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12-20-2016 09:31 PM - edited 12-20-2016 09:39 PM

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:

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:

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.

Any ideas?

Solved! Go to Solution.

1 REPLY

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12-20-2016 09:37 PM - edited 12-20-2016 09:41 PM

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:

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

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

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