Natural Frequency ‘Bump’ Testing

Scenic Acoustic and Vibration Engineering Ltd (SAVE) use Natural Frequency ‘Bump’ Testing techniques to determine the natural frequency of structures. It can also be used for the identification of system damping, mode shapes, modal stiffness, and modal mass, and isolation efficiency. Natural Frequency ‘Bump’ Testing plays a vital role when vibration is diagnosed as an issue. The main uses of Natural Frequency ‘Bump’ Testing and Modal Impact Bump Testing are for Resonance Testing and Resonance control applications, although advance bump testing techniques such as Modal Impact Bump Testing would be required to perform a full resonance test.

There are various methods of Natural Frequency ‘Bump’ Testing. The two most basic methods are Negative Averaging and Peak hold averaging technique. Both methods can be performed on running machines. Negative averaging method averages out the vibration caused by the rotation of the machine from the vibration response due to the impacts. Peak Hold Averaging simply holds the spectral peak at its maximum amplitude but does not average out the vibration due to rotation. Neither of these methods provide the force impact information or the phase information. They simply show the amplification at the frequencies excited by the bump test. These methods are not Modal Analysis techniques.

An ideal impact to a structure is a perfect impulse, which has an infinitely small duration, causing a constant amplitude in the frequency domain, this would result in all modes of vibration being excited with equal energy. The impact hammer test is designed to replicate this, however in reality a hammer strike cannot last for an infinitely small duration but has a known contact time. The duration of the contact time directly influences the frequency content of the force, with a larger contact time causing a smaller range of bandwidth. A load cell is attached to the end of the hammer to record the force. Impact hammer testing is ideal for small light weight structures, however as the size of the structure increases issues can occur due to a poor signal to noise ratio, this is common on large civil engineering structures.

Another method of Natural Frequency ‘Bump’ Testing is the Modal Impact Bump Test using a calibrated Force Impact hammer. Modal Analysis techniques can be used to ascertain the Natural Frequency of a system or component. This method is much more detailed and would be required to perform a Modal Analysis of a structure or component. This method would be applied to ascertain the Impact Force, Coherence, Phase, and Transmissibility and/or the Frequency Response Function (FRF). Providing Dynamic force impact measurements are recorded to calibrate the system response (response and impact measured at the same location in the same direction), the Mode Shapes, Modal Stiffness, Modal Mass, and Damping can be calculated. This process requires processing and analysis of the Real and Imaginary Vibration Magnitude and Phase response data.