The usual method of excitation is to have the hammer impact the column to create an impact elastic wave. After the impact, the hammer body leaves the object to be measured. Therefore, when the hammer body leaves the object to be measured, the external force becomes 0, and the measured object becomes a free vibration state.
In this system, the suppression of free vibration is a method of adding additional mass and additional static load. During the excitation process, the hammer is kept from leaving the surface of the test body and always maintains a certain static load.
In short, the residual vibration can be effectively suppressed by continuously maintaining a static load and additional mass after hitting the top section of the pile. In daily life, we can actually see similar examples. For example, when knocking and drumming, in order to stop the sounding (vibration) in the middle, it is often used on the cymbal, the drum or the hand by a hammer. The mechanical principle is the above theory.