9/29/09 FFT (Optional) 12-19
3. 167.11-167=0.11dBVsubtract 167dB (180-13, dB volts)
4. 0.11+ 26 = 26.11dBgAdd the Linear Scale setting
5. -26.11/20 = 1.3055divide by 20
6. 10^1.3055 = 20.21gtake 10 to this power (result will be
close to the FFT display)
The difference (+0.2%) between the 824 display and this
calculation are primarily due to the precision of math done
in the 824 versus precision of the math functions in the
calculator.
Method 2:
Take any FFT display reading that shows a dB and a linear
level directly from the 824 display.
Calculate the linear reference level (what linear level gives
0dB) using this formula:
R = L / (10^(D/20))
where R is the linear reference level for the current
calibration settings of the measurement,
where L is the linear level displayed (i.e. g or m/s), and
where D is the decibel level displayed.
Here's an example. I have an 824 connected to a signal
generator supplying 0.5 volts RMS, the Linear Units setting
is "g" and the Linear Scale setting is "-26.00" dB. I run the
824 FFT for a while and press stop, the display now shows a
dB level of 121.7dB and a linear level of 50.87 milli-g.
R = L / (10^(D/20))
R =.05087g / (10^(121.7dB/20))
R =.05087g / 10^(6.085)
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