9/29/09 FFT (Optional) 12-13FFT CalibrationThe FFT spectrum always displays the measured level indecibels on a logarithmic scale. In fact, the use of the term“level” implies a decibel value relative to a definedreference value.Acoustic measurements are in dBrelative to 20 μPaWhen the System 824 has been calibrated using an acousticcalibrator, whether from the ISM, SSA, LOG or FFTinstrument, the decibel levels are relative to 20 μPa (20micro-Pascal).Note, however, that it is possible to provide a digital readoutof the amplitude at the cursor position in linear units asshown below in the section “Displaying Amplitude in LinearUnits” on page 12-14Calibrating dB to a Different ReferenceFor measurements of non-acoustical parameters such asacceleration, it is more appropriate to express themeasurement in decibels relative to a different referenceusing units typical for that parameter. This is easily doneusing a calibrator to excite the transducer to a known value.The decibel level of a measured parameter is calculated asfollows:Lv = 20Log10[V/Vref]where V = measured parameterand Vref = reference value, same units as the measuredparameterCalibrating to measure accelerationin decibel units relative to anacceleration 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” indecibels relative to 1 μg. Using an accelerometer calibratorwhich excites the accelerometer at an rms amplitude of 1 g,such as the PCB 394C06 Portable 1g Handheld Shaker, we
