User's Manual

ZONESCAN net 56 / 56
Version 1.4
The two time series signals are mathematically compared with one another,
i.e., correlated with one another. If the leakage sound arrived at the sensors
simultaneously, the same signatures are compared and the signals are
then similar and the correlation is high.
In general, however, the respective signatures arrive at the sensors at
different times and simultaneously recorded signals do not correlate with
one another. Because the signals were digitally recorded and stored, it is
possible to retroactively shift the signals with respect to one another and re-
correlate them with one another.
This is performed systematically. From the results, one obtains a
correlation curve that includes the individual correlation values over the
respective computationally performed time shift.
If, during this process, one reaches precisely the time shift that corresponds
the propagation time difference from the sound source to the two receivers,
the signal signatures match and one obtains a correlation maximum at this
point.
Because this is the comparison of two different signals, it is also called a
cross correlation.
In the context of leakage noises, the goal of the correlation measurement is
primarily to determine the propagation time difference of two signals.
Because the correlation curve also includes spectral information, it is often
of interest to define the character of the leakage noise, particularly for
distinguishing noises that are not related to the leak, provided this can be
detected by means of a frequency analysis (e.g., electrical noise). The
correlation curve can also be mathematically filtered for the purpose of
retroactively separating such interfering noise to better identify the
correlation maximum caused by the leak.