12-18 824 Reference Manual 9/29/09
How can I convert decibels (dB) levels from the FFT option of the 824 into linear units
in my spreadsheet?
There are two ways to calculate the linear units from
decibels.
Method 1:
With the FFT instrument in the 824 the data can be outputted
to the display in linear values such as 'g' (gravity), or meters/
second, volts or Pascal. The 824 makes this calibration using
a setting that the user enters. This setting is the sensitivity of
the unit relative to 1 volt and entered in dB into the Linear
Scale setting (for example a 50mv/g accelerometer would
have a Linear Scale of 20 LOG(1/0.05) or +26.02dB). The
Linear Unit setting is a text field so that the units may be
annunciated as desired.
The Linear Value is calculated in this way:
1. Take the dB reading
2. Subtract off the calibration offset (this makes the number
bigger since the cal. offset is negative)
3. Subtract 167 dB from the previous result and the number
is now in dB volts
4. Add the Linear Scale setting
5. Divide by 20
6. Take 10 to this power, this is the Linear Value
Here is an example:
The calibration offset is -45.41dB
The Linear Scale is +26dB (for a 50mV/g accelerometer)
The FFT displays a level of 121.7dB and 20.17g linear value
1. 121.7dBTake the dB reading
2. 121.7-(-45.41)=167.11dBSubtract calibration offset
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