# What is Modal analysis: Definition and 45 Discussions

Modal analysis is the study of the dynamic properties of systems in the frequency domain. Examples would include measuring the vibration of a car's body when it is attached to a shaker, or the noise pattern in a room when excited by a loudspeaker.
Modern day experimental modal analysis systems are composed of 1) sensors such as transducers (typically accelerometers, load cells), or non contact via a Laser vibrometer, or stereophotogrammetric cameras 2) data acquisition system and an analog-to-digital converter front end (to digitize analog instrumentation signals) and 3) host PC (personal computer) to view the data and analyze it.
Classically this was done with a SIMO (single-input, multiple-output) approach, that is, one excitation point, and then the response is measured at many other points. In the past a hammer survey, using a fixed accelerometer and a roving hammer as excitation, gave a MISO (multiple-input, single-output) analysis, which is mathematically identical to SIMO, due to the principle of reciprocity. In recent years MIMO (multi-input, multiple-output) have become more practical, where partial coherence analysis identifies which part of the response comes from which excitation source. Using multiple shakers leads to a uniform distribution of the energy over the entire structure and a better coherence in the measurement. A single shaker may not effectively excite all the modes of a structure.Typical excitation signals can be classed as impulse, broadband, swept sine, chirp, and possibly others. Each has its own advantages and disadvantages.
The analysis of the signals typically relies on Fourier analysis. The resulting transfer function will show one or more resonances, whose characteristic mass, frequency and damping ratio can be estimated from the measurements.
The animated display of the mode shape is very useful to NVH (noise, vibration, and harshness) engineers.
The results can also be used to correlate with finite element analysis normal mode solutions.

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1. ### Vibration and Modal Analysis: Adding massless springs to a shaft

Hello, I am working on vibrations and modal analysis recently. I have a question that I could not find any answer. Can you please help me? Imagine a shaft. When we run the free free modal analysis lets assume that first bending mode is 600 Hz. Then we add 2 massless springs to the shaft. The...
2. ### Modal mass and kinetic energy in FEM modal analysis

So, I use Ansys (well known FEM software) and get the next output for a modal analysis toy problem (If you happen to know Ansys that's a pre, but I promise it shouldn't matter). The problem is a simple beam, clamped at one end. I used 160 20-node brick elements to solve it (so no Timoshenko...
3. ### A What is the purpose of modal analysis?

Hello all, I have been asking this question, here, and gaining more insight. I think I can finally ask it the way I need. I can: Conduct an eigenvalue analysis Code the Lanczos algorithm. Understand mode shapes Build the solution of set of coupled differential equations from mode shapes...
4. ### I Justification (or philosophy) of modal analysis

Hello I have tried asking this different ways, here. I do not know exactly what I am asking. So I am hoping to try again. Say we have a differential equation for a multi-body system. We cannot solve it because of the coupling (too difficult). So, we turn to a modal analysis (eigen value...
5. ### A Modal Analysis vs. the Solution to the Diff.Eqs.

Hello The figure below is that of a two body problem in oscillation. Whether by Free Body Diagrams or Hamilton's Principle, we can obtain the two coupled differential equations that describe the oscillation. At this point, one could obtain the closed form solution for the displacement...
6. ### Vibrations, Modal Analysis and Mechanical Systems

Hello I have used and enjoyed the textbook: "Theory of Vibrations with Applications," by William Thomson However, many of the examples there are more civil engineering than mechanical. There are many good examples of "buildings" under vibrations (good examples, but civil, nonetheless) The few...
7. ### Automotive Modal analysis - Input and boundary conditions given?

My understanding in modal analysis is very limited. All I know is it helps to find a specific mode of vibration and the natural frequency corresponding to it. While I was discussing about this with my NVH team colleague, he told me that there is no force input or excitation input given to a...
8. ### I What is the Purpose of Modal Analysis in Mechanical Systems?

Consider this system, above. If I studied the mechanics of this system, I get this system of equations. At that point, I choose to study a free vibration problem and drop the damping, and conduct a modal analysis. I understand the process:vvI assume a solution such that each mass moves...
9. ### RVE Periodic Boundary Conditions in ANSYS Workbench Modal Analysis

Hi I have a project regarding micromechanics of composites. I'm starting my analysis on the Fiber Matrix RVE. Right now I'm trying to find the natural frequency of the unit cell. The Unit cell has some unique geometry which I will keep on changing to see how natural frequency changes. I have...
10. ### I Repeatability of necessity: number restrictions?

