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Structural Dynamics and Acoustic Systems Laboratory

Modal Space

Modal Space cartoon
Illustration by Mike Avitabile

In Our Own Little World

These articles were originally published in Experimental Techniques by the Society for Experimental Mechanics and Blackwell Publishing under the title of Modal Space - Back to Basics.
They are now available in Adobe PDF format (see www.adobe.com for free Acrobat PDF reader).

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The following is the list of articles currently available:

  • February 98 Could you explain modal analysis for me?
  • April 98 Could you explain the difference between the time domain, frequency domain and modal space?
  • June 98 Is there any difference between a modal test with a shaker excitation or impact excitation?
  • August 98 Is there any difference between a roving hammer and roving accelerometer test?
  • October 98 Should I always use a hard tip for impact testing so the input spectrum is flat over all frequencies?
  • December 98 Which shaker excitation is best? Is there any difference?
  • February 99 Curvefitting is so confusing to me! What do all the different techniques mean?
  • April 99 I still don't understand curvefitting. How do I get mode shapes from FRFs?
  • June 99 What's the difference between operating deflection shapes and mode shapes?
  • August 99 Are you sure you can get mode shapes from one row or column of H?
  • October 99 I heard someone say "Pete doesn't do windows" What's the scoop?
  • December 99 I'm still overwhelmed by all this modal stuff - give me the big picture!
  • February 00 I showed some mode shapes to someone; they asked if the structural design was OK
  • April 00 Can I use an oblique angle to run a modal test?
  • June 00 How many points are enough when running a modal test?
  • August 00 Someone told me SDM will never work since I don't have all the modes
  • October 00 Why is mass loading and data consistency so important for modal parameter estimation?
  • December 00 I hear about SVD all the time; could you explain it simply to me?
  • February 01 Does it make any sense to use actual loads when doing a modal test?
  • April 01 When impact testing, can the use of the exponential window cause any problems?
  • June 01 Is there any benefit to using multiple references? I thought only one reference was needed?
  • August 01 What are some of the most important things to consider when impact testing?
  • October 01 Should I use all measurements when estimating modal parameters?
  • December 01 Can the test setup have an effect on the measured modal data?
  • February 02 Which window is most appropriate for the various types of modal tests performed?
  • April 02 What is the difference between a complex mode and a real normal mode?
  • June 02 Could you please explain the basic steps to acquire data for modal testing?
  • August 02 Is there any real advantage to MIMO testing? Why not just collect SISO and then move the shaker?
  • October 02 What are some of the major mistakes that people make in modal tests so I don't make the same ones?
  • December 02 Is it better to collect averaged FRF data for a modal test? or collect time data and process it afterwards?
  • February 03 Why is calibration and mode shape scaling important and does it make a difference?
  • April 03 Is it really necessary to reject a double impact? Are they really a problem?
  • June 03 Someone told me that you must have multiple references to identify pseudo repeated roots
  • August 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?
  • October 03 Why do some measurements have anti-resonances and others do not?
  • December 03 What's the difference between local and global curvefitting ??
  • February 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.
  • April 04 Do I need to have an accelerometer mounted in the X, Y and Z directions to do a modal test?
  • June 04 Mode shape scaling requires a drive point measurement. Is there any way to scale shapes without this?
  • August 04 I find overload and underload of the digitizer and range setting difficult to understand.
  • October 04 Do mode shapes of a plate have any particular predetermined order?
  • December 04 Once I have set up a good measurement, is there any reason to watch the time and frequency results for every FRF?
  • February 05 I know that certain shaker excitations have different characteristics - but which one is best to use?
  • April 05 When I perform impact testing, the input spectrum looks distorted - do you think my FFT analyzer has a problem?
  • June 05 Why does my stability diagram of a component on a system show modes that the SUM or MIF don't show?
  • August 05 Sometimes my input force is very smooth just as expected but often looks it is oscillating - Why is that?
  • October 05 I ran a shaker test with a simple beam but the modes don't look right - What's wrong?
  • December 05 Should the measurement bandwidth match the frequency range of interest for impact testing?
  • February 06 Can modal parameters be extracted for heavily damped modes when you can't see them in the measured FRF's?
  • April 06 What effect can the test setup and rigid body modes have on the higher flexible modes of interest?
  • June 06 Is there any problem running a modal test with 2 Khz excitation but only analyzing up to 500 Hz?
  • August 06 What is a good MAC value so I know my model is right?
  • October 06 Someone told me that operating modal analysis produces better results.
