Proceedings e report - Firenze University Press

STRUCTURAL DEFECT DETECTION USING ACOUSTIC HOLOGRAPHY – A PRELIMINARY STUDY
• except for the excitation process, it is a non contact method
• since an important number of grid points (120) can be measured at the same time, the number of
excitation impacts on the structure is strongly limited.
• The measurement time is comparatively very short.
In the present study it took less than 120 mn for recording the set of 15360 point impulse responses to
be processed.
3.2. Experimental set-up
The impulse response of the harpsichord soundboard is measured in the semi-anechoic chamber at the
Musée de la Musique (Fig. 4 (a)). This experiment is described in [5]. In such a room noise level is
seriously attenuated. This condition allows minimizing the soundboard excitation level and also
optimizing the signal to noise ratio for evanescent waves.
Since the instrument has to be kept under tension, the strings are muffled in order to eliminate their
sound production (Fig. 4(b)).
(a) (b) (c)
Fig. 3 NAH experimental set up: (a) holographic microphone array, (b) string muffling (c) impact hammer
The impulse response is calculated from the measured harmonic acoustic nearfield using a NAH
reconstruction process. A point impulse excitation of the soundboard is provided by an automated
hammer driven by an electromagnet that produces a reproducible impact (Fig. 4(c)). The excitation
position is chosen on the underside of the soundboard. The keyboard is therefore removed. The
position of the impact is chosen so as to mobilize significant flexural vibration modes of the
soundboard.
A 12 by 10 electret microphones array, with a 50 mm step, has been used to collect the pressure field.
So as to fit the measurement grid, the array is moved into 8 positions. For each of these positions the
array is also moved according to 16 interleaved positions so as to refine the measurement step grid to
12.5 mm. The 120 impulse pressure responses for each position of the array are collected using a
home-made 128 channels synchronous digital recorder. Each measurement associated to one impact
on the soundboard has to be phase referenced by systematically recording, along with the acoustic
signals, the constant impulse response of an accelerometer positioned on the soundboard. The
resulting acoustic impulse response field is measured over a parallel plane at a distance zh = 72 mm.
This unusually large distance for NAH has been imposed by technical reasons of accessibility. The
field is finally sampled according to a 1162.5 by 1762.5 mm rectangle grid with a 12.5 mm step. The
different sets of measurement finally count 13348 point acoustic impulse responses (Fig. 4).
3.3. Radiated sound pressure
On Fig. 5, we present the pressure level, averaged on the whole measurement plane, from 0 to
2000Hz. This graph shows a significant radiation level for frequencies between 140 Hz and 2000 Hz.
An important modal density in the usable [0 2000Hz] bandwidth can be observed.
The frequency response depends on the geometry, the material, the limits condition, and on the
environmental conditions during the experiments. This frequency spectrum is the response of the
structure for a mechanical state: it is a sort of the identity signal of the harpsichord.
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