Resonance is a term that is familiar to everyone, but what is the phenomenon of resonance and what is the cause of resonance? Also, what is the “resonance frequency” that people often talk about?

Structural [resonance]: When a structure is acted upon by an external force, if the “force frequency” or “excitation frequency” of the external force is equal to or close to the “natural frequency” of the structure, the structure will experience a “large vibration” phenomenon, which is called “resonance”.

There are two terms here, one is the “excitation frequency” of the external force, and the other is the “natural frequency” of the structure.

Please refer to the diagram, [Why can the swing swing higher and higher?], in fact, it is the application of the principle of resonance. The explanation is as follows:

1.  A swing is a structure, so it has a “natural frequency” of its structure. The blue line shown in the figure is the swing trajectory of the swing, which can be used to calculate the “natural frequency” of the swing.
2.  When swinging on a swing, we will grasp the same time point when the swing swings down and swing hard. This force application method, as shown in the figure, with the red arrow representing the time point of force application, can be used to calculate the force application period and the “excitation frequency” of the corresponding external force.
3.  In order for the swing to be able to swing higher and higher, the “excitation frequency” of the external force applied must be close to the “natural frequency” of the swing. If they are equal, the “resonance” effect will be more significant, so the swing will be able to swing higher and higher.

The reason for [resonance] is: when the “excitation frequency” of the external force is equal to the “natural frequency” of the structure, “resonance” will occur. The phenomenon of [resonance] is that the vibration of the structure will become larger and larger, just like a swing will swing higher and higher.

When we talk about the [resonance frequency], in fact, what we are really talking about is the “excitation frequency” of the external force. Because the “excitation frequency” of the external force is exactly equal to the “natural frequency” of the structure, “resonance” is generated, causing the structure to have the phenomenon of “large vibration”.

For general machines, we do not want resonance to occur, so we need to keep the “excitation frequency” of the external force away from the “natural frequency” of the machine structure. This is the basic principle for avoiding resonance.

So what is the “excitation frequency” of external force? How to know the “excitation frequency” of the external force of the machine? To be analyzed next time!