in either or both
of the following consequences from the following figure (a):
The above figure (b) shows the dependence of the damping of sound radiation on
the sound wave resistance for different wood species and other materials. The
presented relations confirm that in sound boards of musical instruments, low
damping due to internal friction w
and high damping due to sound radiation J
are desirable. From the above figure we can see that the ratio E/r
is most advantageous (high) for planed boards and bad (low) for sanded boards.
This is reasonable because the planed boards have a surface with smooth chopped
fibers that have a higher strength (higher E) in comparison to sanded
boards. It is, however, more difficult to discuss the effect of density on the
acoustic properties because we do not know the differences in the density of
surface layers of the board. For a thorough analysis of the role of surface
layers density we would need to know the exact cutting forces on the board,
which is impossible for sanding.
