Each mode will have their own individual rise or decay times. One method to measure the decay time and damping factor of the mode is to measure the half power bandwith and divide that frequencyu distance with the resonance frequency. Tedfious if done manual. However, if a modal analysis is done, the mode fitting process will extract that information from the modes, given a good fit.

A third method could be to measure the damping in oactave or one thiord octave bands similar to the room acoustic reverberation test with impulse sources.

Maybe the -60 dB level can be a limit? There are some «standards« used for this. E.g. for the rise time of transients, limit met at -3dB of max, or something like that.

Anders is perfectly right, knowing the free vibration frequency is not sufficient, what matters to set this transient time right is the associated structural damping. It depends primarily of the nature of this damping: while the tanDelta of a single piece of metal is only few 1/1000 (the material's loss factor), assemblies will increase it to several 1/100 for most industrial structures (friction between interfaces) - the more complex the structure the higher loss factor. It can be easily determined experimentally by measuring the acceleration decay averaged from several shock pulses here and there (rubber hammer for example)- this does not require complex vibration analysis equipment, using simply an oscilloscope with trigger mode is fair enough... It will remain an average/approximative determination compared to a full modal analysis as suggested by Anders, but still sufficient to get the right order of the number of periods you need to consider the vibration level as steadily established

Sometimes it's enought to use the cellphone microphone and hit the structure with a little hammer.

If You are lucky the important natural frequencies are also in the range of good air borne sound emmision.

And all You have to do is to visualize the time data from Your recorded WAV.

Bernard Garnier & Andre Pevestorf

Dear researchers,

I wanted to share some recent findings from my testing involving accelerometers on a concrete beam. Specifically, I utilized the roving hammer method. Following the importation of acceleration and hammer data into Artemis Pro software and the subsequent acquisition of the first mode shapes, I have noticed that the mode shapes are not as smooth as desired.

For this particular 2-meter beam, I employed 22 measurement points. Considering my focus on damage detection, I am uncertain whether the observed noise stems from the measurement process or is indicative of structural damage. Consequently, I am contemplating whether it would be advisable to remove the noise before importing the data into the Modal analysis software (Artemis).

Your expertise and guidance in this matter would be greatly appreciated.

Dear Fahime, only very large structural damages would influence the modal response of a concrete beam! Many people tried unsuccessfully before you..

The noise relate to the very limited energy of your hammer impacts, you must average at least 10-20 shock responses duly synchronized. And use a bandpass filter in your frequency band of interest... Damages are increasing the structural damping but only marginally decrease the modal frequencies... Good Luck !