The natural frequency is the frequency at which a system vibrates when disturbed. A vibrating object may have one or multiple natural frequencies. All objects vibrate when acted on by an external stimulus. The vibrations occur at various natural frequencies and with varying shapes. The modal frequency and the modal shape, together, characterize a vibration mode.
The natural frequency of a system can be altered by changing the properties of the system. For example, a tuning fork for the musical note "A" vibrates at a frequency of 440 Hz. To change the natural frequency you can change, for example, the length of the two tines. Similarly, you can change the length, thickness, or tension of a guitar string to change its natural frequency.
In a modal frequencies analysis, the natural frequencies of all calculated modes are included in a drop-down list near the plot legend. The number of modes that the solver calculates depends on the study settings you choose when setting up the simulation. The frequencies are given in Hertz (cycles per second).
The legend shows the natural frequency and mode number associated with the currently displayed deformed shape. When you access the drop-down list, you can see all vibration modes in order of ascending frequency. When you choose a particular mode, the displacement plot changes to show the deformed shape for that vibration mode choice.