Larson Davis System 824, Technical Reference Manual

Page: 421 / 673

9/29/09 FFT (Optional) 12-13
FFT Calibration
The FFT spectrum always displays the measured level in
decibels on a logarithmic scale. In fact, the use of the term
“level” implies a decibel value relative to a defined
reference value.
Acoustic measurements are in dB
relative to
20 µPa
When the System 824 has been calibrated using an acoustic
calibrator, whether from the ISM, SSA, LOG or FFT
instrument, the decibel levels are relative to 20 µPa (20
micro-Pascal).
Note, however, that it is possible to provide a digital readout
of the amplitude at the cursor position in linear units as
shown below in the section “Displaying Amplitude in Linear
Units” on page 12-14
Calibrating dB to a Different Reference
For measurements of non-acoustical parameters such as
acceleration, it is more appropriate to express the
measurement in decibels relative to a different reference
using units typical for that parameter. This is easily done
using a calibrator to excite the transducer to a known value.
The decibel level of a measured parameter is calculated as
follows:
Lv = 20Log
10
[V/V
ref
]
where V = measured parameter
and V
ref
= reference value, same units as the measured
parameter
Calibrating to measure acceleration
in decibel units relative to an
acceleration unit (e.g. g, m/s
2 , ft/s 2 ,
etc.)
For example, if we prefer to express acceleration in units of
“g”, we may select to represent the “acceleration level” in
decibels relative to 1 µg. Using an accelerometer calibrator
which excites the accelerometer at an rms amplitude of 1 g,
such as the PCB 394C06 Portable 1g Handheld Shaker, we