# Modes of Vibration

1. Mar 23, 2012

Hi,

I'm playing around with ANSYS to find the modes of vibration of a structure. How do I know which are the most important modes? I understand that there are infinite modes of vibration - getting higher and higher in frequency.

I'm guessing that it has something to do with the effective mass of each mode versus the total mass.

Also, when people say "Natural Frequency," which mode does that frequency belong to?

Cheers

2. Mar 23, 2012

### OldEngr63

There is a "natural frequency" associated with each mode of vibration. For most purposes, the lower frequency modes will be the important modes, because it takes more and more energy to excite the higher frequency modes.

3. Mar 24, 2012

Ok. How do I know how many of the lower modes are important?

The reason I ask is because the first 9 are well below forcing frequency. 10th mode is pretty much bang on the forcing frequency.

4. Mar 24, 2012

### AlephZero

If your excitation force is a sine wave at a fixed frequency, the important modes are likely to be the ones close to that frequency. It doesn't matter whether that is the first or the 100th mode.

Actually you can improve on that statement by saying the important modes are also those where the structure moves a lot at the point where the force is acting, because theo more the structure can move, the more work force can do to excite that mode (work = force x distance).

5. Mar 24, 2012

### OldEngr63

In a transient situation, all of the lower modes are important. In steady state, it is more as AlephZero has described. Remember also that an excitation acting at a nodal point of a particular mode cannot excite that mode, no matter what the extent of frequency alignment.