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8.6.5
Type of correlation
The
cross correlation
uses
channel 1 and channel 2 to measure;
the source of noise to be located must be within the measuring
section.
Auto correlation
uses either channel 1 or channel 2. The sec
-
ond signal required for correlation is produced by
reflecting the
noise to be located at a
reflection point. It must be received by
the channel in use.
As the energy from the reflected noise is usually very low, this
procedure is rarely feasible. The chances are usually better if
there are gaseous media in the pipe.
The position of the reflection point must be known for auto cor
-
relation.
The auto correlation function (ACF) always has a symmetrical
progression. Only the negative area of the ACF (delta t <0) is of
interest.
8.6.6
Sampling frequency
The continuous time signal of the channels used is scanned and
digitized at a certain frequency, the sampling frequency.
The lower the
sampling frequency, the less calculating time is
required for averaging. However, the sampling theorem requires
a sampling frequency which is at least twice as high as the high-
est frequency contained in the signal! If the sampling frequency
setting is too low, measuring errors may occur. For this reason,
the sampling frequency should only be reduced in exceptional
circumstances, for noises of very low frequency.
The sampling frequency is at the maximum with “automatic”.
Select “manual” for a manual setting. You can change the sam
-
pling frequency using the arrow keys.
All subsequent measurements will be performed at this sampling
frequency.
