User's Manual

34 I 72
During noise monitoring, the noise level is repeatedly measured in
intervals of a few seconds. During a one-hour measurement
period (e.g., from 2 a.m. to 3 a.m.), several hundred individual
measurement values are collected. The sound level is measured in
dB. If, for example, the sound intensity of 15 dB is measured 120
times, this sound intensity has a frequency value of 120. Other
sound intensity values are measured with a different frequency. If
all of these frequencies are plotted as y values against a scale of
the various dB sound intensities (x axis), one obtains the so-called
histogram as a frequency distribution.
This histogram is a representation of the noise distribution in the
measured period of time. The curve has a maximum, which
represents the most frequently measured noise sound intensity.
For leakage noise monitoring, the quietest noise detected in the
observed period is of particular interest. It is the smallest dB value
for which a measurement value exists. Because a leakage noise is
always present, one assumes that this value most likely
characterizes the leakage noise, while higher dB sound intensity
values arise from the addition of primarily random noise sources
(e.g., vehicle traffic).
The so-determined smallest sound intensity value, found at the left
edge of the histogram curve, is referred to as the dBmin value and
plays a key role in the evaluation of the leakage noise
The signal spectrum can be used to make a statement about a
possible leak.
Figure 36 shows a spectrum with electrical influence. If the curve
has this appearance, one can assume that no leakage noise is
present
Figure 35: Spectrum with electrical influence
Histogram
cont.
Signal spectrum