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# RMS Calculation

Creator

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?

3 REPLIES

# Re: RMS Calculation

Siemens Experimenter

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

# Re: RMS Calculation

Experimenter

Hello,

I want to sum up orders energy correctly and therefore I just read your answer on RMS calculation in LMS and I have some questions:

- why does RMS calculation in LMS use RMS values in the sum, I would have used amplitude values

- why does LMS divide the first and last value by two (assume, it has something to do with negative frequency spectrum?)

p.s.: to calculate sum of orders I use "user metrics" in online processing and herein I have to manually write down the formula (included manual window correction) as rms sum is not provided here.

Best regards, Gregor

# Re: RMS Calculation

Siemens Phenom

This has a link to an FAQ on the RMS calculations on the GTAC support site (you will need your GTAC login):  https://solutions.industrysoftware.automation.siemens.com/view.php?sort=desc&p=1&q=RMS+calc&file_typ...

The values summed depend on the function, it is different for time, Frequency spectrums or Autopower Linear, and Autopower power functions.

As this article, the other one and the FAQ state we only half the first and last value if it does not coincide with 0 Hz.

I don't understand the following as the RMS calculations described calculate the RMS sum between frequency values of a single curve, not adding multiple curves (orders in your case) together.  Why don't you use the Data Block Processing add-in to sum your orders (or use the + sign in User Metrics)?  With the Calculate Automatically option turned on, these calculations are done automatically after the measurement run or after the results are calculated via Time Data Processing.  There are several ways to add your orders together - you can add the Amplitude or dB values.

p.s.: to calculate sum of orders I use "user metrics" in online processing and herein I have to manually write down the formula (included manual window correction) as rms sum is not provided here.

Within LMS Test.Lab we handle all the window corrections, functions, etc. for the RMS calculations.  The problems occur (hence the FAQ) when people try to duplicate the calculations in Excel for example.  If you need more information, your local GTAC support team can assist or write an Enhancement Request (ER) if needed.