  • December 06 What is MRIT? I hear people talk about it for impact testing.
  • February 07 What is the difference between all the mode indicator functions?
  • April 07 How do you select the reference location for a modal test?
  • June 07 How do you interpret the stability diagram?
  • August 07 The Stability Diagram has poles that are not indicated by the MMIF or CMIF. Are they really modes?
  • October 07 Bolted joints are common in structures. Can the frequency change significantly due to joints?
  • December 07 Sometimes the mode shapes appear to be rotated from what is expected. Are the modes wrong? What’s up?
  • February 08 A reference DOF is needed for all modal software systems. Why is the FFT analyzer reference different?
  • April 08 Should more residuals be used to improve a curvefit? The results look better when many extra terms are used.
  • June 08 My coherence is better in some measurements than others when impact testing. Am I doing something wrong?
  • August 08 My accelerometer is not overloaded but my measurement is terrible. What could be wrong?
  • October 08 Double impacts are undesirable. What about multiple impacts?
  • December 08 Can you describe reciprocity? It just doesn’t make sense to me.
  • February 09 Do we really need an accurate updated model?
  • April 09 I made a stiffness modification to a free-free system. The flexible modes shifted down!
  • June 09 I made a stiffness change to the tip of a cantilever beam but I can only shift the frequency so far.
  • August 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?
  • October 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?
  • December 09 Curvefitting still seems like black magic to me. Can you explain transfer function, FRF and parameters to me?
  • February 10 Is there any effect in impact testing when hitting harder or softer with the different tips?
  • April 10 Can the shaker stinger have any effect on the FRF measurements?
  • June 10 I am still confused about how the SDOF in modal space is related to the physical response?
  • August 10 I hear mode participation all the time. What does that really mean?
  • October 10 Hey - I ran a test and got some extra modes I didn't expect. Does the overall test set up have any effect?
  • December 10 Will the support mechanism have any effect on FRFs? Does bungie cord vs fishing line make any difference?
  • February 11 Does the structure need to come to rest between impact measurements? Doesn't the damping window take care of that?
  • April 11 Why can't I run a modal test with one big shaker and just "crank up the signal"?
  • June 11 Is there really a difference running a modal test with MIMO as opposed to SISO?
  • August 11 So if I use multiple reference vs single reference FRFs, is there really a difference in the modal parameters?
  • October 11 Someone told me that you can’t accept a FRF when the input spectrum has more than a 20 dB rolloff
  • December 11 Sometimes when I have a double impact, I switch hammer tips to eliminate it. Is that OK?
  • February 12 I took a measurement one day and the next day the measurement looked different. How can that be?
  • April 12 We compare tests to fully built-in models all the time. Can you really simulate built-in down in the lab?
  • June 12 If you don’t excite a structure at its natural frequency, then how can you know what it is?
  • August 12 How free does a test need to be? Does it really matter that much?
  • October 12 So what really is a drive point FRF? Do you have to impact exactly at the same point?
  • December 12 We updated a composite plate model but the properties seem to not be physical.
  • February 13 Can you explain the Complex Mode Indicator Function (CMIF) again? What are these crossover frequencies?
  • April 13 If the frequencies between test and model are close, then is the model correlated?
  • June 13 Do you really need to measure FRFs? Or is a Transmissibility OK?
  • August 13 If we perform a roving impact hammer test and impact many points, is there any possibility of missing a mode?
  • October 13 So now I have another question…if the hammer impacts a node, then there is no response elsewhere?
  • December 13 Someone told me they used a Hanning window for an impact test. That doesn’t seem right…Can that distort data?
  • February 14 An eigensolution gives us frequencies. But how do we get mode shapes?
  • April 14 Do you need to mount triaxial accelerometers at all locations? That can result in more channels needed and more cost.
  • June 14 Does gravity come into play when testing to find frequencies? Does orientation make a difference?
  • August 14 So I am still confused about the MAC? What is a "good" value for MAC?
  • October 14 So what is the best way to make a free-free test set up ... because nothing is really free-free.
  • December 14 So before you end this Modal Space series ... can you provide some last pieces of advice?

A compilation of the all of the articles is also available.
If you have a question on some aspect of Modal Analysis, just contact us.

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In January 2001, Sound & Vibration magazine published an article entitled Experimental Modal Analysis - A Simple Non-Mathematical Overview which is a very good summary on the main aspects relating to experimental modal analysis.

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A copy of the notes that are used for the Society for Experimental Mechanics, Young Engineer Program is offered every year at the International Modal Analysis Conference is available online:

For any further information please contact us.