For a detailed description see papers:
-SALI Samo, KOPAC Janez.
The influence of
different machining processes on the acoustic properties of wooden resonant
boards. CAS j., 1998, vol. 3, no. 5, series II, p. 15-21.
-KOPAC Janez, SALI Samo. The frequency response of
differently machined wooden boards. J. Sound Vib., 1999, vol. 227, no.
2., p. 259-269. (URL):
http://www.idealibrary.com (article no. jsvi.1999.2246).
How important is the machining process for mechanical (acoustic) properties of
the resonant board for the classical guitar?
The influence of the machining wooden resonant boards for guitars on the
theoretical acoustic properties of this instrument has been studied.
Square-shaped spruce boards (Picea abies Karst.) were selected to represent a
typical portion of the guitar resonant board. Three different machining
processes were used to prepare the test specimens: planing, sanding and milling.
Vibration of the specimens was initiated by impacting them with a small wooden
ball. The resulting oscillations, measured by an accelerometer mounted on the
board, were processed by a frequency analyser. The measured response of the
differently machined boards was analysed statistically in terms of amplitude and
damping of fundamental frequency, and power spectrum, in order to distinguish
between the different acoustic properties of the boards. For the given material,
board shape, and board fixture, it was found that the selected type of machining
had a strong effect on the vibrational, and thus acoustic, properties of the
tested boards, as illustrated by the figures below.
There is a significant difference, after the ball's impact, between the
acceleration of a planed wooden plate and a sanded wooden plate. This difference
is significant at the statistical rejection level better than 5%. The
acceleration corresponds to the fundamental frequency:
There is also a significant difference among the planed, sanded, and milled
wooden plates in terms of the strongest components of the acceleration power
spectrum:
Conclusions:
1. All three tested cutting processes and their technological parameters were
suitable for generating a good surface quality of the specimens made from two
types of wood. In one type of tested wood (spruce, seasoned for 5 years) the
roles of feeding speed and depth of cut in planing were estimated. The
comparison of planing, sanding and milling was performed with specimens made
from another spruce, which was seasoned for 30 years. Sometimes acceleration of
differently machined specimens differed significantly, which was proved with
statistical tests.
2. The cutting process affects the average modulus of elasticity and probably
also the average density of wooden boards.
3. The significant correlation between the cutting process
and the theoretical acoustic properties of a wooden resonant board is
established and explained by the modulus of elasticity and density.
4. Experiments with both types of wood showed superiority of planing with low
feeding speed and small depth of cut.
No implications for guitarmaking could as yet be made. Perhaps the observed
correlation is not even important for making resonant boards of real guitars.
Only future experiments can help us understand the interplay between the cutting
process and the acoustic properties of the resultant resonant board. However,
the current results and experimental set-up provide a good starting point to
address this larger question.
