# Number in the upper lefthand corner of Praat soundwave

What does the number in the purple box (0.4649) indicate? I always thought it was a measurement of amplitude, but when I get the RMS amplitude of the highlighted selection, Praat returns a value of 0.2208 Pascal. Thank you!

## 2 Answers

You was thinking correctly, it is just that this value is the highest amplitude of your visible signal on the window. Whereas RMS (Root Mean Square), like its name indicates it, takes into consideration the average of the amplitudes of your signal. So the RMS value will be always between the highest and the lowest amplitude found in a given interval.

The exact formula of RMS is: √ { 1/(t2-t1) ∫t1t2 x2(t) dt }.

It appears that this is the maximum and minimum air pressure level within the window (where the link says "There are two numbers shown on the left side"). The corresponding Praat manual on sound pressure calibration explains how the integer values are normalized, and how you might calibrate recordings to get true sound pressure level.