Acoustic Emission Results and Spectral AnalysisDue to their complementarity in signal description, the fraction of highamplitude peaks in acoustic signal (>5000, in 16 signed bits coding), namelyRp+, and the average amplitude value of the sound recorded (AAS, measureof the sound intensity or “loudness”), computed during the last 10 s of thefirst compression step, were thought to be of particular relevance. Otherpossible indices for acoustic temporal signal description, as number of peaksand maximum amplitude, were highly correlated to the one or the other. Whenmapping the results of Rp+ obtained for all samples against those of AAS(Fig. 10), the acoustic signals of samples J and E showed low average amplitudes(AAS < 120), but more peaks of larger amplitude (Rp+ > 7%), i.e.,louder ones. Conversely, the group of samples I, A and D emitted fewer highamplitude peaks (<6%) and higher AAS (close to 180). Other samples showedsimilar values of Rp+ ratio as the latter group, but intermediate values of AAS,closer to the first group. Large SDs limit sample classification at this stage(Table 3). In line with the commonly admitted acoustic perception of crispness(Duizer 2001), the lower values of AAS of samples J and E are in agreementwith the stability of the size distribution of flakes, meaning that very fewpieces are broken in compression to 100 N. Conversely, the size redistributionduring the compression of other samples is due to numerous fractures reflected
by the larger number of acoustic events of moderate amplitude.
Spectral analysis has been performed on every sample. Normalized
power spectra are displayed for representative samples of each group formed by multidimensional analysis of sensory data (Fig. 11A–D). All samples display
a similar feature with rather uniformly distributed emission in terms of
frequency, including a gap for a frequency that is slightly higher than 5 kHz.
Only the average normalized power spectrum of sample J shows significant
differences with the other samples, mainly marked by a large peak at 1.7 kHz
(Fig. 11D). For further comparison, the principal component analysis is performed
on the acoustic spectra (Fig. 12A). The first dimension accounts for
56.7% of the variations, and the corresponding eigenvector is close to the
opposite of J spectrum, displaying typical low frequency emission in the
spectral domain of interest (Fig. 12B). The second principal component
clearly opposes samples E to L and G, by a contrast in acoustic emission,
principally in the first part of the spectral range for E (<3 kHz) and in the
second part for the other two samples (close to 7.5 kHz) (Fig. 12C). All other
samples were characterized by similar values of spectral dimension. The
relevance of the acoustic signal analysis in the spectral domain relies in the
evidence of a different behavior for sample J, which confirms a different
fracture mechanism.
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