Norsonic nor140, Instructional Manual

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Norsonic Nor140
Instruction Manual
The corresponding level sensitivity will be:
Typical attenuation through the adapter Nor1447/2 and
the preamplifier Nor1209 is 0,1 dB. Subtract this at-
tenuation from the level sensitivity (E.g: -52,0 dB).
Remember to select “Standard” as preamplifier and
set the preamplifier correction and polarization voltage
to “OFF” in the set-up menu for the instrument.
Velocity and displacement
Vibration is often measured as velocity or displace-
ment. The velocity can be obtained from the accel-
eration by time integration of the signal. In a similar
way displacement may be obtained by integrating
the acceleration twice. No integration is performed in
the sound level meter Nor140. However, if the vibra-
tion signal is sinusoidal with frequency f, an integration
correspond to dividing the acceleration by the angular
frequency
ω
= 2
π
f
. This may also be used for more
complex signals analysed in 1/3-octave bands or an-
alysed by the FFT-option. An example may illustrate
how this is done.
Assume that the vibration level in the 80 Hz band
(1/3-octave is assumed) is 74 dB or 0,1 ms
-2
. The exact
midband frequency for the 80 Hz filter is 10
1,9
= 79,43...
Hz (See ISO 8041). The vibration velocity is therefore:
The corresponding vibration displacement is:
Calculation of frequency
weighted acceleration
Different types of frequency weightings are in use for
obtaining a single value for the vibration severity. The
international standard ISO 8041 specify nine different
frequency weightings related to human response to
vibration. The different time-averaged weighted accel-
eration values may be calculated from the levels in the
1/3-octave bands.
If the acceleration in i
th
frequency band is
a
i
, and
w
i
is
the weighting factor for that band, the weighted accel-
eration,
a
w
, will be given by:
The values for
w
i
are found in ISO 8041 and depends
on the selected weighting.