Lavry AD122-96 MX Operation Manual page 25

AD122-96 MX Appendix IV
Such measurement and calculation may aid in evaluating the noise of a driver device, or a series of
devices. The distribution due to addition of Gaussian signals is still Gaussian. While the 8dB
"translation factor" is true for Gaussian noise, it does not hold true for non-Gaussian noise. Such factor
is 3dB for sine waves and 0dB for square waves. Higher than expected reading may indicate a pickup
of non-random noise such as power line hum.
Explanation for the 8dB factor:
The Gaussian bell shape curve is unbounded. Theoretically speaking, very high amplitude may occur,
but very high amplitudes occur very seldom (it has been said that waiting a few trillion years may
produce a million volt spike). Practical observations show that most of the peaks occur within 3.5
times of the standard deviation (sometimes referred to as a crest factor of seven). The following
calculation:
20 log (3.52/2)=7.871
justifies adding about 7.81dB to the peak hold reading. Given a 1dB meter resolution, we round the
correction factor to 8dB.
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