Click here to purchase the entire book in PDF format. Standard deviation The standard deviation, abbreviated $s$, is the square root of the variance $s = \sqrt{s^{2}}$ It's a calculation of the variability of your measurements which, happily, is in the same units as the measurements themselves. Note that, when the mean is 0 (as it is in the case of most audio signals), the standard deviation is the same as the RMS value of the signal.