Modal Analysis Basics

These articles were originally published by Dr. Peter Avitabile, Professor of Mechanical Engineering at the University of Massachusetts Lowell, and were later published under the title of Modal Space - Back to Basics by Society for Experimental Mechanics and Blackwell Publishing (1998 to 2014).

Feb 98

Could you explain modal analysis for me?

Apr 98

Could you explain the difference between the time domain, frequency domain and modal space?

Jun 98

Is there any difference between a modal test with a shaker excitation or impact excitation?

Aug 98

Is there any difference between a roving hammer and roving accelerometer test?

Oct 98

Should I always use a hard tip for impact testing so the input spectrum is flat over all frequencies?

Dec 98

Which shaker excitation is best? Is there any difference?

Feb 99

Curvefitting is so confusing to me! What do all the different techniques mean?

Apr 99

I still don't understand curvefitting. How do I get mode shapes from FRFs?

Jun 99

What's the difference between operating deflection shapes and mode shapes?

Aug 99

Are you sure you can get mode shapes from one row or column of H?

Oct 99

I heard someone say "Pete doesn't do windows" What's the scoop?

Dec 99

I'm still overwhelmed by all this modal stuff - give me the big picture!

Feb 00

I showed some mode shapes to someone; they asked if the structural design was OK

Apr 00

Can I use an oblique angle to run a modal test?

Jun 00

How many points are enough when running a modal test?

Aug 00

Someone told me SDM will never work since I don't have all the modes

Oct 00

Why is mass loading and data consistency so important for modal parameter estimation?

Dec 00

I hear about SVD all the time; could you explain it simply to me?

Feb 01

Does it make any sense to use actual loads when doing a modal test?

Apr 01

When impact testing, can the use of the exponential window cause any problems?

Jun 01

Is there any benefit to using multiple references? I thought only one reference was needed?

Aug 01

What are some of the most important things to consider when impact testing?

Oct 01

Should I use all measurements when estimating modal parameters?

Dec 01

Can the test setup have an effect on the measured modal data?

Feb 02

Which window is most appropriate for the various types of modal tests performed?

Apr 02

What is the difference between a complex mode and a real normal mode?

Jun 02

Could you please explain the basic steps to acquire data for modal testing?

Aug 02

Is there any real advantage to MIMO testing? Why not just collect SISO and then move the shaker?

Oct 02

What are some of the major mistakes that people make in modal tests so I don't make the same ones?

Dec 02

Is it better to collect averaged FRF data for a modal test? or collect time data and process it afterwards?

Feb 03

Why is cali/pation and mode shape scaling important and does it make a difference?

Apr 03

Is it really necessary to reject a double impact? Are they really a problem?

Jun 03

Someone told me that you must have multiple references to identify pseudo repeated roots

Aug 03

I ran a modal test on a portion of a structure of concern and many modes look the same!  What did I do wrong?

Oct 03

Why do some measurements have anti-resonances and others do not?

Dec 03

What's the difference between local and global curvefitting ??

Feb 04

We talked about the number of points needed for a modal test before. Someone told me that the entire shape may not need to be completely defined.

Apr 04

Do I need to have an accelerometer mounted in the X, Y and Z directions to do a modal test?

Jun 04

Mode shape scaling requires a drive point measurement. Is there any way to scale shapes without this?

Aug 04

I find overload and underload of the digitizer and range setting difficult to understand.

Oct 04

Do mode shapes of a plate have any particular predetermined order?

Dec 04

Once I have set up a good measurement, is there any reason to watch the time and frequency results for every FRF?

Feb 05

I know that certain shaker excitations have different characteristics - but which one is best to use?

Apr 05

When I perform impact testing, the input spectrum looks distorted - do you think my FFT analyzer has a problem?

Jun 05

Why does my stability diagram of a component on a system show modes that the SUM or MIF don't show?

Aug 05

Sometimes my input force is very smooth just as expected but often looks it is oscillating - Why is that?

Oct 05

I ran a shaker test with a simple beam but the modes don't look right - What's wrong?

Dec 05

Should the measurement bandwidth match the frequency range of interest for impact testing?

Feb 06

Can modal parameters be extracted for heavily damped modes when you can't see them in the measured FRF's?

Apr 06

What effect can the test setup and rigid body modes have on the higher flexible modes of interest?

Jun 06

Is there any problem running a modal test with 2 Khz excitation but only analyzing up to 500 Hz?

