Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Sine on Random

Sine on Random

Sine on Random

I am doing a study on Combining Sine and Random vibration.
In test bench, component is tested by starting Sinusoidal testing then followed by Random vibration testing. Harmonic effect continues in Random vibration testing.
I wants to compare my test results with simulation. I performed separately Harmonic analysis and Random analysis then combined using square root of sum of squares method ( SRSS - is not advisable because it lost phase information).
Another method I performed a Random vibration analysis in frequency domain by combining sine and random spectra ( combined in time domain )
Is it possible to compare the combined spectra ( sine and random ) with the test results ( Sequence of harmonic followed by random testing).
If possible, how can the results be justified.

Arumuga Pandian

RE: Sine on Random

Pretty advanced for this forum. You can join Tom Irvine's web page www.vibrationdata.com for $20 or so and look around for it. If anybody can resolve your issue Tom can. Go to his website for his contact information and consult with him if you can't find what you're looking for by browsing his website. He's got gobs of white papers and useful software on his website though.


RE: Sine on Random

@tunalover. Thanks for your suggestion.

RE: Sine on Random


You can model very weird vibration modes through numerical analysis. Work out your applied forces and/or displacements. Do the numerical integration. When I did this, I was curious about non-linear damping.

This may be doable on a spreadsheet. It certainly can be done with MathCAD or Octave.


RE: Sine on Random

What are the frequency ranges for both tests? Certainly the same.

Harmonic effect is due to a too high level of the excitation.
Try to decrease the level of the signal till the harmonics disappear.

To understand, you can imagine a signal with a fixed sinus frequency. The FFT is just one Dirac.
Now, if the gain is too much, then the sinus is notched out. The sinus looks like a square signal and its FFT is a suite of decreasing Dirac (the harmonics).

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close