Proceedings e report - Firenze University Press

STRUCTURAL DEFECT DETECTION USING ACOUSTIC HOLOGRAPHY IN THE
CULTURAL HERITAGE FIELD – A PRELIMINARY STUDY
Sandie Le Conte 1 *, Sylvie Le Moyne 2
1 Laboratoire du Musée de la musique. Paris, France.
2 UPMC Université Paris 6, Institut Jean Le Rond d'Alembert, CNRS UMR 7190. Paris, France.
Abstract
We are now able to measure a dynamic property of the wood [1], the loss angle through the whole audio
frequency range (20 Hz- 20 kHz), when the wood sample is excited with a continue vibration (simulation of the
playing effect). But its disadvantage is that it’s a destructive technique because we need a wood sample. So, in
addition, to know better the dynamical behaviour of a whole wooden structure we use holographic acoustic.
After preliminary acoustical holography experiments realized on a 17 th century harpsichord, a detailed study has
been carried out to measure the wooden structure ageing using this technique. The main objective of this
measurement is to control the stress on a soundboard before the rupture due to string tension, due to vibration
when the musical instrument is played, or due to low hygrometric variations. Indeed, all these modifications in
the structure will modify dynamical response of this structure. We show that this technique is able to detect a
simulated structural defect in the soundboard, by measuring a shift for a few resonance frequencies.
1. Introduction
The Musée de la musique laboratory is carrying research to improve the knowledge of the mechanical
phenomena related to conservation and restoration of the soundboards of musical instruments. Indeed,
the soundboard is one of the most important elements in the sound production of stringed instruments.
Musical instruments preserved in the museum are cultural heritage objects, and not only their material
integrity, but also their sound production function should be preserved, when possible. This goes with
the conservation of the vibrational properties of the wood of the soundboard (Young's modulus, loss
angle and resonance frequencies). Particularly, it is interesting to understand the mechanical effects
due to string tension and/or to hygrometric variations on load structure as the soundboard. How is it
possible to detect cracks, or loading variations on the soundboard through vibrational analysis without
contact? Any structural modification of the vibrating element induces a variation in its acoustic
response.
The playing conditions for a musical instrument induce quick hygrometric variations for the wood. In
fact, the proximity of the musician generates an increase of temperature and humidity in the musical
instrument's environment. Preliminary viscoelasticity measurements of the variation of the loss angle
with hygrometry changes have shown that this property is very sensitive to water content in wood
(Fig. 1) ([2]).
Loss angle variation (%)
Time (min)
Fig. 1 Water content effect on the spruce loss angle at 300 Hz . The grey squares are for a dried sample, and the
black ones for a sample conditioned at 12 %
* E-mail: sleconte@cite-musique.fr
Joseph Gril (edited by), Wood Science for Conservation of Cultural Heritage –Braga 2008: Proceedings of the International
Conference held by COST Action IE0601 (Braga - Portugal, 5-7 November 2008, ISBN 978-88-6453-157-1 (print)
ISBN 978-88-6453-165-6 (online) © 2010 Firenze University Press