5 votes
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Do internal / external stresses affect modal frequency?

Consider a small piece cut out of a structure that has non-zero internal stress. To maintain equilibrium, there must be some forces applied to the boundary of the piece. (Of course when it was part ...
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5 votes
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How to simulate the Natural Frequencies of a Planetary Gearbox?

To obtain the modes shapes and resonant frequencies, you start from your equation of motion with no externally applied forces, which is indeed as you've stated. $$\mathbf M \mathbf{\ddot q} + \mathbf ...
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  • 1,262
4 votes

What is the frequncy range of seismic vibrations?

The frequency and ground acceleration and other pertinent data are recorded in a seismic design espectra which is unique for each site and varies with the type of the soil on the side. Usually ...
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3 votes
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What does multiplicity of a multiple natural frequency mean?

"Multiplicity" usually refers to several different modes with the same natural frequency. For example, ignoring any fault conditions and tolerances, if there are several identical planet gears, each ...
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3 votes

What type of test signal is appropriate for frequency response analysis of string instruments?

First, can you really actuate white noise? If yes, White noise would probably show many natural frequencies and harmonics. However, make sure the actuation bandwidth is sufficient for your frequency ...
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3 votes
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Is a 3 Hz beat frequency from my neighbor's HVAC a sign of a malfunction?

A 3Hz pulsating hum-- noise that is getting louder and softer at 3Hz. Yes, I agree that would be quite annoying. Since this is a residential unit in the US, both the compressor and the fan are run ...
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3 votes

Definition of stiffness for structural dynamics: How would you find the stiffness of an unusual beam directly?

k=F/d is a linear relationship. If you're doing a non-linear analysis, you shouldn't expect a linear response. For this beam, a geometrically non-linear analysis is appropriate if your deflections ...
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2 votes

Generating a Blank Seismic Tripartite Graph

This is the only place on the internet that I could find that actually gave decent resolution images for tripartite plotting paper. However, it had some limits that affected use in our area: 1) the ...
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2 votes
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Generating a Blank Seismic Tripartite Graph

You can try running the following R script which produces the figure below. ...
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2 votes

How to get natural frequencies from accelerometer measurements?

It depends on what your data looks like. I recently got some accelerometer data that looked like this after running it through Python's Scipy fft function: The results are pretty straightforward- ...
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What's the difference of using different state-space forms for solving eigenvalue problem?

Regarding the different state space formulations. First, M might not be invertible. In many cases it will be invertible, but sometimes it will not be. In your specific case, this would correspond ...
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2 votes

What's the difference of using different state-space forms for solving eigenvalue problem?

For any sinusoidal motion, the $\mathbf{q}$ really represents the motion $\Re(\mathbf{q}\,e^{i \omega t})$ in the time domain. $\mathbf{q}$ is complex because in general, the different elements of the ...
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2 votes

What is Experimental and Operational MODAL ANAYLIS?

The most basic purpose of the measurements is system identification - i.e. to determine the modal frequencies, mode shapes, and damping factors for the modes in some frequency range. You can use ...
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2 votes

What is Experimental and Operational MODAL ANAYLIS?

Typically you are not doing modal analysis for its own sake. Typically you have a problem and you are using modal analysis as a tool to solve that problem. The goal of the modal analysis will depend ...
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Modal Testing and Frequency Response Interpretation

You can't make much use of just one FRF, especially if none of the peak responses line up between the model and the test. Fix the accelerometer at the position of one of your model DOFs and then ...
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2 votes

Nastran / Frequency Response / Free-Free

If your structure was literally hung on a piece of (plastic covered) wire then it was constrained vertically, at that point, by the wire. A better method is to hang it from a soft bungee cord, so its ...
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2 votes

Relation between natural frequency and the thickness

We know for undamped harmonic vibration $\omega= angular velocity \space, L=length of brick \ , m=mass \ , \\ I= \text{second moment of area of the brick section } \ , \ h= \text{the height of the ...
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2 votes

Construct "Modal Coordinates Time History" from transfer function and test accel-time data

I assume you will be doing this in a software, matlab or Abaqus for example, it’s not particularly easy thing to do with your given info, but. Perhaps this example method will help Abaqus Infos at ...
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How do I study the frequency response of a physical system with Arduino?

The arduino does impose some limits. But a classical frequency identification is build from a few steps: As the other comments already suggest, the design of a suitable input. As mentioned, a ...
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2 votes
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How to determine the dynamic response of a spring-mass system?

As TimWescott indicated, this problem is more suitable for control theory for a fuller understanding than for digital signal processing, yet there's no control.se as far as I know. Hence you can give ...
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2 votes

How can I find the right resonant frequency of a chassis?

Assuming that the model is adequate and appropriate, then to answer your second question: Which frequency out of those five is right? All of those modes and frequencies are possible in the structure....
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1 vote

Why phase margin and gain margin should be positive?

The gain and phase margins of a system are characteristics that can be obtained by studying one of the following well-known plots: Nyquist Plot, Bode Plot, Nichols Chart, Disk Margin which gives ...
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1 vote

Relation between natural frequency and the thickness

Natural frequencies depend upon stiffness and mass. These two properties depend on the material characteristics and geometric configurations. In general: Increasing stiffness, the natural frequencies ...
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1 vote
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How do I verify the reaction forces of an FE model due to a response spectrum

In response spectrum analysis of MDOF structures, the most common methods of modal combination are: absolute sum (assume the peak response of all modes occurs simultaneously) SRSS (square root of ...
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1 vote

What can be the reason of the constant 50 Hz sound in my home?

Some large buildings have a transformer attached to them for the mains supply for all apartments. The vibrations are probably being transmitted via harmonic resonance due to acoustic amplification. ...
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1 vote
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Base Driven Tuning Fork Mechanical Response Spectrum

You said you applied a constant displacement to the base. In that situation, if the structure doesn't have any modes that can are excited at a particular frequency, the response is going to be ...
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1 vote

MIMO control: static inverse plant decoupling

If a MIMO plant is upper or lower triangular then one can't create an interaction loop which might make the system unstable. So using stabilizing SISO feedback control for each diagonal term should ...
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1 vote

How the amount of bolt's preload in a single lap joint can change the natural frequencies?

I'm unsure about the magnitude of the change, but providing tension by preloading an object definitely alters the natural frequency. It is commonly adopted to avoid resonance from external disruptions....
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1 vote

Eigenfreuency of beam

Are you looking for a single number for one particular beam, or a closed form solution for any possible combination of dimensions? If the former, the easiest will be ANSYS (or Abaqus or NASTRAN or ...
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1 vote
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Undamped forced vibration for a multi dof system

The code in your OP seems to be for a 2 DOF system not a 24 DOF system. You could check that by printing the "frequencies" variable from your eigensolution. The reason you are not getting anti-...
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