Aug 06

What is a good MAC value so I know my model is right?

Oct 06

Someone told me that operating modal analysis produces better results.

Dec 06

What is MRIT?  I hear people talk about it for impact testing.

Feb 07

What is the difference between all the mode indicator functions?

Apr 07

How do you select the reference location for a modal test?

Jun 07

How do you interpret the stability diagram?

Aug 07

The Stability Diagram has poles that are not indicated by the MMIF or CMIF. Are they really modes?

Oct 07

Bolted joints are common in structures. Can the frequency change significantly due to joints?

Dec 07

Sometimes the mode shapes appear to be rotated from what is expected. Are the modes wrong? What's up?

Feb 08

A reference DOF is needed for all modal software systems. Why is the FFT analyzer reference different?

Apr 08

Should more residuals be used to improve a curvefit? The results look better when many extra terms are used.

Jun 08

My coherence is better in some measurements than others when impact testing. Am I doing something wrong?

Aug 08

My accelerometer is not overloaded but my measurement is terrible. What could be wrong?

Oct 08

Double impacts are undesirable. What about multiple impacts?

Dec 08

Can you describe reciprocity? It just doesn't make sense to me.

Feb 09

Do we really need an accurate updated model?

Apr 09

I made a stiffness modification to a free-free system. The flexible modes shifted down! 

Jun 09

I made a stiffness change to the tip of a cantilever beam but I can only shift the frequency so far.

Aug 09

If I run a shaker test with the input oblique to the global coordinate system, how do I decompose the force into the specific components in each direction? 

Oct 09

When the Transfer Function is evaluated along the frequency axis, the damping is zero. Does this mean there is no damping in the system?

Dec 09

Curvefitting still seems like black magic to me. Can you explain transfer function, FRF and parameters to me?

Feb 10

Is there any effect in impact testing when hitting harder or softer with the different tips?

Apr 10

Can the shaker stinger have any effect on the FRF measurements?

Jun 10

I am still confused about how the SDOF in modal space is related to the physical response?

Aug 10

I hear mode participation all the time. What does that really mean?

Oct 10

Hey - I ran a test and got some extra modes I didn't expect. Does the overall test set up have any effect?

Dec 10

Will the support mechanism have any effect on FRFs? Does bungie cord vs fishing line make any difference?

Feb 11

Does the structure need to come to rest between impact measurements? Doesn't the damping window take care of that?

Apr 11

Why can't I run a modal test with one big shaker and just "crank up the signal"?

Jun 11

Is there really a difference running a modal test with MIMO as opposed to SISO?

Aug 11

So if I use multiple reference vs single reference FRFs, is there really a difference in the modal parameters?

Oct 11

Someone told me that you can't accept a FRF when the input spectrum has more than a 20 dB rolloff

Dec 11

Sometimes when I have a double impact, I switch hammer tips to eliminate it. Is that OK?

Feb 12

I took a measurement one day and the next day the measurement looked different. How can that be?

Apr 12

We compare tests to fully built-in models all the time. Can you really simulate built-in down in the lab?

Jun 12

If you don't excite a structure at its natural frequency, then how can you know what it is?

Aug 12

How free does a test need to be? Does it really matter that much?

Oct 12

So what really is a drive point FRF? Do you have to impact exactly at the same point?

Dec 12

We updated a composite plate model but the properties seem to not be physical.

Feb 13

Can you explain the Complex Mode Indicator Function (CMIF) again? What are these crossover frequencies?

Apr 13

If the frequencies between test and model are close, then is the model correlated?

Jun 13

Do you really need to measure FRFs? Or is a Transmissibility OK?

Aug 13

If we perform a roving impact hammer test and impact many points, is there any possibility of missing a mode?

Oct 13

So now I have another question…if the hammer impacts a node, then there is no response elsewhere?

Dec 13

Someone told me they used a Hanning window for an impact test. That doesn't seem right…Can that distort data?

Feb 14

An eigensolution gives us frequencies. But how do we get mode shapes?

Apr 14

Do you need to mount triaxial accelerometers at all locations? That can result in more channels needed and more cost.

Jun 14

Does gravity come into play when testing to find frequencies? Does orientation make a difference?

Aug 14

So I am still confused about the MAC? What is a "good" value for MAC?

Oct 14

So what is the best way to make a free-free test set up ... because nothing is really free-free.

Dec 14

So before you end this Modal Space series ... can you provide some last pieces of advice?