Larson Davis 824 User Manual

9/29/09 FFT (Optional) 12-17
NOTE: The linear units level is displayed in engineering
exponential notation. This notation expresses the exponent with a
single character: “n” for nano ( ), “ ” for micro ( ), “m”
for mili ( ), a space for no exponent, “k” for kilo ( ), “M”
for mega ( ) and “G” for giga
().
Frequently Asked Questions Concerning FFT
What is the reference level for decibels (dB) levels in the FFT option of the 824?
The decibel levels displayed by the FFT option of the 824
are the same as those for SPL in other instruments such as
SSA or ISM and is 20 micro-Pascal (set during your
microphone calibration process). Now, that may not mean
much if you are viewing the output of an accelerometer
rather than a microphone. The linear units are available to
view the output of the accelerometer but the dB level may be
of use.
To calibrate the dB level to some usable value for
acceleration requires the use of an accelerometer calibrator.
We use the PCB 394C06 Portable 1g Handheld Shaker. Set
the calibration level to something like 140dB and run
through the calibration procedure. Then the 824 FFT will
read 140 dB when it is excited with 1g. This provides a
reference of 0.1 micro-g (also 1 micrometer per second
squared).
How do I get the DNA software to read FFT data in linear units.
To get DNA to read FFT data in a linear unit such a g or m/s
the data must be converted from the decibel numbers that
DNA receives from the 824 to that linear units value. To do
this you need to know a reference level for the decibel
reading that the 824 provides. It will change depending on
the current calibration offset in the 824. The answers to the
next two questions will help you accomplish this
calculation.
10
9–

10
6–
10
3–
10
3
10
6
10
9

Larson Davis 824 Specifications

General

Brand	Larson Davis
Model	824
Category	Measuring Instruments
Language	English