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RMS Calculation

[ Edited ]

I exported some spectral data from LMS Test.Lab to Excel, and tried to reproduce the RMS/Overall calculation according to this formula I found in the online help of LMS Test.Lab:

online_help.png

 

I don’t get the same RMS value in Excel as I do in LMS Test.Lab!

 

Here is a screenshot from LMS Test.Lab:

 TestLab.png

 

 

The total RMS in LMS Test.Lab is 0.02554 g. In Excel, using the online help formula, it is 0.0443203 g. A screenshot of my Excel calculations is below.

Excel.png

 

 

Any ideas?

 

 

 

 

 

 

 

 

 

 

 

 

1 REPLY
Solution
Solution
Accepted by topic author TBake
‎12-20-2016 09:39 PM

Re: RMS Calculation

[ Edited ]

Hi,

 

Noticed that you had peak data values with a Hanning window.  The data values are amplitude corrected (factor of 2) for a Hanning window.

 

The RMS total is based on individual rms values which are energy corrected (factor of 1.633). Other windows (Flattop, etc) have different amplitude and energy correction values.

 

To get the correct total RMS value you must:

  • Multiple the peak values by 0.707 to convert to rms
  • Divide by 2 to remove amplitude correction
  • Multiply by 1.633 to energy correct
  • Square all the values
  • Divide the last and first squared value by 2
  • Sum all the squared values and take the square root

Here is a screenshot from Excel, with one of the key formulas showing:

Excel2.png

 

The rms and energy correction is also mentioned in the online help, although an example is not given:

online2.png

 

Doing the extra rms and energy correction with the same 10 values you showed in your example gives the correct RMS total of 0.02554 g