Modal testing

The transfer function (or frequency response function (FRF)) is often curve fitted to estimate the modal parameters; however, there are many methods of modal parameter estimation and it is the topic of much research.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.

When the signal is transmitted through the piano wire or the stinger, the object responds the same way as impact testing, by attenuating some and amplifying certain frequencies.

For large civil engineering structures much larger shakers are used, which can have a mass of 100 kg and above, and are able to apply a force of many hundreds of newtons.

Shakers have an advantage over the impact hammer as they can supply more energy to a structure over a longer period of time.

However, problems can also be introduced; shakers can influence the dynamic properties of the structure and can also increase the complexity of analysis due to windowing errors.