Proceedings e report - Firenze University Press

STRUCTURAL DEFECT DETECTION USING ACOUSTIC HOLOGRAPHY – A PRELIMINARY STUDY
modification due to a default or a load will appear in [K] or in [M] and the solution x to the problem
will be different. For this reason, we have simulated a default in the harpsichord by adding a mass on
the bridge. As a preliminary experiment to simulate a structural defect, a mass of 110g was cautiously
deposited on one bridge (Fig. 7). The impact acoustic response of the harpsichord was measured in the
same conditions (hygrometry, temperature and string tension) as before in the limited field near the
keyboard (Fig. 7).
Fig. 7 Artificial added mass on the soundboard
Fig. 8 shows the frequency variations on the average level measured after the added mass. We are able
to detect variations induced by the mass addition. Indeed, the second mode around 80 Hz is lower in
the case of the mass addition than without. And we can see that a lot of resonance frequencies (marked
on the spectrum from the Fig. 6) are shifted of a few Hertz.
radiated pressure (dB)
60
55
50
45
40
35
30
0 200 400 600 800 1000
frequency (Hz)
1200 1400 1600 1800 2000
218
with added mass
without added mass
Fig. 8 Added mass effect on the radiated pressure spectra
Measured area
with added mass
4. Conclusion
These first experiments have shown the reliability of the acoustical holography in the cultural heritage
survey. The main advantage of this technique is the non contact aspect of the measurement. The
preliminary experiment of the structural defect simulation indicates that acoustical holography is a
very promising technique for the structural defect detection in vibrating structure. Future work will
include the evaluation of the detection limit.
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