Svantek SV 200A, Руководство пользователя

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198 SV 200A User Manual
D.3 S
TATISTICAL LEVELS
–
L
NN DEFINITION
The noise level L(t) is the continuous random variable. The
probability that the temporary noise level L(t) belongs to the interval
L L , L
k k
 +
is called the class density and it can be expressed by
the equation:
( )
 
P t L L t L L P
n
1 i
i k k k

=
 =  +  
where:
i
t

- time intervals, in which the noise level
( )
L L , L t L
k k
 + 
occurs,
L

- so-called class interval or distribution class of the series,
P - total observation period.
In case when the class interval approaches infinity, the probability of L(t) tends to the probability of
k
L
. In
practice, L

value is strictly determined and it depends mainly on the dynamics of the measurements
performed in the instrument. There are 120 classes in the instrument and the width of each class is equal
to 1 dB. The histogram is the set of the class density values calculated for all classes.
The statistical distribution function, which determines the probability
(expressed in %) of the noise occurrence on the level equal or less
than
L L
k
 +
is given by the formulae:
( )
 
( )

=
= 
j
1 k
k j
L P L t L P
The cumulative density function, expressed by the equation:
is directly used to determine so-called statistical levels Lnn or
position parameters of the distribution.
( )
 
( )
 
j j
L t L P 1 L t L P
 − = 
The Lnn is the certain boundary level surpassed by the temporary noise level values in not more than nn%
of the observation period.
Example:
Let us assume that L35 is equal to 76.8 dB. It means that during
the measurements the noise level 76.8 dB was exceeded in not
more than 35% of the observation period.
The cumulative density function for the exemplary data is presented
in Figure on the right side. In order to determine the
Lnn level one
has to draw the horizontal cursor and find out the crossing point
between the cumulative density function and the cursor. In the
instrument the user can determine 10 statistical levels - from
L01 to
L99 (1% step of observation period).
The display in the instrument presents only first statistical level N1
(set to: L01 up to L99).
The statistical level LN%
value, the profile’s number the statistics
are taken from, the RMS detector ( Lin. , or Exp. : Fast , Slow or
Imp
.), the filter’s name (
A , C or Z ) and real time are displayed in
the top-right side of the display in one-result view mode.
Exemplary cumulative density