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the design of the classifier. On the other hand, very often, the archaeologist does
not know the period of all the available specimens that could be used to form the
training set of observation: semi-supervised training is a requirement in this
application. Even more interesting is that the expert archaeologist can assign some
probabilities of classes (ranging from 0 to 1) to part of the pieces of the training
set, scenario that we will call PSS. Most of the semi-supervised classifiers are not
capable of dealing with PSS, only if the assigned probabilities to the labelled
feature vectors are 0 or 1. Therefore, we used the Mixca procedure explained in
Chap. 3
which meets the required conditions for this classification problem.
6.1.4 Experiments and Results
Two identical transducers (one in emitter mode and the other one in receiver
mode) with a nominal operating frequency of 1.05 MHz were used to obtain the
through-transmission signals. This operating frequency was selected after per-
forming different tests, as the most appropriate to achieve small ultrasound
attenuation with resolution enough to separate different kinds of ceramics. Sam-
pling frequency was 100 MHz and the observation time was 0.1 ms (10,000
samples) for every acquisition. To reduce observation noise 16 acquisitions were
averaged. The size of the transducers was also important since the ceramic pieces
were small (a few centimetres in height and length, and less than one centimetre in
width, see Fig.
6.2
).
The ceramic pieces were measured using a device where the ceramic piece is
placed between two cases that adjust to the curved surfaces of the piece (see
Fig.
6.3
). this device was implemented to perform controlled and repeatable
measurements, thereby improving the manual recording. A rubber adaptor was
used as coupling medium to match the acoustical impedance of the transducer to
the piece. The adaptor has a good coupling to the surface of the material and be
innocuous to the piece. The emitter is located in a case on the lower side of the
piece and the receiver is located in case on the upper side of the piece. Note that
the transducers are embedded into a case that has a pressure control that allows the
force that is applied to the material to be the same for each measurement. Since the
propagation velocity is an important feature for classification, the device has a
mechanism that allows that piece thickness to be measured and transmitted to the
signal processing system simultaneously with the ultrasound measurement.
The distribution of the pieces was: 47 Bronze Age, 155 Iberian, 138 Roman,
and 140 Middle Ages. Thus, a total of 480 pieces were used in the experiments
from deposits at the Valencian Community in Spain. The features were selected
from the features defined in
Sect. 6.1.2
. A total of 11 features were considered. The
first four were the time averages over the whole acquisition interval of the 4
ultrasonic signatures defined in Eqs. (
6.1
-
6.4
). The squared magnitude of the Short
Term Fourier Transform was used to estimate P
k
ð
f
;
t
Þ
.
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