The necessity quantifier (aka Provability quantifier, or ~◊~, or Belief, or... instead of the usual square I will be lazy and call it "N") is often allowed to be repeated as many (finite) times as one wishes, so NNNNNNψ is OK. Is it possible to somehow include into the axioms some restriction on...
11. N

### Modal analysis - results exported - need to compute deflecti

Hello, I have got results for a modal analysis (mesh of 150 shell elements) in ascii format. I have also got (from another process) forces to apply at each element; how do it compute the deflection? I get that it's a form of summation of all the modes, but I am stuck. Thanks! BB
12. ### How to eliminate modes in Ansys modal analysis

I am studying the modal analysis of a mechanism and I am only interested in the transverse modes, I wonder if there is a way to formulate my problem so that the results show only the transverse modes without the torsional and longitudinal ones, this would help in the post-processing a lot.
13. ### Can I use symmetry in modal analysis in FEA

I am doing FE analysis for symmetric shape in boundary conditions and geometry, I would like to know if I can use symmetry in my analysis or not. I have tried to run trials, the results are very similar but I am not sure if this just a coincident
14. ### Rotor Analysis -- Big deformation problem

Hi everyone, i am currently doing a rotor analysis with "modal analysis" on Ansys and even though my rotor's specs are L=145mm D=10mm and the rotating velocity is 10000rpm i get a deformation of 100mm at most of the modes. I have read that this might be cause of a mass-normalized value but i am...
15. ### A Uncertainty Propagation in Coupled Oscillator

I am a senior physics and mathematics major, and this is my last semester. As a result, I am taking advanced physics lab, which feels more like a grad school experiment than an undergrad. One of the labs deals with the modal analysis of three spring-mass systems placed vertically as shown in the...
16. ### Large deformation in modal analysis

Hi all, I have been simulating the following rhombus compliant mechanism using Ansys Modal analysis to have a quick understanding about its mode shapes,,,, the problem is that the deformation is extremely large you can see this as the scale is (*10^-5 ) . This result is for sure unrealistic but...
17. ### B Modal Analysis & Mode Shapes: Real-World Applications

What is modal analysis and what are mode shapes. Plz explain along with a real life problem for better understanding
18. ### Boundary conditions of that beam.

Hi all! I have to calculate the natural frequency of the system. Any idea of boundary conditions of this case? There is beam supported by two springs on the left side.
19. ### Modes orthogonality in a dielectric slab

A typical mode in a dielectric slab like this, with propagation along x, uniformity along z and refractive index variation along y, is represented by the following function: f (y) = \begin{cases} \displaystyle \frac{\cos (k_1 y)}{\cos (k_1 d)} && |y| \leq d \\ e^{-j k_2 (y - d)} && |y| \geq d...
20. ### Axial Fan Modal Analysis ANSYS

Hi, We have an axial fan in the lab that apparently vibrates badly ("wobbles") at certain frequencies. To find the resonance and the bad frequencies, I decided to do an ANSYS modal analysis. I am still pretty new to it, and have a few questions. 1) Regardless of how many modes I pick...
21. ### Experimental Modal Analysis using FDD?

I am a beginner to Experimental Modal Analysis and I have a scaled miniature frame and my objective is to conduct modal identification under the structure's operating condition using Frequency Domain Decomposition (FDD). However, after I expose the structure and take the reading, the results...
22. ### Nonlinear Mass Spring Damper with Euler Bernoulli Beam

I'm trying to find a solution to a system in which a clamped free Euler-Bernoulli Beam system rests on top of a mass-spring-damper system. The MSD system has nonlinearities in both the spring and the damper and is of the form: I have extended the nonlinear restoring force to its 3rd term and...
23. ### Modal Analysis of a Rotor Blade Using Simpack

Hi everyone, I am relatively new to Simpack and would appreciate some help with getting started. Basically I am trying to obtain the displacement of a rotor blade as it rotates (I already have the model). I want to have the displacements of each modal shape along the entire blade radius. Now...
24. ### What is the correct way to compute physical displacement in FEM modal analysis?

Hi all, I have an FEM model that I am doing a modal analysis of. I wanted to check that how I am computing the physical displacement is the correct way, as I've read a lot of about normalising modes, participation factors, effective masses, etc. and I'm not 100% sure on it. I've got the...
25. ### Welded Versus Solid Piece Modal Analysis

Looks like it is time for round two... So for any of you who were following my other post (https://www.physicsforums.com/showthread.php?t=762560) about whether or not to glue my model or make a solid piece for a modal analysis, you know that I was having a lot of trouble deciding. It turns out...
26. ### Glued Versus Solid Model in Modal Analysis

I am performing Modal analysis on a fairly simple structure (a square aluminum rod with some pillars attached to the top of it) and need to get some basic modal analysis results. Making a solid aluminum piece would be rather expensive and time consuming to design and then mill out, so the...
27. ### Modal Analysis for Safe Region in Automotive Vibrations

can i locate region in an automotive, which is safe from vibrations, by modal analysis? can i conclude that blue region in mode shape a safe region?
28. ### Modal analysis of an automobile

hi. I need to locate a safe region in an automotive, where vibration level is very low. i have done modal analysis of that vehicle. In the result i got DMX valve. which is maximum deflection value. what does it actually represent? Does it means the amplitude of vibration. Can i conclude...
29. ### Modal Analysis: Interpreting ANSYS Results

I have been easing my way into ANSYS and doing some modal analyses. The component I am modeling is a body of revolution with N=15 slots in it. The cross section looks much like the one in post #1 of this thread. I started out by modeling the full 360° ring and requested the first 6 Modes...
30. ### Modal Analysis of AC Unit: Refrigerant Considerations?

