CEMB USA N500 - Manual (Page 39)

FFT analyzer mode 6 - 7
One of the most frequent uses is the so-called Impact test: A hammer is used to stress a
structure and to cause it to vibrate in order to determine its natural frequencies. For such
purpose, a sensor should be placed in the zone to be examined and a threshold value
chosen, which is higher than the background noise read, but lower than that produced by
the hammering with which the structure is stressed.
N.B.
After enabling the trigger and selecting the required settings, press
to return to the Measurement page in which the mode selected for the trigger is
specified by a specific icon:
–
continuous mode
–
"single measurement" mode
Now merely press , and wait for the trigger threshold to be exceeded.
If it is required to stop the procedure manually (before or after exceeding the threshold),
just press again.
1. Modes
This is the parameter which indicates whether the trigger is:
–
OFF (disabled) :
the measurement is started and stopped manually by the operator on pressing
–
Cont. (enabled in continuous mode) :
acquisition is started when the signal exceeds the trigger threshold, and continues
until the operator stops it manually (by pressing
)
–
Single (enabled in “single measurement” mode) :
When the signal exceeds the trigger threshold, a single measurement is made (duly
observing the parameters set for the FFT), then the acquisition is stopped
automatically; this is the most frequently used mode because it allows analyzing
phenomena of transitory type; by suitably presetting the FFT parameters, it is
possible to obtain an acquisition time sufficiently long for containing all the
important information.
Subsequent acquisitions would only succeed in capturing noise, therefore they
would be counter-productive.