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Natural frequency measurements w/ XL2 - post processing

Dear Forum,

I thought I might ask if you are aware of options for software that I might use in analyzing natural frequency of floor slabs with my NTI and accelerometer kit.  At a former employer, we would use the NTI to measure acceleration from a heel drop or weight drop, recording an uncompressed wav file, then post process in the 01dB dBFA software suite.  I don’t have that software anymore so wondering if you are aware of other options consultants may use?  I don’t believe NTI has a built in solution?  The important frequencies are in the 0 – 10 Hz range, so reviewing on screen during an FFT measurement does not work for this since frequency axis starts at 7.0 Hz.


Hello,


A couple ways I can think of getting the natural frequencies:

 

Use Audacity to do the post analysis

Identifying resonant frequencies does require accurate amplitude scaling.

 

You can use Audacity’s spectrogram capability.  You can download Audacity for free for PC, MAC, Linux at https://www.audacityteam.org/

 

Make an uncompressed WAV file on the XL2, uncompressed WAV recording requires the XL2 Extended Acoustic Pack Option.

 

Import the WAV file. Press the down arrow (1), and switch to the Spectrogram view (2). By default it will show spectrograms out to 20k Hz so you will want to adjust the Spectrogram Settings (3)

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I prefer the spectrogram in Audacity since you can control setup better and you can visualize the damping by seeing how quickly each peak decays.


You could also uses Audacity’s Plot Spectrum function.

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Set Algorithm = Spectrum



Use the new XL2 Vibration Option to log down to the 0.8 Hz 1/3-octave

We just added the Vibration Option which lowers the lowest 1/3-octave of the RTA from 6.3 Hz to 0.8 Hz.

 

If you have XL2 Extended Acoustic Pack, you can log 1/3-octave at up to 100ms rate.

 

You would need to import the logged RTA data in Excel to identity the resonant frequencies.

 

An advantage of logging the 1/3-octave is you also can calculate the damping https://www.bksv.com/media/doc/bv0044.pdf

 

One issue with the XL2 Vibration Option is that it is very recent (April 2018). There is not currently a version of XL2 type approved firmware that has the vibration option. A Type Approved firmware that supports the XL2 Vibration Option be available in the next couple months It is possible to switch back and forth between the Type Approved and Vibration Option firmware.

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