For doing modal analysis of suction and discharge side of an air-conditioning unit, is it needed to consider the inside refrigerant?
31. ### Modal Analysis - Interpretation of Stability Diagrams

Hello! I'm doing a modal analysis and I'm trying to extract modal parameters from my aqcuired data. I'm using the Pulse Reflex Software. I got to the point where the Software provides me with a MIF and a stability diagram and I am to choose the parameters. However, I am unsure how...
32. ### Some questions about solving eigenproblems to do modal analysis

So for my graphics class I am currently reading "Stochastic Dynamics: Simulating the Effects of Turbulence on Flexible Structures" by Jos Stam However, while starting to implement it, I've realized my understanding of math is not strong enough to fully understand what is going on so I have a...
33. ### Mode shapes in experimental modal analysis

Hi, I am doing experimental modal analysis of a cylindrical casing. I have 64 FRFs. I have curve fitted dominant modes and obtained a synthesised FRF. Now when I see the animation of mode shapes corresponding to each of the frequencies in FRF, I am finding a not-so-significant (amplitude...
34. ### Need an help in Modal Analysis

Need an urgent help in Modal Analysis I am doing an modal analysis of audio system attached to an Instrumental panel in a car using ANSYS 13 version software. I have used SHELL281 element for meshing and Black Lanczos method to extract the modes. This shell element has six degrees of freedom. I...
35. ### What is role of mass participating factor while doing Modal Analysis?

What is role of mass participating factor while doing Modal Analysis and how can we come to a conclusion that how many number of modes are required for a particular Modal Analysis. Please help me .
36. ### Getting Started with Free Modal Analysis in ANSYS

Hi all, If we do free free modal analysis in ANSYS shall we need constrain the model or not? Where can I see the first 6 modes which have frequency equal to zero. I am confused can anybody please help? Regards Shankar
37. ### Modal analysis -reduced -fluid80

Hi, i have been modeling tank filled with water for modal analysis in ansys. fluid is modeled by fluid80 and tank by shell 63. i have used reduced method as it is the only acceptable method. i am getting very high values for mass participating in each direction which is much greater than the...
38. ### Modal Analysis of Plate using lines and beam elements

ANSYS Help: Simple Modal Analysis of Plate I am modeling a plate (36"x3"x.25") in ANSYS. When I construct a line out of two keypoints and give the line a rectangular section, the analysis works and I can output modal displacements just fine. With these modal displacements I can plot the mode...
39. ### Exploring Vibrational Modes in Rifle Barrel Design

I am not a weapons designer, this is purely to satisfy my own curiosity :) I would like to simulate a gun shot in a rifle barrel and determine the mode and freq. of vibration using a FE program (Code_Aster if anyone is interested), but I don't really know what transient load to apply. What...
40. ### Modal analysis of a pin-jointed frameworks

Hi I'm trying to find the modal frequencies of a pin-jointed frames, to validate an ansys model. So far I've used direct stiffness method to generate a global stiffness matrix for the framework in Matlab. Is it as simple as mutiplying the stiffness matrix with an inverted mass matrix and...
41. ### Modal analysis for a CANTILEVER BEAM

Hi there, I am a beginner and I need to do analytical derivation of a beam and model it in matlab. the beam is cantilever and it is excited at its cantilever end any hints to start? Cheers
42. ### More experimental modal analysis

Hi, I'm trying to use experimental modal analysis to determine the modal parameters of a beaker. I'm using a shaker and have successfully obtained FRF's (I think!) Has anybody got any idea what I've to do from here? I've read what feels like a million articles but can't figure out how to...
43. ### Is an Impact Hammer Setup Suitable for Lightweight Modal Analysis?

Hello, I need some help with setup/equipment used for a study to determine the natural frequencies on some small, lightweight samples (12in x 1.5in x .12 in.) in a catilever beam setup. The samples are made of carbon fiber and kevlar so they are fairly flexible to begin with. I am working on...
44. ### Unconstrained Modal Analysis: Application and Considerations

Hi all, I am new to this forum. I have a doubt. I have a doubt regarding modal analysis. Where exactly does unconstrained modal analysis find its application? Thanks in advance. Regards, Shankar
45. ### Modal Analysis for CE Structures

Not too long ago, there was a thread in the General Physics section (I think) where someone was asking about how large buildings are evaluated from a modal analysis perspective. It was my experience that large structures had a completely analytical approach with some scale modeling. This...