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3.2.5 Correlation Graph
Figure 26: Correlation Graph
Correlation is a mathematical method for comparing two time synchronized signals with one
another.
A leakage noise is simultaneously recorded by two sensors at different locations which are
represented by the black lines at either side of the graph if the pipe data is known. The sound
emitted by the leak spreads in the water pipe at a defined sound velocity. If the acoustic
event were to be brief and occur only once, e.g., a blow with hammer, it would be simple to
measure the arrival at both receivers. In this case, one would only need to compare the
arrival times at the two receivers with one another. If these are the same, the sound has
traveled the same distance to the receivers, i.e., the sound source lies in the middle of the
pipe between the receivers.
If the arrival times are different, it is possible to calculate the distance to the two receivers
from the measured time difference. The sound arrives at the closer sensor before the more
distant sensor.
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