Modal Theorical FAQ

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The following articles have originally been published in SEM Experimental Techniques. It is write by Dr Peter Avitabile head of The Structural Dynamics and Acoustic Systems Laboratory (SDASL) at the University of Massachusetts

Modal Generality

What is modal analysis?.
Common mistakes.



Basic steps to acquire data
Can the test setup have an effect on the measured modal data ? Do the setup boundary conditions and accelerometers have an effect?
How many points are enough when running a modal test
is it better to collect averaged FRF data for a modal test? Or collect time data and process it afterwards?
We talked about the number of points needed for a modal test before.

Sensor and reference

I ran one test with an x-excitation and can see some modes and another test with a y-excitation and see some different modes could I use an oblique angle instead ?
Do I need to have an accelerometer mounted in the X, Y and Z directions to do a modal test?
Is there a difference between a roving hammer and roving accelerometer test?
Someone told me that you must have multiple references to identify pseudo repeated.
Is there any benefit to using multiple references?I thought only one reference was need.
Should I always use a hard tip for impact testing . . .so the input spectrum is flat over all frequencies
double impact question


Is there any difference between a modal test with a shaker excitation or impact excitation?
Is there any real advantage to MIMO testing? Why not just use SISO and then move the shaker?
Which shaker excitation is best? Is there any difference?

results interpretation.

I showed some mode shapes to someone. They asked me if the structural design was ok. What should I tell them ?

Modal theorical


When impact testing, can the use of the exponential window cause any problems?
Which window is most appropriate for the various types of modal tests performed?
I heard someone say Pete doesn't do windows. Whets the scoop.


Curvefitting is so confusing to me!
What's the difference between local and global curvefitting ?
I still don't understand curvefitting ...

others questions

What's the difference between a complex mode and a real normal mode?
Could you explain the difference between time domain, frequency domain and modal space?
I ran a modal test on a portion of a structure of concern and many modes look the same!
Someone told me SDM will never work
Are you sure you can get mode shapes from one row or column of the H matrix?
I hear about SVD all the time Could you explain it simply to me?
I'am still overwhelmed by all this modal stuff Laplace, Fourier, FRFs, and all that!
Does it make any sense to use the actual operating loads as the excitation force for a modal test if the force is a random signal ?
Why is calibration and mode shape scaling important? And does it make a difference?
Mode shape scaling requires a drive point measurement.Is there any way to scale shapes without this ?
What's the difference between operating deflection shapes and mode shapes? sometimes they look the same to me!
Why is mass loading and data consistencyimportant for modal parameter estimation?
Should I use all collected measurements when estimating modal parameters ?
Why do some measurements have anti-resonances and others not ?