User's Manual

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Version 1.4
5 Appendices
5.1.1 Correction Explained in detail
Leakage noise is generally a wild mixture of tones, each of which can be
associated with individual frequencies (tone pitches, oscillations). The
distribution of this frequency content is referred to as a spectrum.
First, the time series signal is recorded. These time series values can
undergo a mathematical transformation, Fourier analysis.
A particularly efficient computational technique of Fourier analysis is the
FFT (Fast Fourier Transform). This is used to calculate the frequency
content and, in particular, identify the presence of dominant individual
frequencies. Using this information, it is possible, for example, to draw
conclusions on disturbances (e.g., electrical machines).
Frequencies are specified in units of Hz (Hertz) as the number of
oscillations per second. For leakage noises, one usually finds a uniform
mixture of frequencies, primarily located in the range from 50 to 1000 Hz.
The knowledge of the spectrum, i.e., of the frequency response, is
particularly helpful for the computational use of filters, by means of which
one can separate the useful and the interfering portions of the noise
measurement spectra.
Correlation is a mathematical method for comparing two time series signals
with one another.
A leakage noise is simultaneously recorded by two sensors at different
locations. 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.
Leakage noise, however, is not a one-time acoustic event, but it is also not
completely monotonous. Small, irregular changes in sound intensity and
frequency occur. These changing signatures travel to the two receivers in
the same way as a hammer blow and may arrive at the sensors at different
times.
Because the identification and comparison of an individual signature is not
as simple as in the case of a hammer blow, the two time series signals are
first recorded for a period of time; the time series includes many individually
measured signal values.